House Extension Cost London (2026): Prices, Planning & Tips

How much does a house extension cost in London?

Extending is the default move for London families who have outgrown their home. With the average London property selling for around £550,000 (HM Land Registry, July 2026) and stamp duty and fees on top of any move, adding a kitchen-diner or an extra room at the back of the house is often the cheaper way to get more space. It is not cheap, though. Labour, access, licences and party wall work all push London prices above the rest of England.

Published 2026 London rates vary depending on what each source includes:

Source	London figure	What it covers
HomeOwners Alliance	£2,300 – £3,400 per m²	Single storey extension, London
BestBuilders (outer London)	£2,900 – £3,700 per m²	Side return or rear extension, mid-tier finish, ex VAT
BestBuilders (inner London)	£3,300 – £4,200 per m²	Side return or rear extension, mid-tier finish, ex VAT
Humphreys & Sons (London builder)	£1,800 – £3,000 per m²	Single storey extension, London average
Homebuilding & Renovating	25–35% premium	London and the South East vs national rates

The spread is wide because some figures are building-only prices while others include glazing, finishes and fees.

London extension prices by type: our estimates

Our calculator applies a London multiplier of 1.25 to national build rates, then adds design fees (4–7%), a structural engineer, building control, a £274 lawful development certificate and 20% VAT. Use these as a planning budget, not a quote.

Project	Size	Basic	Standard	High-end
Single storey rear extension	20m²	£58,000 – £75,500	£70,000 – £91,500	£86,000 – £117,000
Kitchen extension (mid-range kitchen)	25m²	£90,000 – £129,000	£105,500 – £149,000	£125,000 – £181,000
Side return extension (excluding kitchen)	15m²	£48,500 – £65,000	£60,000 – £79,500	£74,000 – £101,000
Wrap-around extension (excluding kitchen)	30m²	£90,500 – £115,500	£109,000 – £144,500	£137,500 – £183,000
Two storey extension (both floors)	40m²	£101,500 – £130,000	£123,500 – £159,000	£151,500 – £204,000

For a loft rather than an extension, see our separate London loft conversion cost guide.

Rear extension London: cost by size

Floor area	Typical layout	Basic	Standard	High-end
16m²	4m × 4m, PD depth on a terrace	£46,500 – £61,000	£56,500 – £74,000	£69,000 – £94,500
20m²	5m × 4m, open-plan dining area	£58,000 – £75,500	£70,000 – £91,500	£86,000 – £117,000
25m²	Kitchen-diner on a semi	£72,000 – £93,000	£87,500 – £113,000	£107,000 – £145,000
30m²	Large family room, often with prior approval	£86,000 – £110,500	£104,500 – £134,500	£128,000 – £173,000

See also our rear extension and single storey extension cost guides.

Inner vs outer London extension costs

Borough matters, but mainly through access and specification rather than a fixed price list. BestBuilders’ 2026 model puts mid-tier side return and rear extensions at about £3,300–£4,200 per m² in inner London and £2,900–£3,700 per m² in outer London, both before VAT, so the midpoints differ by roughly 14%. The reasons are practical:

• No rear access. Most inner London terraces have no side passage or back lane, so every bag of muck and every steel goes through the house.
• Controlled parking zones. Builders’ vans need suspended bays or permits, and skips on the road need a licence.
• Specification. In zones 1–3 the typical brief is a steel-framed, frameless-glazed kitchen; in outer suburbs a brick extension with bifold doors is more common.

Side return extension cost in London

The side return is London’s signature extension. Victorian and Edwardian terraces were built with a narrow rear wing (the outrigger or closet wing) and a thin alley of garden beside it. Infilling that strip, usually with a glazed or part-glazed roof, turns a dark galley kitchen into a full-width room.

