The honest answer before any caveats: a mid-spec single-storey rear extension in inner London lands between £3,400 and £4,600 per square metre once professional fees, statutory costs and contingency are included — excluding VAT. For a 25 m² kitchen extension in Hackney or Wandsworth that means roughly £90,000 – £130,000 plus VAT.
Step 1 — Fix your footprint and finish level first
Every credible budget starts with two numbers: how many square metres of new floor area, and which finish tier you are buying. Everything else is derived. Write both down before you talk to anyone.
| Tier | Glazing | Kitchen | Floor / heating | Per m² ex VAT |
|---|---|---|---|---|
| Value | Off-the-shelf aluminium bifolds, standard rooflights | £10k–£15k trade kitchen | Engineered oak, rads or basic UFH | £2,900–£3,500 |
| Mid | Slim aluminium sliders, 2 large rooflights | £18k–£28k with integrated appliances | Large-format tile, zoned UFH | £3,400–£4,600 |
| High | Bespoke steel-look or structural glass | £45k+ bespoke joinery | Stone or micro-cement, multi-zone UFH | £5,000–£7,500 |
Step 2 — Pick the right per-m² base rate for your type
London costs vary more by finish and complexity than by extension type — but some types carry structural or planning overhead that shifts the rate.
Step 3 — Apply your borough tier multiplier
London labour and logistics are not uniformly priced. We quote against three tiers:
| Tier | Examples | Adjustment | Why |
|---|---|---|---|
| Prime central | K&C, Westminster, prime Camden | +15% to +20% | Permit-only parking, restricted access, conservation detailing, premium subcontractors |
| Inner London | Hackney, Islington, Lambeth, Southwark, Wandsworth, Tower Hamlets, H&F, Haringey | +5% to +8% | Party Wall Act near-universal, rear-only material access, tight site setup |
| Outer London | Barnet, Bromley, Croydon, Ealing, Hillingdon, Richmond | Baseline (0% to −5%) | Side gate access, on-plot parking, easier deliveries and spoil removal |
Step 4 — Add the costs that sit on top of the build
An estimator quoting only shell plus fit-out is quoting incompletely. Add every one of these:
- 1
Professional fees
10–14% of buildArchitect (RIBA stages 1–4), structural engineer, planning consultant where needed, and project management. Design-and-build firms bundle these; independent architects invoice separately by stage.
- 2
Planning fees
£129–£258£258 for a householder application in England (2026); £129 for a Lawful Development Certificate. Pre-application advice, where offered, runs £250–£800 extra.
- 3
Building Control
£850–£2,200Council or Approved Inspector. Includes plan check and site inspections through to the completion certificate — insist you receive that certificate.
- 4
Party wall
£900–£3,500 per neighbourOne agreed surveyor is cheapest. If your neighbour appoints their own, you pay both. Terraces with two adjoining owners can reach £3,000–£7,000.
- 5
Structural engineer
£750–£1,800Beam and foundation calculations plus construction details. Cheaper if instructed early alongside the planning drawings rather than after tender.
- 6
Contingency
10–15%Hold it as real cash, in your account, not in the builder's price. On pre-1930 London stock, ground conditions and hidden rot are normal, not exceptional.
- 7
VAT at 20%
Standard-ratedExtensions to existing dwellings are standard-rated. Some works on listed properties or long-empty homes vary — assume 20% until an accountant proves otherwise in writing.
- 8
CIL
Usually £0Community Infrastructure Levy is normally nil under 100 m² of new floor area, but check your borough's charging schedule and file the exemption form on time.
Step 5 — Test the shape of your budget
Once you have a total, sanity-check its distribution. A London extension budget that puts more than a quarter of the money into glazing, or under 10% into fees and contingency, is a budget that will move.
