London extension costs by borough in 2026: what house prices tell you (and what they do not)

Londoners planning an extension often ask for "the Kensington rate" or "the Croydon rate" as if there were a published tariff. There is not. What does exist are carefully hedged national and London cost bands from named publishers, and an official monthly house-price series that shows how far property values diverge across the capital. Used together - and with a sceptical reading of any quote - they are enough to keep you from being sold a fairy story. Sources were checked on 28 October 2026.

Start with published build-cost bands, not folklore

Checkatrade's house extension cost guide (extension-type table last updated June 2026; reviewed with estimators who are RICS and CIOB members) puts a full extension at about £1,800 to £3,000 per m² excluding VAT, with a shell-only build around £1,200 to £1,700 per m². A 20m² single-storey is illustrated at roughly £40,000 to £56,000 nationally. A wraparound or two-storey sits higher still because of structure, scaffolding and roof work.

HomeOwners Alliance publishes the same UK £1,800 to £3,000 per m² band and states that a single-storey extension in London may cost from about £2,300 to £3,400 per m². The Federation of Master Builders has separately observed that London and South East labour and materials can run some 30 to 50 per cent above the national average in some cases. None of those sources then subdivides London into thirty-two borough rates. Anyone who does, without naming a survey, is guessing - and that guess is often used to justify an unexplained premium or an improbably low headline.

What the official house-price index actually shows

The UK House Price Index for England: August 2026 (published by HM Land Registry / ONS on 21 October 2026) is the right place to look for comparable, dated borough averages. Sale prices are not build costs, but they do show the market context in which contractors, surveyors and planners operate.

• Kensington and Chelsea: average about £1,204,072 in August 2026, down 13.8% year on year - still the most expensive local authority in England on this series.
• City of Westminster: about £887,206 (−14.3% year on year).
• Richmond upon Thames: about £813,931 (flat year on year).
• Croydon: about £396,718 (−1.5%).
• Barking and Dagenham: about £369,881 (+4.7%) - the cheapest London borough on the same table.

London as a whole saw average prices fall 2.2% in the 12 months to August 2026, with flats (about half of London sales) down 5.0%. Provisional figures are revised as more transactions register, so treat the latest month as a snapshot, not a verdict. Use it to understand why a contractor may price inner-London logistics and finish expectations differently from an outer-borough plot with driveway access - not as a formula that multiplies your floor area by a postcode factor.

Why the same drawings often cost more in prestige and inner areas

Without inventing a borough tariff, the published sources and common London site conditions point to the same cost drivers:

• Labour and logistics. Higher day rates, parking restrictions, Congestion Charge and ULEZ impacts on vans, and longer waits for skips all appear in contractor pricing for central and inner London.
• Access. Checkatrade's assumptions include "a good level of access". Many terraced rear gardens are reached only through the house or a narrow side passage; crane lifts for steel and glass add cost a suburban plot may avoid.
• Planning and heritage. Conservation areas, listed buildings and Article 4 Directions are concentrated in boroughs such as Westminster, Kensington and Chelsea, Camden and Richmond. They often remove or narrow permitted development and demand matching materials that take longer to source.
• Party Wall density. Terraces and semis mean more adjoining owners. HomeOwners Alliance cites an average party wall agreement cost around £1,000, varying with complexity - and that multiplies with each neighbour.
• Finish expectations. Prestige postcodes often specify larger glazed openings, underfloor heating, bespoke joinery and stone or engineered floors that sit outside a "simple plastered box" assumption.

So a 20m² single-storey that a national guide places at £40,000 to £56,000 may, on the HomeOwners Alliance London band, sit nearer £46,000 to £68,000 before VAT and before kitchen or bathroom fit-out - and a constrained, high-spec job can go higher still. Outer boroughs with easier access can sit closer to the national band, but poor ground, Thames clay or a basement neighbour can erase that advantage overnight.

How to use borough prices without being misled

1. Anchor every conversation in the Checkatrade / HomeOwners Alliance £/m² bands, dated and linked.
2. Use the UK HPI borough averages only as context for labour markets and finish expectations - never as a multiplier you apply to the floor area.
3. Insist on itemised quotes against drawings you own: preliminaries, foundations, structure, envelope, glazing, first and second fix, finishes, attendance on consultants, VAT and programme.
4. Ask what assumptions the quote makes about access, steel, brick matching and Party Wall surveyors.
5. Compare at least three written quotes for the same scope. A price far below the published London band with no thinner specification is a red flag, not a bargain.
6. Keep a contingency of around 10 per cent for soft ground, drain diversion or hidden asbestos - the same contingency HomeOwners Alliance and Checkatrade both recommend.

Fees that sit outside the build rate

HomeOwners Alliance and Checkatrade both flag costs homeowners forget: architect or designer fees (often around 3 to 15 per cent of construction cost, or a fixed fee), structural engineer visits, planning fees (England householder application to enlarge a single dwellinghouse listed at £548 on the Planning Portal fee schedule; prior approval for a larger home extension £249), building control, Party Wall and that contingency. VAT at 20 per cent on most domestic building work is another line that should never be "assumed included" without it being written down. Soft costs and statutory fees can easily add several thousand pounds before a single brick is laid.

Vetting before the deposit clears

• Match the limited company name and number on Companies House to the quote.
• Confirm any TrustMark or Federation of Master Builders claim on the scheme's own register.
• Ask for public liability and employer's liability details, and for photographs of comparable London extensions at steel and roof stage - not only the finished glossy shots.
• Citizens Advice advises keeping deposits as low as possible and not agreeing more than 25 per cent, preferably with deposit protection or an FCA-registered escrow arrangement.
• Put the specification and programme in a written contract. Under the Consumer Rights Act 2015, services must be performed with reasonable care and skill; under the Consumer Contracts Regulations 2013, many off-premises contracts attract a 14-day cooling-off right.

A borough average is a conversation starter. A dated, itemised quote against your drawings is the only figure that matters. If a salesperson leans on "everyone pays Kensington rates around here" without showing the line items that drive that claim, ask them to walk you through the preliminaries instead - and walk away if they cannot.

Sources checked on 28 October 2026: UK House Price Index England: August 2026 (published 21 October 2026); Checkatrade, How much does a house extension cost? (June 2026 type table); HomeOwners Alliance, House extension costs; Planning Portal, Fees for planning applications in England; Citizens Advice, Before you get work done on your home. House prices are provisional averages, not build rates. Ranges are published guide figures, not quotations. Nothing here is legal, tax or surveying advice.