How much does a house extension cost in London?

How much does a house extension cost in London?

This article discusses house extension costs in London, along with tax considerations and consultants’ fees. It explains the options that are available and outlines approaches to making savings.

As a headline summary, an architect-designed house extension in London may cost between £3,000 and £5,000 per square metre. Elsewhere in the UK, you can expect to pay between £2,000 and £3,500 per square metre.

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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.

House extension costs in 2026

While many factors 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, measure the size of the 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. They do not include VAT or professional fees.

Stone House extension by Architecture for London

Single-storey house extension costs

As outlined above, a single-storey extension in London will usually cost between £3,000 and £5,000 per square metre. £3,000 per square metre would represent cost-effective finishes and products. Elsewhere in the UK, you can expect to pay between £2,000 and £3,500 per square metre.

Two-storey extensions

To approximate the cost of a 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 extensions

Due to construction complexity, a basement is more costly. A basement extension in London will typically cost between £6,000 and £8,000 per square metre.

What will your house 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.

Hackney House extension by Architecture for London

Consultant fees for a house extension

Architects

Architects are often the first point of contact for a project and provide a range of services for their fees. 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 house extension 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. The cost of all professional services, including architects, will typically be 15%-20% on top of the construction cost.

Barnsbury House extension by Architecture for London

Additional costs

Planning

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 work. 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.

Insurance

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.

VAT on house extensions

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 whether you can benefit from a reduced rate.

Barnsbury House extension by Architecture for London

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 specification of the glazing, the thermal performance, 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 ability to open up living spaces to gardens and bring in lots of natural light into the extension can dramatically improve your enjoyment of the space. Large sliding or folding doors can completely change how the extension can be used in connection with the garden in the 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. You may also wish to consider switching to a heat pump for a more sustainable option.

Site constraints

House extension costs will increase if there are any issues with the site of the extension. For example, the soil type, or the proximity to trees might mean that more expensive foundations are required.

If your property is in central London on a tight site, such as a terraced house, then builders will need to price in the extra cost of access and waste removal issues, increased skip and parking charges etc.

Alterations to existing services, including underground drainage, incoming meters, etc, will further add to your extension costs.

Sites may also have planning and heritage constraints. If you have a listed building or a property in a conservation area, then you should allow for higher consultant and construction costs. These are due to the additional consents required and a likely need for specialist building products and craftspeople for the refurbishment of historic sash windows and shutters, decorative plaster work etc.

General specification

House extension costs will be influenced by the level of specification you require. You can choose from simple, standard products or completely bespoke items. Your selected 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.

House extension in Dartmouth Park by Architecture for London

External works

Also consider the cost of external works, for example, improvements to garden landscaping, patios, pathways, driveways and boundary walls.

House extension costs as a percentage

The following examples give a breakdown as a percentage of the total project budget:

The cost of builders, material and construction: 60%
Architect: 10% (or 15% as a percentage of the construction cost)
Other consultants: 5%
Planning permission and administration: 1%
Party wall agreements: 2%
Insurance: 1%
VAT: 20%

How an architect can help with budgeting

An architect will consider all of the issues affecting building costs for your extension. They will be able to propose cost-saving alternatives where required, and advise on phasing or limiting the scope of the project. By commissioning detailed surveys, site assessments, opening up works and trial pits, architects will be able to assess most potential cost issues before they occur.

Good quality, detailed drawings will help builders to price the project accurately and should help to reduce provisional sums. An architect will also help you assemble a shortlist of builders for your project.

House extension in Highbury by Architecture for London

House extensions – 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 the enjoyment of your living spaces. They can also make your home perform better thermally and therefore make internal spaces more comfortable.

Do I need a quantity surveyor?

A quantity surveyor is an expert in construction costs, and their appointment is advisable, particularly for more complex extensions with a larger budget.

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 plan. Using this document, you can remove items or change the specification 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 a house extension is that 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.

House extension in Kensington by Architecture for London

Ten house extension cost considerations:

What is the goal of your house extension?
What are your budget constraints?
Will an extension add value?
Is it cheaper and easier than moving home?
Do you have a contingency in case of unforeseen items?
Do you want to extend 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 projects page to view our residential architecture projects in London.