- Shell — groundworks, structure, roof, envelope42%
- Glazing, rooflights and external doors13%
- Kitchen, joinery and appliances17%
- M&E — electrics, plumbing, UFH, ventilation12%
- Fees, statutory costs and contingency16%
Worked example: 28 m² rear extension, mid-spec, Hackney
Open-plan kitchen-diner, bifolds, two rooflights, underfloor heating, party walls both sides.
| Line | Basis | Amount |
|---|---|---|
| Shell | 28 m² × £3,600/m² | £101,000 |
| Fit-out modifier | Open-plan + feature glazing | £28,000 |
| Professional fees | ~11% of build | £15,000 |
| Statutory | Planning, BC, party wall ×2, structural | £6,000 |
| Contingency | 12% | £18,000 |
| Total ex VAT | — | £168,000 |
| VAT | 20% | £33,600 |
| Total inc VAT | — | £201,600 |
Same brief in Ealing: typically 6–10% lower. In Kensington & Chelsea: 15–20% higher, mostly labour, parking suspensions and heritage detailing.
Step 6 — Compare quotes on a like-for-like basis
- Is VAT shown separately?You
A net quote next to a gross quote is a 20% illusion. Normalise both to ex-VAT.
- Are provisional sums identified?Builder
PROV and PC sums for kitchen, glazing and landscaping should be listed with the allowance stated.
- Who pays statutory fees?Builder
Planning, Building Control, party wall and skip/parking permits — named, not assumed.
- Is muck-away and spoil removal included?Builder
London spoil disposal is £180–£320 per tonne removed including permits.
- Is scaffolding priced for the full duration?Builder
Ask for the weekly hire rate beyond the allowed period.
- Is making-good to the existing house included?Builder
Plastering, decorating and floor levelling where new meets old — the classic variation.
- Is there a stage payment schedule with triggers?Both
Payments against completed work, never against calendar dates.
- Is retention held?You
2.5–5% for 6–12 months after practical completion.
The cheapest quote on the table is almost always the least complete one. Price the same scope three times before you price three different scopes once.
— What we tell every homeowner on the first call
The three costs homeowners forget
| Forgotten cost | Typical London range | When it lands |
|---|---|---|
| Kitchen and appliances | £12,000–£25,000 mid-spec | Week 14–16, after the money feels spent |
| Landscaping and garden reinstatement | £3,000–£8,000 | Final two weeks on site |
| Temporary accommodation | 8–14 weeks of London rent | First fix, when dust and no kitchen collide |
How to bring the London number down without cheapening it
- Design for efficiency — a well-planned 22 m² beats a badly planned 32 m² and saves £30k+
- Repeat details: the same rooflight and door spec twice is cheaper than two bespoke ones
- Use standard glazing module sizes; bespoke prices 30–50% higher for the same finish
- Buy design-and-build under one contract to remove the coordination gap that generates variations
- Order long-lead steel and glazing at contract signature
- Value-engineer the shell to fund a showpiece rooflight
- Change the design after tender — every post-tender change is priced without competition
- Leave the kitchen out of the budget so the headline looks lower
- Let contingency live inside the builder's price
- Accept a lump-sum quote with no schedule of works behind it
How our estimator handles all of this
The calculator layers borough tier, extension type, finish, features and party wall logic on top of shell and fit-out benchmarks, then adds fees, statutory costs and contingency automatically. The quote page shows the full itemised breakdown — nothing bundled — with VAT separated so you can compare like-for-like against builder quotes.
+Is £2,500 per m² ever realistic in London?
Only for a value-spec extension in outer London with side access, standard glazing, no party wall complications and a trade kitchen supplied by the homeowner. Treat it as the floor, not the benchmark.
+Should I hold contingency or let the builder price risk?
Hold it yourself. A builder pricing risk inside a fixed sum will price it at the pessimistic end and keep the difference; you will never see it back.
+Do I pay VAT on architect and engineer fees too?
Yes — professional fees are standard-rated. Budget VAT across the whole project, not just the build contract.
+How much does an extension add to a London house value?
Typically 6–11% on a mid-market terrace for a well-executed kitchen extension, and less in prime central where the £/m² of existing space is already very high. Never treat uplift as a guaranteed return.
+When is the cost fixed?
Realistically at contract signature on a full technical package — structural details, glazing schedule, kitchen and electrical layouts. Fixing a price off planning drawings alone invites variations.