What it should cost: a London price & red-flag guide (2026)

Nobody can quote your project from a web page, and anyone who gives you one firm number is selling, not estimating. What this guide gives you instead is more useful: honest low-to-high ranges for every part of a London project in 2026, and the framing to read a quote against them. Because the most dangerous number in building isn't the high quote — it's the low one. A price 40–60% below the rest of the field almost never means a bargain; it means something has been left out, and you'll meet it later as a variation.

How to read what follows. Every figure is a range, dated to 2026 and for London, and — unless stated — build costs exclude VAT and professional fees, which you add on top. The ranges are triangulated from published industry data: the RICS Building Cost Information Service (BCIS), the HomeOwners Alliance cost guides, the Federation of Master Builders, and the official Planning Portal / GOV.UK fee schedules for statutory costs. Where the spread is genuinely wide and project-specific, we flag the figure as directional rather than well-sourced.

Build cost per m² by project type

Fit-out allowances

Build cost usually takes you to a watertight, plastered shell. Kitchens and bathrooms are separate allowances on top, and they swing widely with specification:

• Kitchen (units, worktops, appliances, fitting): roughly £15,000–£38,000+ — the single biggest variable in most extension budgets.
• Bathroom / en-suite (sanitaryware, tiling, plumbing): roughly £6,000–£20,000 depending on size and finish.
• Watch the "prime cost" and "provisional" sums in a quote — these are placeholders for items not yet chosen (kitchen, tiles, flooring). A cheap quote often hides a low placeholder you'll have to top up.

Professional fees

Statutory fees (England, as at 2026)

VAT and contingency — the two everyone under-budgets

• VAT: most building work to an existing home is standard-rated at 20%, and so are the professional fees. The belief that loft conversions are zero-rated is a myth. Reduced (5%) and zero rates exist only in specific cases — see the VAT guide. Budget 20% unless you can prove otherwise.
• Contingency: hold 10–20% of the build cost back for the unknowns — and in London's older Victorian and Edwardian stock, where what's behind the wall is a surprise, 15% is a sensible default rather than a luxury. A budget with no contingency is a budget that will be broken.

The red-flag check

• Compare like with like: ask every builder to price the same detailed specification, not their own interpretation.
• Check what's excluded as much as what's included — VAT, fees, kitchen, decoration, scaffolding, skips, party wall.
• Treat a large up-front deposit, cash-only pricing and no written contract as red flags in their own right (see the who-to-hire guide).
• A realistic all-in budget for a typical 20m² single-storey rear extension in London — build, fees, statutory, VAT and contingency — lands in the order of £85,000–£120,000. If a number is far below that, find out why before you celebrate.

How this connects to your Planning Permission Checker report

Planning Permission Checker's dossier carries indicative build-cost ranges calibrated from real project data, alongside the sold-price comparables that tell you whether the spend is worth it — both as ranges, never single numbers, and both cited. What this guide adds is the framing to read a real quote against the market. For a measured cost model on a specific scheme — a proper estimate rather than a range — enquiries route to Hampstead Chartered Surveyors, an RICS-regulated practice (Firm Reg. 923064), who can bring in a quantity surveyor where the project warrants it.