Home Addition: Cost Guide & Contractor Quotes

We put together a home addition cost guide to help you understand budgeting when it comes to building a home addition. Free contractor quotes included.

At some time you have probably wished your home was larger. Whether you have just had another child and need more bedrooms, children growing into their teens and need more living space or an elderly relative needing to move into a self-contained apartment joined onto your house. Whatever the reason, there comes a time when the choice is between moving house or building an additional room or rooms onto your existing home. If you have the choice and you have no other reasons to move house, I would always go for building extra rooms.

Once you have made the commitment to build an addition (in the UK this is called an extension), then you are probably quite happy with where you live. You are happy with the neighbourhood, the schools, the shopping areas and the recreational facilities. The rest of your home suits all your purposes except for the fact that it is too small.

Why would you need a house addition?

First of all you must decide what you want the additional room for.

Extra living space. Everyone wants to have more space in their home. People like to have space to entertain and space for the kids to have their friends around. Wouldn’t you rather have your kids bring their friends home rather than hang out on a street corner somewhere?

Extra bedrooms. These days, more and more people are choosing to move in with their parents or with their grown up children. The cost of rented accommodation and the difficulty in obtaining a mortgage means that your grown up children who would otherwise have their own apartment now choose to live with their parents for financial reasons. Likewise elderly parents who find it difficult to manage on their own and who cannot afford a nursing home are moving in with their children. In both these cases extra bedrooms, guest bathrooms and living space will be required.

An apartment for your elderly mother (known in the UK as a Granny annexe). Often your elderly parent wants to be self-sufficient but you still would feel more comfortable to have them nearby in case of emergency. This is when a self-contained addition, whether detached or adjoined, comes into its own. You can quite easily build an extra living room, bedroom, shower room and small kitchen which would serve this purpose.

Extra rooms for hobbies. Most Americans and Europeans love to hoard things, whether they are family treasures, memorabilia, hobby stuff, gym equipment, bikes. All these equipment and keepsakes need space to store and space to use them. A home addition can significantly add to the usable floor area in your home by just adding three or four feet.

Extra room for your business or to work from home. With more and more people needing only a computer and a phone to earn an income, it makes sense to work from home. You will however need a dedicated room where you can go to shut yourself off from the distractions caused by the rest of family. A simple office is all that is required, with access to internet and enough room for a desk and some shelves.

Rent out your additional room. If you choose to build an extra bedroom with its own bathroom, there is no reason why you cannot take in lodgers and rent the room to provide an extra income. This isn’t a very common reason to have one built specifically to rent, but it isn’t one to overlook.

Something to help when you come to sell your home. If you intend to move house in the not too distant future you will need some additional space to your home to make it stand out from all the others in your street. Extra room and a bit of modernisation is the ideal solution to increasing kerb appeal. Building an addition will not necessarily increase the value and provide a significant return on your investment but it will help to sell the place.

Building an addition can be cheaper than moving house. Not only will the cost of an addition be cheaper than all the fees you have to pay when moving, it will also remove the hassle of packing your belongings, finding a removal company, and settling into a new house. In fact usually most of the addition can usually be completed before it needs to encroach on your existing house.

Have a sunroom installed. Sunrooms (in UK, think ‘conservatory’) are a good way to gain extra living space at a fraction of the cost of a properly built addition. You can buy sunroom kits suitable for DIY installation so it can be considerably cheaper than hiring a contractor. Sunrooms have the added advantage of giving you natural sunlight all the year round and if built on the sunny side of your house, they can significantly decrease your heating costs by warming up the trapped air inside.

While you are experiencing the upheaval of building work in your home, and for not a major increase in cost, you can incorporate a few of these additions, if not all into your project. The only limitation is to have done what you can afford.

What type of addition do you want?

There are many types of additions you can plan for:

  1. Ground floor additions.
  2. First floor additions.
  3. Detached additions.

Each type has its own benefits, drawbacks and cost implications depending on the layout of your existing house, the size of your existing house and what extra room you have available on your land.

When you want an addition built you will have to contact:

The local planning office at the city hall. These are one of the first people to contact to explain your ideas. The local authorities are the people who administer and enforce the local building regulations, building codes and permits. They will be the people who accept or decline your plans. They know what is allowed and what is not allowed to be built and their word is law. It is therefore important that you listen to what they have to say and do what they advise.