At our London rates a 15m² side return costs about £60,000–£79,500 at a standard finish before the kitchen, and BestBuilders says inner London projects can push past £85,000 once the kitchen is in. Side returns cost more per m² than a simple rear box because nearly all of the work is structural: removing the outrigger’s side wall, inserting steel beams, raising the party wall and handling drainage that typically runs right down that alley. See our side return extension cost guide for a national breakdown.

Planning is not automatic. Lambeth’s Design Guide SPD (Part 4, August 2023) says single storey infills are “potentially acceptable” where they stay subordinate, should be “mostly glazed” so the original return keeps visual primacy, and on heritage assets should be set back from the corner of the main return. It also says wrap-around extensions will be resisted on heritage assets and in conservation areas. Expect other inner boroughs to apply similar tests.

Kitchen extension cost in London

Most London extensions are really kitchen projects. Budget the building and the kitchen separately: our calculator allows £15,000–£30,000 for a mid-range kitchen and £30,000–£50,000 for a high-end one, and London showroom kitchens can go well beyond that. A 25m² kitchen extension with a mid-range kitchen comes to about £105,500–£149,000 all-in at standard finish. More in our kitchen extension cost guide.

London clay, foundations and trees

Much of London sits on shrinkable clay, which swells when wet and shrinks in dry summers, especially near thirsty trees. That shows up in your extension budget in three ways:

• Deeper foundations. NHBC Standards (chapter 4.2) set minimum depths of 1.0m in high-shrinkage clay where trees are outside the zone of influence, rising to 1.5m where planting is restricted. Near large trees depths increase further, and anything beyond 2.5m must be designed by an engineer. Piled or mini-piled foundations are common on London clay sites close to mature planes or oaks.
• Trees you cannot touch. Many street and garden trees in London are protected by tree preservation orders, and in conservation areas you must give the council six weeks’ notice before most tree work. Your engineer will need the species, height and distance of nearby trees, including the neighbours’.
• Drains. Victorian terraces often share a combined drain running behind the houses. Building over or near it means a CCTV survey and, for a public sewer, the water company’s agreement.

Foundations are the biggest unknown in a London quote, so expect them to be priced as a provisional sum until trial holes are dug.

Access, parking and licences

Every London borough licenses skips, scaffolding and hoardings on the highway, and most of the capital is covered by controlled parking zones. Examples of current charges:

Borough	Licence	Fee
Lambeth (2026/27)	Skip licence	£80
Lambeth (2026/27)	Scaffolding or hoarding, 6 weeks	£765
Wandsworth (2025/26)	Skip licence, 28 days	£72.20
Wandsworth (2025/26)	Scaffold, single or double frontage, 28 days	£213 plus £51.50 inspection

Parking bay suspensions are charged separately. A good London quote lists all of these rather than leaving them as surprises.

Party walls on London terraces

A side return usually means building on or against the boundary, and digging foundations within 3m of a neighbour’s footings. Both are notifiable under the Party Wall etc. Act 1996. On a mid-terrace you may need to serve notice on both sides if the rear extension runs full width. BestBuilders estimates £1,200–£2,400+ in surveyor fees for two awards on a London terrace; our calculator allows £1,000–£3,000 per neighbour. Written consent from a neighbour costs nothing. Read our guides to party wall agreements and party wall surveyor costs.

Planning permission for a house extension in London

England’s permitted development rules apply in London as elsewhere: a single storey rear extension of up to 3m on a terraced or semi-detached house (4m detached) without planning, or up to 6m (8m) through the prior approval scheme, with a maximum height of 4m and eaves no higher than 3m within 2m of a boundary. Side extensions must be single storey and no wider than half the original house. Our extension planning permission guide covers the full list, and a lawful development certificate (£274) is the safe way to prove a project complies. A householder application costs £548.

