London House Extension Cost in 2025:
10 things to consider before you start your house refurbishment
The biggest question you’ll ask yourself before deciding to extend your home. This article will give you key advice before you jump in and potentally save you £1000 or more.
June 2025
5 minutes read
AUTHOR
Fabian Danker
In this article:
House Extension Overview:
Discovering the potential costs of extending your house in London can involve various considerations.
The foremost factor that significantly impacts the expenses is the size of the extension you want to build.
This size, often measured in terms of floor area in square meters (m²) , directly corresponds to the overall project cost.
When determining the size of your extension, it’s essential to keep London House Extension planning policy regulations in mind.
Your architect will provide insights into the maximum feasible size within permitted development rules (if applicable) or through a planning application tailored to your property.
Once you’ve established the required extension size, we can initiate cost estimates using standard square meter rates. The following indicative prices are to serve as a rough starting point. These figures exclude VAT and professional fees.
Breakdown of London House Extension Cost:
• Single Storey – A London single storey extension usually falls between £2,500 and £4,000 per sqm.
• Two Storey – To estimate a two-storey extension’s cost, add £3,000 to the single-storey guideline price. Efficiency is gained with two-storey extensions, requiring only one set of foundations. However, obtaining planning consent might be more intricate.
• Roof Extension – A London roof extension usually falls between £2,500 and £4,000 per sqm.
• Basement – Due to complexity and risk, basements incur higher expenses. In London, a basement extension typically ranges between £5,000 to £7,000 per sqm.
It’s worth to note that builders quotes will vary depending on a number of factors, these figures are to act as a guideline to gain a ball-park figure of how much a London House Extension can cost.
Purpose of the london house Extension:
Extensions often include kitchens or bathrooms, which can raise costs compared to additional bedrooms or living areas.
• Kitchens – If including a kitchen within your extension could add £15,000 to £40,000. The lower range covers basic appliances and units.To achieve a bespoke look on a budget, you can purchase the carcasses (main cabinets) of a kitchen and have a seperate joinery company produce bespoke doors.
• Bathrooms – A bathroom extension starts around £10,000 to £20,000, with variations based on fixture quality.
As a rule of thumb, large format tiles take less time and less cutting vs mosaic, and is reflected in the price. However very large slab tiles can be logistically difficult to install which is the exception.
Marble slabs can be selected from specialist suppliers, however the level of co-ordination and skill required to install is reflected in the price and can increase the cost of the bathroom by two or three times.
Quality of sanitartyware, taps, mixers can give you the sense of saving money. However its recommended to avoid very cheap fittings, as they don’t tend to last as long. When this happens, spare parts may not be easy to source or available to purchase from the manufacturer. In this case buying cheap fittings can be a false economy.
Consultant Charges for House Extensions:
Professional services generally amount to 15%-25% over construction expenses: Architects are pivotal in projects, managing design and planning permission. They help translate ideas into reality, creating precise plans for accurate builder quotes. Architects propose cost-effective alternatives and optimise existing spaces when suitable.
Other specialised consultants, including quantity surveyors, planning consultants, heritage consultants, structural engineers, mechanical engineers, party wall surveyors and specialist designers might be needed depending on the project’s complexity.
Additional Considerations:
Planning – The application fee to local authorities for extensions is £528, and even if planning permission isn’t required, obtaining a Lawful Development Certificate is recommended for future property transactions.
Insurance – Notify insurers before construction begins to avoid liability for structural damage during renovations.
VAT – Regular VAT rate is 20%, but reduced rates might apply to certain cases (When a property has been vacant for over two years or converting a single house to seperate flats)
OTHER Costs:
• Windows and Doors – Larger windows carry higher costs per square meter, the level of insulation, material and thickness of the frames can also increase the cost.
• Heating – Consider upgrades to your central heating system for larger extensions. When extending a property, the cost of installing underfloor heating vs installing central heating to the extension can be comparable. This would also be an appropriate time to upgrade the insulation throughout the house to reduce the energy use over the next several decades. An architect can help guide to as to whether a heat pump, or alternative heating system would be suitable for your home.
• Site Constraints – Soil type, relocation of utilities, and property listing affect costs.
• Specification and Finishes – Material quality, lighting, and electronics can increase costs.
• External Works – Landscaping, driveways, external drainage, lighting, external living areas and kitchen can also increase the cost of the works. However its good to plan for these spaces from the start to allow the house to be designed holistically.
Added Value:
London house extensions can enhance property value by up to 50% and augment living space quality. You can use our free house extension property value calculator to see how much your extension may increase the value of your home. As well as property value, there is the added value you’ll enjoy living in your home. Extensions facilitate tailored spaces and specialised rooms like studios, library, cinema rooms or playrooms. Your London House Extension can be shaped according to your preferences and needs.
Quantity Surveyors:
Quantity Surveyors can be a great assett to your building team.
At the earliest stages of the project, you can appoint a Quantity Surveyor to apply market rates to potential design(s) which allows you to make a more strategic decision over the project. A concept cost plan can cost between £500-£2,000 but would go into more detail than square meter rates to give a better indiction of project costs. If you have a strict project budget, its possible to allocate the budget to phase the works so that the project is future proof and implemented over the course of a few years.
A Quantity Survey can assist after planning has been granted during RIBA Stage 4 Tender, and can be appointed to create the Schedule of Works (A documet listing the builder’ work for a builder to price from.)
For larger, complex builds, or if the value of the works are over £150,000 a Quantity Surveyor can assist during construction with taking monthly site visits to assess and value a contractors work. This would be alongside the Architect where the Architect concentrate on assessing quality of the build, a Quantity Surveyor would assess cost claims.
How to Find a Trusted Architect:
Architects are registered with the ARB (Architects Registration Board) and have to meet a their set of criteria to maintain their names on the register. If you are in doubt that the company you are speaking with are Architects, you can check via the ARB Register website.
Frequently Asked Questions:
£2,500 – £4,000 per square meter for construction, plus professional fees and VAT.
£5,500 – £7,500 per square meter for construction, plus professional fees and VAT.
£2,500 – £4,000 per square meter for construction, plus professional fees and VAT.
£5,000 – £7,000 per square meter for construction, plus professional fees and VAT.
construction cost calculator for london home extensions 2025:
Try our Construction Cost calculator below to work a rough cost of your next London House Extension project.
(NOTE: The cost of building depends on several factors, including but not limited to the complexity of the design, extent of structural work, and the quality of materials. The cost doesn’t cover professional fees, legal fees, statutory fees, environmental audits, other third-party fees, Total VAT or planning obligations. The prices might change from time to time, but they might not account for inflation or international events. We advise you ask your architect and costing team to conduct due diligence on your timescale and budget. Prices last updated June 2025. See our disclaimer for more information.)
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AUTHOR Fabian Danker
BA(Hons) MArch ARB RIBA
BA(Hons) MArch ARB RIBA
Fabian is the Director of Studio Colab and a Chartered Architect. He has over 10 years experience working on high-end residential house extensions and refurbishments in London. Prior to becoming an Architect, Fabian spent several years working as a contractor and a joinery company giving him a great insight into the building process and costings which feeds into his designs. Fabian has worked internationally including Melbourne and Thailand on sustainable Net Zero high-end houses.