Their job is to allow building works for the benefit of all the residents not just you, so if you get them on your side right at the beginning then they can be a fund of good advice and information. Ask their advice about what they think of your ideas. If they say no to everything then there is no point employing anyone else. Ask them what would be acceptable and change your ideas to suit. Normally their services and advice are free but you will have to pay for a permit to work and for your plans to be inspected.

All these questions, and more, must be answered by a qualified architect and structural engineer. These professionals will probably know what type of additions the local permits are issued for

An architect. This person will be able to draw up plans for approval with the local planning office. The architect will also be able to ensure your ideas are practical and that the design looks in keeping with the remainder of your home and with the other homes in your area. You will also need plans to give to your chosen general building contractor so he can calculate a cost for constructing the addition. The question as to whether we actually need an architect will depend on how complex the project is. If it is just a simple room with one or two dand one window, then possibly the architect is not necessary unless major structural work is expected. Usually the general contractor would be competent enough to handle the job.

A qualified architect will tell you what additions are feasible and which ones are not.

  • If you have limited space available on your land then you will want to build upwards.
  • If you live in an area of single storey dwellings then you probably won’t be able to get a permit to build upwards.
  • Are the foundations of your existing home sufficient to take the extra load of another storey?
  • The architect will often act as your representative when dealing with the permit office.

Although an architect will add to the up-front costs of your project, they will often be able to take your ideas and make them into something great. Not only that but with an architect on board, the job often goes a lot smoother especially if the project is complex. Without doubt, an architect’s touch will definitely make the project add value to your house. If you are in doubt as to whether you need an architect, speak with the local planning office to find out if they require proper architect’s plans for your proposed work.

A structural engineer. Sometimes the architect will organise this for you. The structural engineer’s job is to :

  • Inspect the existing building methods and materials to see if they are strong enough to build on.
  • Inspect the existing foundations to ensure additional storeys are safe.
  • Inspect the ground conditions to decide whether the addition can be safely built.

If there is nothing structural expected to change then you may not need this person. Their input depends on the requirements of the planning office and the complexity of the project.

A general building contractor. This can either be a single contractor who can do some of the work himself and subcontract out the other work, or it can be a company who specialises in this kind of work and who already has the required tradesmen employed by them. Often the general contractor will also act as project manager, although you can decide to employ an independent project manager if you prefer.

Specialist contractors. All additions will require some specialist contractors to do specific work.

A carpenter will almost certainly be needed to fit doors and windows as well as install flooring, drywall and ceiling. Kitchen fitting is another task that a carpenter will do as well as fitting roof trusses, joists and any timber frame work that is required.

An electrician will be needed no matter what type of addition you choose. You will always need to have lighting, at least. The other jobs an electrician will be needed for include:

  • Install ceiling lighting sockets and circuits.
  • Wall mounted power sockets and circuits.
  • Electrical heating controls.
  • Underfloor electrical heating.
  • Fire alarm circuits.
  • Fans.

If you are installing photovoltaic cells on the new roof, then an electrician will be required to connect these to the breaker board. He will also need to install new breakers and tie the new circuits into the existing house circuits.

A plumber will be needed if you are considering additional kitchen or bathroom space. The plumber’s job can consist of:

  • Installing solar water heating system.
  • Installing mains drainage and a septic tank.
  • Surface water drainage.
  • Bathroom furniture including bath, shower, toilet, basin.
  • Installing water heaters, gas or oil.
  • Installing water purification systems.
  • New water pipes.
  • Installing gas pipes.
  • Install a sink and drainage.

A decorator will be needed to finish off the painting work unless you are happy and confident enough to tackle that job yourself. Depending on the size of the addition and whether you are building upwards will depend on whether it is important to hire a professional decorator.

A qualified mason or bricklayer will be needed to deal with the concrete foundations and floor slab. If you are having solid walls or stone sidings then a mason will be essential.

HVAC technician will be required to install the extra ducting and possibly upgrade the equipment to cope with the additional load.

A gardener, garden designer or landscape architect will be needed to make good the grounds after the building work has finished. You could save money by doing this work yourself.