The London-specific catches are:

• Conservation areas. Greater London has over 1,000 of them. Side extensions are not permitted development in a conservation area, so most London side returns in these areas need full planning.
• Article 4 directions. Planning Aid for London lists Article 4 directions in all 33 London councils, covering home alterations, office conversions or HMOs. Haringey, for example, has four directions in conservation areas removing rights such as changes to windows and doors. Check your borough’s map before assuming anything is permitted.
• Borough design guides. Most councils publish householder guidance. Lambeth’s Design Guide SPD (Part 4, 2023) is detailed on infills and wrap-arounds; Hackney has been replacing its 2009 Residential Extensions and Alterations SPD with a new version consulted on in 2025.
• Flats. Converted flats have no permitted development rights, so a ground-floor flat extension always needs planning and usually the freeholder’s consent.

Basements in London

If you are thinking of digging down instead of out, check borough basement policy first. Kensington and Chelsea’s Policy CL7 limits basements to a single storey under no more than 50% of the garden, bans excavation under listed buildings and requires at least 1m of soil above any basement beneath a garden. Several other central boroughs have similar policies. Our basement conversion cost guide covers prices.

How long does a London extension take?

1. Measured survey and design: 3–8 weeks.
2. Approvals: 8 weeks for a lawful development certificate or householder application, longer in conservation areas; party wall notices run alongside and need at least one month for foundations, two months for work to the party wall itself.
3. Build: MyJobQuote gives 9–12 weeks for a side return and 10–14 weeks for a kitchen extension.
4. Kitchen fit and sign-off: 2–4 weeks.

How to cut the cost of a London extension

• Stay within permitted development and confirm it with a certificate rather than a full application where your street allows.
• Talk to neighbours early and ask them to agree one surveyor, or consent in writing.
• Limit structural glass. Frameless roof glazing is one of the most expensive items in a side return; a solid roof with rooflights costs less.
• Plan the finance before committing. Our guide to financing an extension compares remortgaging, further advances and loans.

Does an extension add value in London?

London’s average price of about £550,000 hides a huge range: HM Land Registry puts Kensington and Chelsea at £1,232,640 and Barking and Dagenham at £375,604 (July 2026). London prices also fell 3.3% in the year to July 2026, so value gains are not guaranteed. In higher-value boroughs, an extra 20m² of well-designed living space is likely to be worth more than it costs; in lower-value outer boroughs, a £90,000 kitchen extension may not be fully recovered on sale. Compare sold prices of extended and unextended houses on your street before choosing your spec.

London house extension FAQs

How much does a house extension cost in London?

A 20m² single storey rear extension in London typically costs £70,000–£91,500 at a standard finish, including fees and VAT. Basic builds start around £58,000 and high-end glazed extensions can exceed £115,000.

What is the cost per m² of an extension in London?

Published 2026 figures range from about £2,300 to £3,400 per m² (HomeOwners Alliance) up to £3,300–£4,200 per m² ex VAT for inner London side returns (BestBuilders). Our standard London build rate is £2,750–£3,375 per m² before fees and VAT.

How much does a side return extension cost in London?

About £60,000–£79,500 for 15m² at a standard finish on our calculator, before the kitchen. With a mid-range kitchen and party wall costs, an 18m² side return can reach £91,000–£134,000.

How much is a kitchen extension in London?

Around £105,500–£149,000 for a 25m² extension with a mid-range kitchen, including fees and VAT. A high-end kitchen and structural glazing can take it to £180,000.

Do I need planning permission for a rear extension in London?

Not always. Single storey rear extensions within permitted development limits do not need planning, but conservation areas, Article 4 directions, flats and two storey extensions usually do.

Do I need a party wall agreement for a London extension?

Usually. Building on the boundary or digging foundations within 3m of a neighbour’s footings is notifiable under the Party Wall etc. Act 1996. If neighbours consent in writing, there is no surveyor fee.

Costs in other areas: London lofts · Beckton · Leatherhead · Portsmouth · Maidstone · Essex · Peterborough · Bristol · Birmingham · Nottingham · Manchester · Leeds · Liverpool · Llanelli · Glasgow · All areas compared

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