How much does a house extension cost?

  • September 2018
  • Kieron Peaty
How much does a house extension cost

At the moment, many are choosing to extend their homes instead of moving house. Extending is often cheaper than upsizing and this, coupled with the stress associated with moving to a new house, makes an extension the sensible choice. However, when looking at house extension costs and budget, there are numerous considerations to account for and it is important to remember that extensions in London and the surrounding area, carry a premium comparative to the rest of the UK. This is due to uplifts in material and labour costs, as well as restrictive construction sites being more common place. 

In this article, we will look at what to expect when considering an extension.

Guideline extension costs

Remember that all guideline prices listed below are for construction only and are not inclusive of VAT or fees associated with professional services. Despite the raft of factors that can affect extension costs, the most important to remember is size. So when considering your extension, try to retain an idea of the approximate size of space you are looking to add.

Extension types

  • Single storey - ground floor extensions will likely cost between £1,200 and £1,500 per square metre, dependent upon your level of specification. £1,200 will typically provide you with a standard finish and low-range products. In London prices will be higher and you can expect to pay between £2,000 and £3,000 per square metre.

  • Two storey - can appear better value than single storey when we consider that the industry often calculates the cost of a two-storey extension by adding 50% to the guideline price of a single storey extension. Materials for structural support such as foundations and roof, account for some of the primary costs associated with a single-storey extension but, once these are in place, they need not be paid for again for a second storey. With that said, achieving planning consent for a two storey extension can be a thorny process, comparative to single storey.

  • Basement - structural complexity and construction difficulty translate to high risk and thus, greater cost, typically between £4,000 and £5,000 per square metre. Occasionally, older Georgian or Victorian properties possess existing basements requiring conversion or partial extension, which would need less work. In these instances, the project would be more cost-effective.

Define the need 

The most popular type of extensions include new kitchen or bathrooms. Understandably these rooms add value to a property but they will also add cost to your extension comparative to bedrooms or other forms of living space.

  • Kitchens - expect an additional £10,000 as a minimum, for a low-mid range design complete with units and some appliances.

  • Bathrooms - will cost anything from £5,000 with this price likely including WC, bath, wash-basin and shower however, this price is defined by quality, so expect to pay more for higher spec fixtures and fittings.

Consultant fees

Professional services will typically equate to 15%-20% on top of the construction cost:


Architects are an invaluable investment for your project, trained in managing project risks and skilled at navigating tricky matters such as planning permission, local authority issues and a raft of other potential problems. Typically architects are the first point of contact for your project but perhaps most importantly, they are the key to your extension idea becoming reality. Architects will create the detailed plans you need for contractors to be able to provide accurate construction quotes.

An extension may not be the right answer for your project needs and an architect will be able to provide different solutions, which may prove more economical by reducing costs or possibly facilitate more of your home being renovated for the same cost. Often re-configuring poorly planned existing spaces can unlock a homes potential for less, proving to be cheaper, simpler, quicker and more beneficial than simply adding a poorly conceived large extension.

Other consultants

Quantity surveyors, services engineers or other specialist consultants may need to be involved with your project. Obviously, this is dependent upon your project type but an architect will identify and discuss at what stage these are required.

Additional costs


A house extension planning application fee will cost £206. Planning permission is not always required, as it may fall under Permitted Development. However, in these instances it's still advised to apply for a Lawful Development Certificate. The certificate confirms that the works do not need planning, which will be particularly important when you come to sell the property, after completing the works. It would not be unreasonable for a prospective buyer to expect proof that the works are lawful. The application fee for a Lawful Development Certificate is half the planning application fee.


It is important that you notify your home insurance providers prior to the commencement of any building work, as many policies will not cover you whilst work is in progress, potentially leaving you liable in instances of structural damage, for example.

For your own peace of mind, you may wish to take out an additional policy as cover, whilst work goes on. As an example, this may take the form of non-negligence insurance, which can offer protection should any structural damage occur to a party wall, as a result of your extension.


For an extension you will pay a full VAT rate, which is currently 20% although there are exceptions where the VAT rate will change. For example, when a property has been vacant for more than two years it may be eligible for a 5% VAT rate.