Finally a roofer will be required to ensure your addition and your existing house are both watertight and the roofs are properly joined and sealed.
Project manager. This role is to ensure:

  • All the materials have been ordered and delivered ready to be used.
  • The work stays on schedule.
  • The work stays within budget.
  • Acts as a liaison between the customer and the other professionals.
  • Acts as a co-ordinator of the team.
  • Ensures the bills are paid on time.
  • Apply for appropriate permits and liaise with the local planning office.

Sometimes the project manager role can be fulfilled by the customer, the general contractor, the architect or an independent project manager, whichever the customer chooses. It is important, whoever is chosen, that they are skilled in project management, able to earn respect from the other team members and understand the pitfalls and problems that occur when building an addition.

If the project is small and no logistical problems are expected then the general contractor can be expected to handle this role himself.

Building upwards or outwards

The first big question you need answered will be “Do I build upwards or outwards?”

They both have their pros and cons but you need an appreciation of these before you can decide.

Building upwards

Building upwards means that you intend adding another storey onto an existing building. Usually you won’t be adding a third storey onto a second (Let me just explain, the first storey is the ground floor, the second storey is the first floor and the third storey is the second floor. Ok, if you understand now, we can move on). As I was saying, you won’t normally add a third storey. Structurally there is no specific reason why you can’t, it is just that a three storey house may look a bit odd and probably won’t match with any nearby houses. It will also cost a considerable amount of money. The only way you could build a third storey would be to use the roof space and convert that into a living area of some kind. You are not lifting the roof line so there won’t be any major additional weight on the foundations and the planning office will be more sympathetic.

If you already have a single storey building, you must understand that the foundations will have been designed to accommodate the weight of a single storey building. If you want to build another storey then the foundations may have to be strengthened to accommodate the extra weight.

Problems with building upwards include:

  • Existing foundations may need to be strengthened
  • There may be regulations limiting the height of your house
  • You will need to add a staircase which uses up a significant amount of living space (usually about 80 to 120 square feet)

Building outwards

If you are intending to build outwards, then you are increasing the building’s footprint and need additional foundations. The addition may be single or double storey, either way you will need to build the appropriate foundations. You will find that some states prevent you from building within a certain distance from your boundary. This is not just for an aesthetic reason but is mainly to provide a firebreak between properties. You will therefore probably not be able to encroach onto that fire break. You will probably not be able to extend the front building line as it will disrupt the aesthetics of the overall neighbourhood. The only place you will be able to build is out from the rear of the property. This will eat into your garden but that is the price you have to pay.

Building out usually just involves adding an extra room at ground level. This is the most common type of addition. All building work can be accomplished independently and will not cause any upheaval to the rest of the house. Everything can be done until the final moment when the doorway is knocked through from the new to the existing.

As stated earlier, the problems with this kind of additions are:

  • New foundations need to be poured
  • You will lose part of your garden
  • Local regulations may not allow variance on the building line or an increase in foot-print

Building a sunroom

One way to get around the foundation problem and provide extra room for living or hobbies is to build a sunroom (in the UK this would be called a conservatory). If the roof is made from a polycarbonate type plastic then these are usually classed as temporary buildings, have less weight and will need far weaker foundations than a normal addition.

A sunroom is usually built with either half or whole glass walls and with a roof made from either glass or clear polycarbonate roofing material.

A sunroom is far cheaper to build than a normal additional room and often prefabricated self- assembly ones are available at very reasonable prices.

You will have to decide whether the sunroom is intended for four season use or for three season use as the type of heating requirements will be different.

Whichever type of sunroom you decide on, you will need to have it assembled, painted or varnished, power, lighting and possibly heating which means hiring electricians, carpenter and painters just as you would a normal addition.

If you decide on building a sunroom kit rather than an addition, you will require someone to come and take accurate measurements and levels otherwise it will be a very expensive project if you find it doesn’t fit.

Building a detached addition

If you already have a detached garage, seriously consider remodelling the garage into living space. The costs to build a detached addition are similar to the cost of building a detached garage.

You can have this one built in a similar fashion to a normal house or you can purchase a prefabricated detached addition. They come in many different styles such as a:

  • Pioneer log cabin.
  • Scandinavian cedar hut.

And many more styles to suit your budget and existing house design. All that is needed is to pour a concrete slab and arrange for mains drainage and services to be available. The cabin will arrive either in sections or ready assembled and all that is needed is for the building to be levelled and fixed to its base followed by the services connected.

