Dartmouth Park house extension costs in London, proposed 3D visual

Updated for 2024.

How much does a house extension cost in London? The article explores house extension costs, along with tax considerations and consultants fees. It explains the options that are available for various budgets and outlines the best approaches to making savings.

Building a house extension is often cheaper than upsizing and avoids the stress of moving. London extensions do however have a cost premium when compared to the rest of the UK. This is due to higher labour and material costs, and the difficulties of compact construction sites.

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House extension costs in 2024

While there are many factors that could affect the costs of an extension in London, the most important factor to consider is the size of the extension. This is because size (generally measured as floor area) directly correlates with the total cost of the project.

You should, therefore, have a reasonably accurate idea of the size of space that you are looking to add or transform.


There may be planning policy issues to be aware of when deciding the size of your extension. Your architect will be able to advise you of the likely maximum size possible under permitted development regulations (if relevant) or with a planning application for your property.

Once you know the required size of your extension, we can begin to estimate costs using some standard square metre rates. All of the guideline prices listed below are for construction only, updated as of 2024. They do not include VAT or professional fees.

Highbury house extension: north London architects

House extension costs:

  • Single storey – A single storey extension in London will usually cost between £2,200 and £3,300 per sqm (£200 and £300 per square foot). £2,200 per sqm would represent cost-effective finishes and products. Elsewhere in the UK, you can expect to pay between £1,320 and £1,650 per square metre (£120 and £155 per square foot).
  • Two storey – To approximate the cost of two-storey extension, add 50 to 60% to the guideline price of a single storey extension. There are economies with two-storey extensions as only one set of foundations is required for example. Two storey extensions can be more complex to achieve planning consent for.
  • Basement – Due to construction complexity and risk, a basement is more costly. A basement extension in London will typically cost between £4,400 and £5,500 per square metre (£405 and £510 per square foot).

What will the extension be used for?

Many people include a kitchen or a bathroom in their extension plans. These will naturally increase the overall cost when compared to additional bedrooms or living spaces.

  • Kitchens – If you are looking to add a kitchen as part of your extension, then expect to pay an additional £15,000 to £40,000. £15,000 will typically cover a low to mid-range design complete with units and some appliances.
  • Bathrooms – The additional cost for a bathroom will begin at around £5,000 to £10,000. The price will increase based on the quality of fixtures and fittings.

Kensington house extension in a conservation area

Consultant fees for a house extension

The cost of all professional services will typically be 15%-20% on top of the construction cost:


Architects are often the first point of contact for a project and provide an invaluable investment. They will navigate the key project elements including design and gaining planning permission.

Perhaps most importantly, an architect helps to turn an idea into reality. They create the detailed plans required so that builders can give accurate quotes.


An architect can also provide alternative solutions that may be more economical and help reduce costs. Sometimes reorganising and improving existing spaces can be cheaper and more beneficial than a poorly conceived house extension.

Other consultants

Depending on the project it may also be necessary to involve other specialist consultants. An architect will discuss and identify when these are required. Specialist consultants typically include quantity surveyors and engineers.

Additional costs


The planning application fee for house extensions is £206. Some house extensions will not require planning permission, as they may fall under Permitted Development. In this case, it is still advised to apply for a Lawful Development Certificate which confirms that the works do not need planning. This is particularly important if you plan on selling after completing the works. A prospective buyer is likely to expect proof that the works are lawful.

The application fee for a Lawful Development Certificate is half the planning application fee.



Many home insurance policies will not cover you while there are building works. This could potentially leave you liable should there be any structural damage during renovations. Therefore it is important to notify your insurers before beginning work and you may also need to take out an additional policy while construction is taking place for your peace of mind. For example, non-negligence insurance can help to protect you, should there be any structural damage to your neighbour as a result of your extension.


Extensions usually attract a full VAT rate, currently 20%. There are some projects where the rate of VAT will change. For example, any property that has been vacant for more than two years may be eligible for a reduced VAT rate of 5%.

