The honest answer, before any caveats: a mid-spec single-storey rear extension in inner Londontypically lands between £3,400 and £4,600 per square metre once professional fees, statutory costs and contingency are included, but excluding VAT. For a 25 m² kitchen extension in Hackney or Wandsworth that means an all-in build cost of roughly £90,000 – £130,000, plus VAT.
That number is 20–30% higher than the national average, and there are specific London reasons why. This guide walks through the full cost stack, the borough-tier uplift, what to budget for on top of the headline number, and the two or three line items that most homeowners forget until they arrive as a variation.
Per-square-metre ranges by extension type
London construction costs vary far more by finish and complexity than by extension type. That said, some types carry structural or planning overhead that shifts the per-sqm cost.
- Side return extension (10–20 m²): £3,300–£5,000 per sqm. Small footprint, but a high proportion of expensive elements (structural steel, roof glazing, party wall works).
- Rear extension (20–35 m²): £3,000–£4,800 per sqm. The most efficient £/sqm rate because the footprint is bigger relative to the shell cost.
- Wraparound (30–45 m²): £3,300–£5,200 per sqm. Combines a side return and rear, with more complex roof geometry.
- Double-storey rear (40–70 m²): £3,500–£5,500 per sqm. Steel and foundations scale up; roof and cladding costs are shared over two floors, but plumbing and party wall complexity increase.
Borough tier makes a real difference
London labour and logistics are not uniformly priced. We group boroughs into three tiers when we quote:
- Prime central (Kensington & Chelsea, Westminster, parts of Camden and Notting Hill): expect a +15–20% uplift on the ranges above. Restricted access, permit-only parking, conservation-area conditions and premium subcontractor rates all bite.
- Inner London (Hackney, Islington, Wandsworth, Lambeth, Southwark, Tower Hamlets, Hammersmith & Fulham, Haringey): typically +5–8%. Party Wall Act almost always applies and rear-only material access is standard.
- Outer London (Barnet, Bromley, Croydon, Ealing, Richmond, Hillingdon and similar): roughly on par with the national London figure, sometimes slightly below because side access via gates removes carry-through logistics.
What sits on top of the headline cost
Estimators quoting only shell + fit-out are quoting incompletely. For a realistic all-in London budget, add:
- Professional fees (10–14%) — architect, structural engineer, planning consultant, project manager. Design-and-build firms often bundle these; independent architects charge separately.
- Planning fees — £258 for a householder application in England (2026), £129 for a Lawful Development Certificate.
- Building Control — £850–£2,200 depending on scheme size and whether you use the council or an Approved Inspector.
- Party Wall surveyor(s) — £900–£2,400 for a terrace or semi-detached property. If neighbours dissent and appoint their own surveyor, you pay for both.
- Structural engineer — £750–£1,800 for calculations and construction drawings.
- Contingency (10–15%) — non-negotiable on London Victorian stock. Ground conditions, hidden rot and out-of-tolerance walls are normal, not exceptions.
- VAT at 20% — extensions to existing residential property are standard-rated. A small share of labour on listed properties may zero-rate, but assume 20% until proven otherwise.
- CIL (Community Infrastructure Levy) — usually zero for under 100 m² of new floor area, but check your council's charging schedule before assuming.
Worked example: 28 m² rear extension, mid-spec, Hackney
Assumptions: open-plan kitchen-diner, bifold doors, two rooflights, underfloor heating, terrace-house party walls to both sides.
- Shell (28 m² × £3,600/sqm mid): ~£101,000
- Fit-out modifier (open-plan + features): ~£28,000
- Professional fees: ~£15,000
- Statutory (planning, BC, party wall × 2, structural): ~£6,000
- Contingency @ 12%: ~£18,000
- All-in ex-VAT: ~£168,000
- VAT @ 20%: ~£33,600
- All-in inc-VAT: ~£201,600
For the same brief in Ealing (outer London), the same specification typically comes in 6–10% lower. In Kensington & Chelsea, expect 15–20% higher — mostly labour, parking permits and heritage detailing.
The three costs homeowners forget
- Kitchen and appliances. Often carved out of the builder's contract and forgotten. A mid-spec London kitchen with integrated appliances lands at £12,000–£25,000.
- Landscaping and garden reinstatement. Skips, spoil removal and re-laying a patio easily add £3,000–£8,000.
- Temporary accommodation. If you move out during first fix, budget London rental rates for 8–14 weeks.
Ways to bring the London number down (sensibly)
- Design efficiency, not shrinkage. A well-designed 22 m² kitchen extension can outperform a 32 m² badly-planned one — and cost £30k less.
- Standard glazing sizes. Bespoke bifolds price 30–50% higher than off-the-shelf modules with the same finish.
- Repeat detailing. The same rooflight spec on both openings costs less than two bespoke ones.
- Design-and-build under one contract. Removes the finger-pointing that generates variations and disputes.
- Skip the show-off features. Structural glass rooflights, curved corners and pocket doors are lovely and expensive. Pick one.
How our estimator handles all of this
The calculator layers borough tier, extension type, finish, features and party wall logic on top of shell + fit-out benchmarks, then adds professional fees, statutory costs and contingency automatically. You see the full itemised breakdown on the quote page — nothing bundled or hidden. VAT is shown separately so you can compare like-for-like against builder quotes that quote net.