If you are not buying a prefabricated addition then you will need architects drawings prepared, the site cleared and foundation trenches dug.

Home Addition Cost

The costs of building an addition will obviously depend on the scope of the work. You will have to consider many different factors which will affect the cost. The main factor is the size of your addition, meaning the footprint and the height.

The average costs of building an addition can vary considerably so we will only talk about approximate prices here.

ProjectAverage costNotes
Average cost of ground floor addition$40,000
Low end$3,000For a single room
High end$45,000For a complete floor
Detached guest addition$50,000Minimum
Detached garage or low end guest house$30,000
Detache shed d$2,800
Sunroom. Average$15,000
Low end$500
High end$40,000
Sunroom kit. Average$12,000

Other factors which have to be considered are as follows. We will initially talk about each factor followed by a summary of the average cost:

  • Architect’s fees to ensure the design matches the style of your existing home. Typically, these fees will account for about 10% to 18% of the total project costs.
  • Site clearance, excavation of foundation trenches and possibly demolition costs (groundworks) will vary significantly depending on the scale of the project and the amount of work required to prepare the ground.
  • Concrete foundations and a concrete sub-floor slab will be necessary.
  • If you are doing anything other than a detached addition, then the supports and roof trusses will have to be incorporated into the existing house. As well as size, these costs depend on the price of timber and labour.
  • Roof covering is another essential and will vary depending on the size of the roof and the style of the finish (asphalt shingles, metal roofing, and slates). This cost will incorporate galvanised flashing as well.
  • You will need some kind of wall covering to match the existing house. Let us assume that you have chosen vinyl siding complete with trim.
  • Interior lining is done using drywall for the ceilings and walls.
  • Don’t forget the insulation needed behind the drywall.
  • Doors will be needed. Once again the cost of these will depend on the number required and the style.
  • Windows are also a necessity. Number, size and style will be factors in this cost.
  • Moulding and coving around the ceilings will finish off the ceilings nicely.
  • Any electrical work requires a licensed electrician. The work will be to run all the cables for lights and power points together while ensuring the existing circuits are up to code.
  • The floors need to be finished off and depending on what you choose will vary the price.
  • Costs for extending the existing HVAC to incorporate the addition will vary depending on size.
  • While you have the upheaval of building an addition it might be worthwhile updating your existing house layout to blend the addition into the whole.
Summary of costs
Architect’s fees10% to 18% of total budget usually about $5 per square foot
Structural engineer$100 to $150 per hour
Groundworks (removing a wall)$300 to $500
Groundworks (excavation)$2,500 average
Pouring pre-mix concrete$75 per cubic yard
Tie in new roof to existing$15 to $30 per square foot
Roofing$80 to $100 per square foot for cheaper materials
Vinyl siding and trim$8 per square foot
Dry walling ceiling and walls$10 per 8’ x 4’ sheet. Approximately $2,000
Insulation$2 per square foot. Approximately $1,300
Door$300 minimum per door
Window?300 to $700 each
Ceiling moulding & coving$5 per linear foot
Electrical work (labour only)$50 to $100 per hour. Approximately $1,200
Plumbing work%50 to $150 per hour
Floor covering Ceramic tiles$1,650 average
Carpeting$1,500 average
Laminate$2,800 average
Vinyl$1,200 average
Stone tile$1,800 average
Wood flooring$4,500 average
Tying in HVAC to existing$1,000 average
Cost of permits$100 to $500

Will the project add value to your home?

Probably the last thing you will want to do after going to the trouble of having an addition built is to sell up and move on. There will come a time however when you decide to move, maybe because of changing job or more probably when the children have all left home and you want to downsize. Then is the time that the addition will add not only value to your home but also kerb appeal. The additional value may not be as much as you paid out for the work to be done, but depending on what extra space you have created, there might be a substantial percentage of the total price as a return on your investment.

It is widely recognised that different types of addition will give you a different amount of return on your investment (ROI). The following table shows what kind of ROI you might expect when you resell your property.

Types of additionROI
Master bedroom suite63.00%
Two storey with upstairs master bedroom and downstairs living area65.00%

What will definitely increase is the kerb appeal. The added room will make your house stand out from the crowd and hopefully allow yours to be sold quickly and without too much negotiation.

How do I go about getting the work done?

