How much does a house extension cost?

A house extension could consist of a loft conversion, side extension, rear extension or basement etc. So how much does a house extension cost? Well, there are no set rules as to how much you should pay but how the project is planned and managed will play an important role.  After putting your hard-earned money into savings for years it’s important to get it right.  You need to find the right design team and builder to ensure you stick to your budget and finish the job.  Any house extension you add to your home, you will only ever do it once.  No one has endless cash to keep ploughing into a project.  And the project should be completed as per your vision from the start not some kind of patched up botch job!  In this blog, we are going to talk about the potential pitfalls, how to avoid them, where you shouldn’t be cutting costs and build costs and how to control them.

house extension cost - carter designs - architect Harrogate

At our very initial meeting, we will ask what is your budget? We do not ask this question lightly as it can be a sensitive topic. If the question is answered (it’s better in the long run if you do) it is kept in the strictest of confidence and this information will not be shared with any other parties. The reason we ask this or should ask this as architects and designers is to establish whether your budget fits the design brief. And when I say design brief this is not just whether you can afford to build a house extension it’s also to establish if you should add an extension. If, for example, you purchased a 3-bedroom house at £250,000 and wanted to extend the property. You could add a loft conversion for a 4th bedroom and en-suite, possibly add a rear extension to the ground floor to provide a bigger kitchen/dining area. If you then obtain quotes from a builder at £200,000 plus VAT to build those extensions, that puts your purchase price and build costs in excess of £450,000. If the house was purchased at £250,000 and the most recent property sold in the street with the same extensions are valued at say £350,000. Chances are you are going to make a loss and effectively put you in negative equity, in particular, if you are £100,000 over investment than the rest of the street!

So, all being well, and your house extension is worth pursuing, how much does a house extension cost? Based on our experience this depends on several factors:-


Type of extension
material finishes
where the project is located
access to the project
how busy the market is
your decision making
how much information is provided


Type of extension

If your extension is of a standard form, then I would suggest it’s safe to say your costs should follow the normal trend. When I say standard form, I mean extensions that have a pitched or flat roof, brick, render or timber cladding etc. Outside of the normal trend would be irregularly shaped extensions, Corten or aluminium cladding, dome roof etc. A typical builder who may not be familiar with some of the building work or materials would most likely raise their fees to accommodate a non-standard form as they sometimes have to employ specialist sub-contractors and allow additional time taken to complete the work.


Material Finishes

Material finishes can have a huge implication on costs. If you choose a high end engineered flooring at £200/m2 it’s going to quickly eat into your budget. We can suggest alternatives that still deliver a high-end finish at a fraction at £50-70/m2 and chances are you would not be able to tell the difference. Try and buy materials if you see sales, I see prices fluctuate all the time for bathrooms, there are always offers on and generally, most leading suppliers offer very similar styles so it’s best to keep an eye out for deals. If you have space store them carefully, ready for when you need them it may even be cost-effective to rent a storage unit for a few months rather than pay extra for on the spot items.


Where the project is located

Project location should not be a factor but if you live in or near to the more expensive part of town then chances are you will pay higher build costs. It pains me to say it, but I have witnessed this time and time again. More affluent areas attract higher priced projects which could see a 30-50% increase in costs in comparison to areas that are less privileged.  We can analyse tender returns and have a good understanding of build costs to know if a project is being overcharged.


Access to the project

A builder will require sufficient access that will allow them safely to get materials and plant onto the site. If a potential development is mid-terrace with no separate access to the rear to build a rear extension, then the builder will have to factor in additional time. Foundations will most likely have to be dug by hand rather than a machine which means increased labour costs. The same applies for bringing in materials in particular steelwork may have to come in through windows which need careful handling.  Agreeing on these costs upfront is vital and having well-prepared drawings and a contract can help the builder determine what equipment and materials are needed before they start on site.


How busy the market is

A busy market is a builder’s market. They can pick and choose which projects they want to take on and leave the ones they don’t. You may have to wait, in some instances 18months for a recommended builder, so it’s best to plan your project early. Some builders may just put in high estimates on the chance if they get it great, if not they didn’t need the project in the first place.

Your decision making

Spend more time making decisions on paper rather than on-site. Deciding you do not like the proportions of a room or not thinking about electrical positions can cause delays and cost you more on-site. Your builder would well be within their rights to charge for additional labour and materials to start moving walls around once they have been constructed. Not everyone can visualise how space will look.  We can help by explaining plans in detail, providing mood images and even 3d images to help you decide what kitchen to go for or what colour you should paint the walls etc.


