House Extension Costs & Planning Permission in London

An extension is one of the best ways to add space and value to a London home, but costs and planning rules vary widely. This guide covers what extensions cost per square metre and by type in 2026, when you need planning permission, current approval rates by borough, the party wall and VAT rules to watch, and how long the whole process takes.

How much does a house extension cost in London?

A house extension in London typically costs £2,000 to £3,500 per square metre for standard work, rising to £3,000 to £5,000 per square metre for premium finishes, excluding VAT and professional fees, according to Architecture for London's 2026 figures. So a 20 m² single-storey rear extension commonly lands between £45,000 and £90,000 once design, structural work and finishes are included.

Extension costs by type

The right type of extension depends on your property and how you use it. Side returns and rear extensions are the most common in London's terraced housing, while wrap-arounds and double-storey extensions deliver the most space.

• Single-storey rear extension: the classic way to enlarge a kitchen-diner, at £2,000 to £2,800 per m² for standard work.
• Side-return extension: fills the narrow alley beside a terrace. A typical London side return costs £55,000 to £85,000, and £64,000 to £102,000 for larger or premium builds (Resi / Multiply).
• Wrap-around extension: combines rear and side return for maximum ground-floor space, at a correspondingly higher total cost.
• Double-storey extension: adds rooms upstairs and down, with a lower cost per m² because the roof and foundations are shared across two floors.
• Loft conversion: an alternative route to more space that often avoids using up garden, and is frequently permitted development.

Almost every extension that opens up the ground floor needs a steel beam where a wall is removed. Our guide to RSJ installation explains that element in detail. See also our house extensions service.

Do you need an architect and drawings?

For anything beyond the simplest extension, yes. You will need measured drawings for both the planning application (where required) and Building Control, plus structural engineer's calculations for beams and foundations. Professional fees, including design, structural engineering and any planning or party wall surveyors, typically add 10 to 15% on top of the build cost. A design-and-build contractor can manage all of this under one roof, which avoids the gaps that appear when separate parties hand off to each other.

Planning permission vs permitted development

Not every extension needs planning permission. Many single-storey rear extensions fall under Permitted Development rights, which allow you to build within set limits without a full application. However, most London side returns, projects in conservation areas, listed buildings, and double-storey extensions usually require full planning permission from your local council.

Because the rules turn on plot width, conservation status and how much you have already extended, the safest first step is a professional assessment. We check your property's permitted development potential and planning history before recommending a route.

Conservation areas and listed buildings

Large parts of London, especially in boroughs like Kensington & Chelsea, Westminster, Camden and Islington, sit within conservation areas, and many homes are listed. In these areas, permitted development rights are usually restricted or removed, materials and design are scrutinised more closely, and listed buildings need separate listed building consent for many changes. It is still very possible to extend, but the design has to respect the character of the area, and early conversations with the planning department pay off. We work across every London borough and tailor each application to local conservation policy.

What are the planning approval rates in London?

The odds are good. Local authorities across England granted around 90% of householder planning applications in the quarter ending December 2025, and that rate has held between 88% and 91% for five years, according to government planning statistics. Rear extensions are approved at above 85% in most councils.

Approval rates do vary by borough, though. Kensington & Chelsea approves around 92.5% of applications, while Brent sits closer to 76%. A well-designed application that respects the local character and neighbours is far more likely to succeed, which is why design quality matters as much as ambition.

Party wall agreements and hidden costs

Two costs catch London homeowners out. The first is the Party Wall etc. Act 1996: if you build on or near a boundary, or excavate close to a neighbour's foundations, you must serve notice, and a party wall award can add a few weeks and surveyor fees. The second is VAT: most extension work is standard-rated at 20%, so always confirm whether a quote includes it. Other costs to budget for include building control fees, structural calculations, and any drainage or utility diversions.

How long does an extension take?

Allow time for the full journey, not just the build. A single-storey London extension usually takes 10 to 16 weeks on site, preceded by several weeks for design, planning and structural calculations. A full planning application typically takes around 8 weeks for a decision once validated.

• Design & drawings: 2 to 4 weeks
• Planning decision (if required): around 8 weeks
• Structural calculations & building control: 1 to 3 weeks
• Party wall notices (if required): allow up to 2 months
• Construction: 10 to 16 weeks for single-storey

GS Renovation manages the entire process, from drawings and planning through to structure and build, across London. For a free assessment of what your property can support, contact us or call 07472 424 226. Adding a kitchen-diner? See our kitchen renovation cost guide.