Let’s assume the project is simple and we don’t need an architect, a structural engineer or a project manager. All we need is a general building contractor.

Look around the area for a likely candidate or two. Ask friends and family if they have had any dealings with either of them. Maybe someone else in your road has had an addition built. If so, get in touch with them and ask what kind of a job the contractor did and whether he acted responsibly and professionally. After you have some idea of how the candidates work, you can contact them and ask them to visit the site (your house).

Show them where you want the addition built and discuss your requirements. If you have any photos or sketches of something similar then bring out those as well. After the contractor is clear about what you want ask him for an estimate for the work to be done.

If he is experienced at his job, he will know whether permits are required and how much they will be. The estimate will list:

  • The total estimated price for the work to be done.
  • A detailed list of materials together with specifications.
  • Whether permits are required. If so, which ones?
  • To what building codes must the work be done?
  • What certification has the contractor got?

If the job needs specialist contractors such as decorator, electrician, plumber, carpenter or an HVAC technician, it will list what work needs to be done by them.

  • What insurance cover he has and whether it is adequate for the project.
  • Whether he has worker’s compensation insurance.
  • Estimated start and finish date of the project.

When the deposit, stage payments and final instalment are due. Never pay the total cost up-front. It is reasonable however for a contractor to ask for a deposit to ensure you are committed to the project. It is also reasonable for money to be paid at agreed stages of the job. These are called ‘milestone payments’ or ‘stage payments’ and will be paid at agreed points during the job. For example, typical milestones may be:

  • To pay for the groundworks and concrete after the foundations have been approved by the building control inspector.
  • To pay for walls and roof, external doors and windows when the addition is watertight.
  • To pay for electrical work, HVAC and plumbing when that work is completed.
  • To pay for the remainder at the end of the job.

These are only examples of when stage payments may occur; it is up to you to agree the details with the contractor. Be aware that paying out too much money in one go, may put you at risk while not paying out enough may put your contractor at risk.

Find out how the contractor plans on handling extras to the contract. In every project, no matter how well organised and planned, there will come a time when something happens which was unforeseen. The ground conditions are different from expected and you have to dig foundations deeper; it is unfair to expect the contractor to pay out for the extra labour, hire of digging equipment and extra concrete. No reasonable person could have expected this so the extra work and extra concrete will have to be paid for by you. The extras will be logged using a ‘variation order’ (variation from the original contract) and this is then treated as an addition to your basic contract. You will be expected to pay for this when you receive your next stage payment invoice.

Maybe you decide you don’t like your choice of bathroom suite and want it changed. You will have to pay for any additional costs associated with the change. However you decide to handle variations, the method will have to be agreed at the start before any work starts. If you get everything sorted beforehand then there will be less chance of friction between you and your chosen contractor.

Tips for making the project go smoothly

As a homeowner, building an addition can be one of the most expensive projects you are likely to do. There are always little things that you maybe didn’t realise or didn’t understand and it is now that we should iron out these problems.

Will the addition look part of the house or will it look like it is an afterthought?

This depends on how good your ‘design eye’ is. Professionals such as an architect or a designer will always make sure that the addition looks like it is meant to be there rather than as an afterthought or an add-on. Skilled professionals can take your ideas and cleverly create the space you need while making it tie into the existing structure so it looks a coherent ‘whole’.

What if there are any restrictions in my neighbourhood?

There may be zoning restrictions in your area. Always check with the local building control offices whether there are any restrictions or covenants, or whether you need a permit for your addition. In some towns and cities you might not be able to build within a certain distance from the front, side or rear property boundaries. Some places have restrictions on:

  • The percentage of your property that may be covered with buildings.
  • How close you can be from the street.
  • How close you can build to a protected space or river.
  • How high you can build.
How do I know I am choosing a competent contractor?

This is always a problem. There is no practical way to know if the person you choose to do your project is going to be competent until they have finished the job! The only things you can do are to ask them for documentation and references.

  • Always check they are licenced and insured for the work you are asking them to do.
  • Ask them how much experience they have with building additions.
  • Ask for photographs of their previous work together with the names and addresses of their previous customers. Contact them and if possible go to see what kind of a job they did. Recommendations are always subjective and just because someone else says the contractor’s work is good, does not mean that you will think so. Go and look for yourself.
  • Ask how they choose specialist subcontractors such as carpenters and electricians.
  • Get estimates from two or three different contractors and see how the costs differ. Don’t always go for the cheapest option, someone who charges slightly more may be able to get the work done in less time. Choose the option that works best for you.
How do I get started?

