How Much Does a House Extension or Loft Conversion Cost in London in 2026?

If you've started asking builders for numbers, you've probably already had three quotes that don't seem to be pricing the same job. That's normal — and it's one of the reasons homeowners across South West London end up overspending before a single wall comes down. Here are realistic current ranges, and what actually moves them.

Single-storey rear extensions

A straightforward rear extension in London currently runs £2,800–£5,500 per square metre, depending on specification. For a typical 15–30m² addition, that's a total of roughly £80,000–£150,000 all-in, including VAT.

• Basic spec: £2,800–£3,200/m² — standard glazing, mid-range fittings, a simple flat roof
• Mid-range: £3,200–£4,200/m² — the bracket most Victorian terrace extensions in this area land in
• High-end: £4,200–£5,500+/m² — extensive glazing, bi-folds, underfloor heating, premium finishes throughout

A wraparound — rear plus side return — typically costs 15–25% more than a simple rear extension, mostly down to the added corner complexity and structural work. Expect £90,000–£170,000+ for a 25–45m² wraparound.

What pushes the number up: a pitched roof instead of flat (add roughly £4,000–£7,000), poor ground conditions needing deeper foundations, difficult access on a mid-terrace build, structural glazing, and — something we see missed constantly — appointing separate party wall surveyors for each side of a shared wall instead of one agreed surveyor.

What keeps it down: a simple rectangular footprint, standard rather than structural glazing, good access, and — on a double-storey addition — keeping walls aligned between floors so less steel is needed.

Loft conversions

Loft conversions scale by type rather than area, since the structural work varies so much between them. A rooflight or Velux conversion typically runs around £27,500 total (roughly £920/m²) and takes four to six weeks — it needs the least structural alteration, since there's no dormer box and no change to the roofline. A dormer conversion typically comes in around £50,000 (about £1,670/m²) over ten to twelve weeks. Hip-to-gable conversions run to roughly £60,000 (around £2,000/m²) over eight to twelve weeks, and a mansard — usually the most involved — averages £65,000 (about £2,170/m²), also eight to twelve weeks.

Once you're changing the roof shape — dormer, hip-to-gable, mansard — you're paying for steelwork, roof alterations and usually a bathroom, which is where budgets creep. Older South West London houses often need unexpected strengthening work once the roof structure is opened up, and that rarely shows up in an initial quote.

Why the quotes you'll actually receive vary so much

Two builders quoting the "same" extension can be £30,000 apart and both be right — for different scopes. One has priced a shell; the other has priced it finished, with a contingency, and with your bathroom fittings included. Comparing quotes properly means comparing scope line by line, not just the bottom number.

A sensible contingency to hold back is 10–15% of the total build cost — industry guidance consistently points to this range, and in our experience it's rarely wasted.

Where a project manager actually changes these numbers

None of the above is a guarantee — every property and every builder's pricing is different, and we won't pretend otherwise. What we do guarantee is scrutiny: catching the omissions before they become variations, sourcing materials at trade rather than retail prices, and making sure the quote you're comparing against three others is actually comparable. On a project this size, that's usually the difference between the fee paying for itself and it being an added cost. See also client-side PM vs architect contract administration and does a project manager fee pay for itself.

Methodology and sources

These figures are ranges, not quotes for your house. They reflect typical all-in build costs discussed with homeowners and builders on South West London residential work around 2026, cross-checked against commonly cited UK cost guides for extensions and loft conversions. Individual tenders move with specification, access, ground conditions, party wall complexity and how complete the drawings are when builders price.

SW London caveats worth naming: Victorian and Edwardian terraces often need deeper foundations than the original house, shared drains that only show up once excavated, and party wall awards on one or both sides — none of which sit neatly inside a per-square-metre average. Mid-terrace access, conservation constraints and higher finish expectations in this part of London also push jobs toward the middle and upper bands more often than a national average would suggest.

We have not invented new pound figures for this footnote. If a line above looks precise (for example a loft-type average), treat it as a planning guide for conversations with your architect and builders — then compare real tenders on a like-for-like scope. For that step, see how to compare builder quotes and the printable comparison checklist. For structuring an indicative budget before tenders firm up, use the extension / loft budget planner. Once a builder has given you a figure, the extension cost overrun calculator works out what that figure realistically becomes by completion. On whether client-side help is worth it for your job size, read when a client-side PM makes sense in Wandsworth & Fulham.

Professional help is a separate line in the budget: what a project manager costs shows my 10–12% fee against a worked 20 m² extension.

Run your own numbers: the extension cost calculator and loft conversion cost calculator give a sourced London build range by type and specification (ex VAT), and the renovation fees calculator adds every professional and statutory fee on top.

Side-return on a terrace? Side-return extension cost in South West London (2026) covers what access, the party wall, foundations and glazing do to the price.

Local notes

I have put local planning, party wall and fee notes on the area pages, including Earl's Court, Balham, Earlsfield, East Sheen and Richmond (which also covers nearby Barnes).