Architects will know the varying rates and whether your project could benefit from a reduced rate, so ask them for assistance if you have any questions about the cost of VAT for your house extension. 

Major item costs 

Windows and doors

Windows can account for a considerable portion of the project cost. As a general rule, remember that the bigger the window, the greater the cost per square metre of the unit. Window price will also be affected by the desired level of glazing, amount of insulation, and style and fitting of the window.

Large sliding patio or bi-folding doors will also add expense, if they are something you wish to incorporate into your extension design. However, the cost of these items needs to be considered against the level of light they will afford your property once installed. Maximising daylight would significantly benefit the living space and the opportunity to have that space open to the garden, will dramatically alter and improve how spaces are used in summer months.

Central heating

You need to consider whether your existing boiler and central heating system can cope with the additional rooms and space created in your extension. You might need to upgrade your system or add a second heating system to the property to ensure the new, additional space is comfortable.

Site constraints

There a many factors that may increase cost. Soil type, for instance, could result in the use of different and potentially more expensive building materials. Access in such cases as terraced properties, may mean contractors look to add to the price to cover the cost of access for workers, materials and equipment. A need to move trees, drainage, pipework, steel work, gas meters, sheds, conservatories or fuse boxes can all add to your extension costs. 

If your property is in a conservation area or is a listed building then you should allow for higher consultant and construction costs. There are many things to consider. 

General specification

Clearly, standard off-the-shelf items will cost less than high-end products so the desired level of finish has a significant bearing upon the cost of a house extension. Tiles, paint and wallpaper all play their part in adding to cost and of course flooring too.

Aside of the those items listed above, you may also want to consider whether you want any joinery and whether it might be bespoke or standard cabinetry with furnishings. Equally, in the 21st Century of technology, lighting and electronics can increase cost, particularly when smart home systems are desired for heating, electricity, audio and visual.

Finishing touches and external work

Once your extension is complete you may need to renew other areas of the your home, both inside and out. Externally, you may wish to landscape the garden or rebuild pathways and driveways, etc. Internally, you may want to decorate and of course fill your new space with furniture.

Total project budget

The following example provides an overview of all the factors you need to consider, in addition to providing an idea of how much you should spend on each:

  • Cost of builders, material and construction: 65%

  • Architect: 10% (15% on top of the construction cost)

  • Other consultants: 5%

  • Planning permission and administration: 1%

  • Party wall agreements: 3%

  • Insurance: 1%

  • VAT: 15% (20% on top of the construction cost and consultant fees)

How an architect can help with budgeting 

A good architect will be able to identify most of the issues that have potential to affect cost, in regard to your extension. Additionally, they will be able to advise of where to focus the work and cost, and propose cost-saving alternatives when necessary.

Should I use a quantity surveyor? 

Quantity surveyors are experts in construction costs and should the budget for your extension exceed £100,000 then it would be advisable to appoint one.

Once you have a design outline a quantity surveyor will be able to furnish you with a cost estimate. As the project evolves he/she will provide up to date prices on items and an accurate cost breakdown. With this information you will be well positioned to assess the cost and either add or remove more details, or change materials as necessary.

Added value 

Undoubtedly, a house extension can improve a living space, in turn creating a better home. However, there are clearly financial benefits too with typical extensions adding anything between 10-30% to the value of a home.

Extra, tailored space

Extending provides an opportunity to adapt a house and truly make it your home. It could facilitate you owning a more sociable kitchen area with greater connection to the garden for instance. Or more tailored spaces, such as a home studio or workshop, or a kids playroom in plain sight of the kitchen that isn't tucked away at the other end of the house.

Extensions, should meet your family needs rather than simply adding another room to a property. 


  • What are you looking to achieve with a house extension?

  • Do you want to expand down, out or up?

  • Will an extension add value?

  • How much space do you want to add?

  • Is an extension cheaper and easier than moving home?

  • What is our budget?

  • Do you have a contingency budget - in case of emergency?

  • Do you need planning permission?

  • What is your timeframe and do you have a completion date in mind?

  • How long will it take and will you need to move out?

Further reading 

How much will a Planning Application cost?
How much do architects cost? How are fees calculated?
How much does it cost to build a house?

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