Key Takeaways
- London extensions cost roughly £2,000 to £2,800 per m² for standard work (Resi, Checkatrade and MyJobQuote, 2026), and Architecture for London quotes £3,000 to £5,000 per m² for architect-led premium schemes.
- A single-storey rear extension starts from about £40,000, and a typical London side-return extension costs £60,000 to £100,000 all in. See our house extensions service in London for how we price and build them.
- English councils granted 91% of householder planning applications in the quarter to March 2026, a rate stable between 88% and 91% for five years (MHCLG planning statistics).
- Many extensions fall under Permitted Development and need no full planning application, but many London side returns need one, because permitted development limits a side extension to a single storey, 4 m high and half the width of the original house, and excludes it in conservation areas.
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, 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 £2,800 per square metre as a build rate for standard work, excluding VAT and professional fees. Single-storey rear extensions start from about £40,000, and a 20 m² one commonly lands between £45,000 and £90,000 once design, structural work and finishes are included. Architecture for London's 2026 guidance quotes a higher £3,000 to £5,000 per m² for a London single-storey extension, and states that its figures are for construction only, excluding VAT and professional fees. Both are build rates, so the gap is specification and location rather than what is counted: the upper end reflects architect-led schemes with a high finish in inner London, while the lower end reflects standard work across Greater London. Ask any quote which end of that it sits at, and whether VAT and fees are inside or outside the number, before you compare it with another.
| Finish level | Cost per m² (London) |
|---|---|
| Standard | £2,000 to £2,800 |
| Mid-range | £3,100 to £3,700 |
| Premium | £4,300+ |
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.
| Extension type | Cost per m² | Typical London total |
|---|---|---|
| Single-storey rear | £2,000 to £2,800 | From £40,000; £45,000 to £90,000 for 20 m² all in |
| Side return | £2,400 to £3,200 | £60,000 to £100,000 |
| Wrap-around | £2,400 to £3,200 | £80,000 to £130,000 |
| Double-storey | £1,800 to £2,600 | From £75,000; mid-range £100,000 to £160,000 |
| Loft conversion (alternative) | £1,250 to £2,500 | £25,000 to £100,000+ |
A double-storey extension has a lower cost per m² because the roof and foundations are shared across two floors, which is why it is often the better value per room; our double storey extension cost guide breaks that down. A loft conversion is the alternative that does not use up garden, and it is more often permitted development. If going down is the only direction left, see basement conversion costs.
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 in London.
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 15 to 20% on top of the build cost (Architecture for London, 2026). A design-and-build contractor can coordinate all of this under one contract, 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, many London side returns (permitted development limits a side extension to a single storey, 4 m high and half the width of the original house), side, two-storey and roof extensions in conservation areas, and double-storey extensions usually require full planning permission from your local council, and listed buildings need listed building consent as well.
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. Our guides to permitted development rights and getting planning permission in London set out each route, including the larger home extension prior approval route, and a lawful development certificate is worth £274 to prove the work was lawful when you sell.
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 a conservation area, permitted development still allows a single-storey rear extension within the limits, unless an Article 4 direction removes it, but side extensions, two-storey rear extensions and roof extensions need planning permission, and materials and design are scrutinised more closely. A listed building needs listed building consent for any works, inside or out, that affect its character, and building without it is a criminal offence. 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 London and the surrounding counties, and the application is prepared by your designer to local conservation policy.
What are the planning approval rates in London?
The odds are good. Councils in England decided 33,500 householder applications in the quarter to March 2026 and granted 91% of them, and that rate has held between 88% and 91% for five years, according to MHCLG planning statistics.
Approval rates do vary by council, and the government's live planning application tables break householder decisions down by local authority if you want your own borough's figure. 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.
Source: gov.uk, MHCLG Planning applications in England: January to March 2026, published 19 June 2026.Party wall agreements and hidden costs
Two costs catch London homeowners out. The first is the Party Wall etc. Act 1996: if you build a new wall on the boundary, or excavate close to a neighbour's foundations, you must serve notice one month before starting, and work to the party wall itself needs two months' notice, and a party wall award adds a few weeks and surveyor fees. Our party wall guide explains the notices. The second is VAT: most extension work is standard-rated at 20%, so always confirm whether a quote includes it, and ask whether an exception to VAT on renovations applies, such as the 5% rate for a property empty for two years or more.
| Fee or extra | Cost | Notes |
|---|---|---|
| Householder planning application | £548 | England, from 1 April 2026, flat rate |
| Prior approval, larger rear extension | £249 | The 6m and 8m permitted development route |
| Lawful development certificate | £274 | Proof the work was permitted development |
| Planning Portal service charge | £75.83 plus VAT | Avoidable if your council takes payment directly |
| Party wall surveyor, extension | £900 to £2,700 | Per adjoining owner, single agreed surveyor (HomeOwners Alliance, 2026) |
| Professional fees | 15 to 20% of build cost (Architecture for London) | Architect, structural engineer and consultants |
Also budget for building control fees, structural calculations, drainage or utility diversions, a Thames Water build over agreement where a public sewer runs close by, and a contingency of at least 10%.
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 builds house extensions across London and the surrounding counties: we coordinate the designer, engineer and building control, then carry out the structure and the build. 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.