To help you make sense of the cost of VAT for your house extension, ask your architect for assistance. Architects will know the varying rates and if you can benefit from a reduced rate.

house extension ideas interior

Major item costs

Windows and doors

Windows can be expensive, as a general rule, the bigger the window is, the greater the cost per square metre of the unit. The price of windows will also be affected by the level of glazing you want, the amount of insulation, and the style and fitting of the window.

Doors will also add expense, particularly if you are looking for large sliding patio doors or bi-folding doors, for example. However, the level of light your property has could significantly increase your enjoyment of the space and the ability to open up living spaces to gardens can dramatically improve how spaces are used in summer months.

Central heating

Depending on the size of your extension, your boiler and central heating system may not be able to cope with the extra rooms and space. This may mean that you have to upgrade your system or add a second heating system to the property to cater for the additional space.

Site constraints

If there are any issues with the site of the extension, then this is likely to increase the cost. For example, the soil type could mean that more expensive building materials need to be used.

If your property is difficult to access, such as a terraced house, then the builders may add an extra to cover the cost of access for workers, materials and tools.


You may also need to move trees, drainage, pipework, steel work, gas meters, sheds, conservatories and fuse boxes, all of which can add to your extension costs. If you have a listed building or a property in a conservation area, then you need to allow for a higher consultant and construction costs.

General specification

House extension costs will be influenced by the level of specification you want to achieve. You can choose from simple, standard off-the-shelf products or high-end luxury. Finishes including tiling, paint, wallpaper, and flooring can substantially affect the cost.

Lighting and electronics also increase costs, particularly if smart home systems for heating, electricity, audio and visual control are required.

Finishing touches and external work

Also consider the cost of external works, for example, landscaping for the garden as well as the rebuilding of pathways and driveways. Internally, you may need interior design services to finish the spaces.

Lambeth architect home extension plans

House extension costs as a percentage

In terms of total project budget, the following example will give you a good idea of all of the factors you need to consider and how much you should be spending on each:

  • The cost of builders, material and construction: 60%
  • Architect: 10% (15% on top of the construction cost)
  • Other consultants: 5%
  • Planning permission and administration: 1%
  • Party wall agreements: 3%
  • Insurance: 1%
  • VAT: 20%

How an architect can help with budgeting

A good architect should be able to highlight most of the issues that could affect the building cost for your extension. They may be able to propose cost-saving alternatives or advice on where to focus the work and costs. By assessing your property and having a detailed understanding, most architects will be able to gauge any potential problems before they occur that could increase your budget unexpectedly.

Good quality detailed drawings will help builders to price the project accurately and should help to reduce provisional sums.


An architect will also to help you assemble a shortlist of builders for your project.

Added value

A rear extension, especially in prime areas of London, can increase a home’s value by as much as 30%. In addition to the financial benefits, a house extension can significantly improve your living space. It can make your home a better and more comfortable place to live.

Do I need a quantity surveyor?

A quantity surveyor is an expert is construction costs, and they are advisable, particularly for extensions that are upwards of £100,000.

Quantity surveyors will be able to provide a cost estimate early in the project, once an outline design is complete. They can then provide up to date prices on items and give an accurate project cost breakdown. With this, you can then assess the cost and add more details, remove items or change materials as necessary. A quantity surveyor, therefore, has a key part to play in helping you to make savings on your house extension project.

Extra space & tailored space

The main benefit of extending is it provides an opportunity to customise your house to help create your ideal living space based on your interests and needs.

Your extension could enable you to have a more social kitchen space better linked to your garden. It could also provide more tailored spaces such as a home studio, workshop or kids playroom.

Holland Park house rear extension

Ten house extension considerations:

  • What is the goal of your house extension?
  • What is your budget?
  • Will an extension add value?
  • Is it cheaper and easier than moving home?
  • Do you have a budget in case of an emergency?
  • Do you want to expand up, down or out?
  • How much space do you want to add?
  • Do you need planning permission?
  • How long will it take and will you need to move out?
  • What is your timeframe and do you have a completion date in mind?

Visit the Architecture for London residential projects page to view our residential architecture projects in London.