Draw a sketch and a plan of what you want the finished addition to look like. Choose a contractor that you think you could work with. Find out what permits you will need (Ask at the building control offices yourself rather than take the contractor’s word for it).

Are there any ways to make things cheaper?

Building an addition can be a very expensive project. There are however ways you can save money here and there.

Have you thought about remodelling an existing room rather than build something new?

You could convert the basement or the roof space into another bedroom with en-suite bathroom.

With better organisation it may be possible to do away with the need for a new room completely. Rather than extending the kitchen you may be able to buy updated kitchen units and make better use of the space you already have.

If you think you need more space for storage, why not go through your stuff and clear out the clutter. You may find you don’t need to build an addition after all.

Rather than build a detached addition for your mother in law to live in when she is old, you could consider converting the garage into a bedroom, living room, kitchen and shower room with toilet. You already have the structure; all you need to do is make it habitable.

Buying a prefabricated detached cabin will be cheaper than building an addition to your existing house. These could also be used as a guest bedroom or as an office.

If the choice is between building a detached addition or an attached addition, always choose the detached option. The difficulty of joining the addition to the existing house and making it weather tight will completely overshadow the relatively easy tasks of laying new foundations and a slab for the detached option.

Buying the materials yourself will save a lot. If the contractor is left to purchase the materials he will quite legitimately add on a percentage of the cost to make it worth his while. If you approach the building supplies merchant beforehand and explain what you intend to do, they will often be more than happy to open a credit account and give you tradesman’s preferential discounts.

Apply for the permits yourself. By taking this job away from the contractor you will be saving money. You must apply for the permits in good time so everything is in place before work starts and you must tell the planning office when each stage of the work has been done so that inspections can take place if required.

Funding the project. Either you already have the funding saved up and set on one side or you will have to ask for funding from a third party. Usually a bank or a mortgage lender will offer to fund the project depending on your individual circumstances and your ability to pay back the loan. It can save money if you can agree an interest free or a reduced interest loan with a friend or family member.

Although I wouldn’t advise anyone to build an addition if they have not had any experience, it may be possible to save money in other ways:

  • Lay the flooring yourself.
  • Do the painting and decorating.
  • Doing your own demolition.

Work as a labourer with your contractor. Sometimes this can be difficult especially if you are not used to manual work or if you cannot keep up with the contractor. If you have some DIY experience, often the contractor will accept your offer of help.

Do the project managing yourself. Although this can be a difficult job and you need to have a good relationship with your contractor, it can save a lot of money if you organise delivery and pay for the materials yourself. The contractor won’t have to worry about it and you will be saving his profit on the materials.

If you have a garden that has endured major upheaval during the building work then you will need to hire someone to reinstate the garden. If you love gardening or if your garden hasn’t been hurt too much, you might like to save some money and do this yourself.

The money you can save by following this advice can be quite considerable. The following table gives you an idea of how much.

Convert basement$3,000
Better organise kitchen with modern custom built units rather than build a few square feet onto the kitchenCan save anything from $13,000 up to $60,000
Do your own demolishingSave $500 minimum

To finish

Building an addition to your home will help provide more space for you and your family to live and play without being under each other’s feet. There are a variety of different types you can choose from and different ways to extend your property depending on your budget, your existing property, your available land and the zoning regulations of your neighbourhood. We talked about the differing reasons for wanting an addition. We talked about the kinds of contractor and professional you would hire to do the various jobs needed to make the project run smoothly and provide you with an addition you can be proud of.

We talked about the various factors that can affect the price of the work and a few ways to cut costs and make the total budget cheaper. We talked about the various restrictions that some areas have in force and how these might affect your project. We also suggested that you should graciously accept any advice and recommendations that the building control office are willing to make in order for your project to continue smoothly. These should be no problem to compromise with your designs if it means that the project can get done.

We decided that it would always be cheaper to convert or remodel an existing room, basement, garage and roof space to give you the extra room you require rather than build from scratch. We also said that if you can fulfill your requirements by building a sunroom then that would be cheaper as well.

Let us know what you think of the information provided in this article.