How much information is provided

The more information you provide the builder the better. Always hire a professional that can develop the scheme from concept to completion not just one element of the design process. You need a design that matches your vision if you’re unsure we can help provide new ideas. Once that design has been agreed it needs to be developed so a builder can understand what you are trying to achieve. If a builder is only provided with basic planning drawings, then chances are you need to fill in the blanks. You could find the builder is having to make decisions on-site without your approval.  Or is unable to carry on with work without knowing what you want, so the work stalls and delays the project. The longer the project is on-site the higher your costs. Also, the builder will not be able to provide you with an accurate quote and most likely only a ballpark figure. If you do not agree on costs beforehand then once the project has started on site there is no incentive for the builder to provide competitive quotes. You could experience delays on-site whilst agreeing design and face larger costs than anticipated meaning you’re likely to exceed your budget. Planning permission will last for a period of 3 years and applications can always be made for an extension of time if the permission is due to run out. This also allows you time to get all of your paperwork in order, planning permissions, tender drawings, finishes schedules etc. The more time spent planning and detailing will lead to more efficient and budget-friendly development.

So how much does a house extension cost?

There is no exact science as to how much a house extension costs as there are too many factors to take into consideration. As a general rule of thumb, we advise most clients to work with build costs of around £1750/m2 for a basic finish excluding VAT. This is purely to try and ascertain if your budget matches your expectations. If you have a budget of £30,000 and want to build a 50m2 extension and completely refurbish the house, then it’s completely unrealistic. That’s not to say you shouldn’t have any work done to improve your home. Part of our design ethos is to firstly identify the problem. Adding more space isn’t always necessary and re-configuring the layout can greatly improve your home. Just look at our Tooting Bec project for inspiration, no extension but by reconfiguring the layout meant the property went from 1 bedroom to a 2-bedroom flat, which also greatly added value.

If your project does involve any major building work then we cannot stress enough, the need for a full design package. Finding the right architecture company that can deliver a full design package is vital. Appointing an architect that only provides designs and no technical drawings means your concept may not be delivered on-site. If they do not understand construction, then they could design something that cannot be built. The same can be said if one firm is appointed to develop working drawings after inheriting another firms design. Chances are part of the design could be missed, misinterpreted or simply not feasible.

If you cut costs at an early stage by thinking a cheaper design company is saving you money, it could actually be costing you thousands in the long run. Or worse your design ideas quickly become lost because the information was not clear, and you have to accept how it is on-site if it’s too expensive to put right. The same can be said if you fall into a dispute with your builder if the correct protocols are not followed then both parties could find themselves in a legal dispute. If no working drawings and/or contract has been signed, then both parties may lose out.  The same can be said if you have little experience taking on the work yourself.  Designing and drawing is probably only about 30% of the work designers do on a project.  The rest of the time is reading planning laws, regulations, calculations, spreadsheets, contracts, research etc the list goes on.


Whilst trying to determine how much a house extension costs its also just as important if not more to control how the project is managed. We provide a full service from concept to completion. Taking into consideration the design, construction and costs from the very outset. By understanding your requirements, we can quickly understand if your budget can be met and controlled once on site. We can also advise if we think an extension is worth it.  We take initial design concepts and develop these into drawings for planning or detailed design. Once that’s completed, we then develop these drawings further by providing clear, detailed and accurate working drawings for a builder to price and work from. The more information the builder is provided the more accurate the quote you should receive. Once you have agreed on a final price and start date with the builder, we will set up a contract between both parties. This will always be a JCT contract for us, you can see the typical form of contracts here.  Referring back to detailed drawings on-site with a detailed quote from the builder removes any ambiguity as to what has or has not been included as part of your quote. The less information the higher the likelihood of exceeding your budget. This may seem a blunt somewhat vague description of how much a house extension costs but the reality is it varies greatly. Whilst there are some cowboy builders out there, there are also some good ones. They can only provide you with a price based on the information provided.  Typically, once the contract sum has been agreed a 10% deposit might be paid but in all circumstances, retention should be applied to the contract value.  Most domestic projects will retain 5% but this can be changed depending on the level of contract issued.  You should always avoid if a builder is asking for lump-sum payments, £10,000 one week, £15,000 the next.  Paying your builder upfront leaves them with no incentive to come back and finish the project.  What if they are using money from your job to pay for another and the go bust.  You are left high and dry!  Once your build has started on-site, we can monitor the work in relation to the contract and contract value. We take the detailed quotation and apply percentages of completed work against those values.  A certificate of payment will be issued based on the work that has been completed not what is left to complete.  This is fair to both client and builder, if a builder does not agree to this then it would suggest they are not right for your project.  On completion of the project, we will carry out a snagging list where we check the property for defects and the list will be sent on to the builder.  Once the builder has achieved Building Control Approval, handed over all warranties, user manuals etc. and completed the snagging list.  Only then will the final retention money be released.


This is why it’s important to find a local architect or architectural technologist that can design your house extension from concept through to completion.  Spending a few thousand pounds at the beginning could save you tens of thousands of pounds later.