Compare Extension Builders: How Many Quotes in 2026?
In 2026 the sweet spot is 3–5 quotes. Two leaves you blind to the median price — if one quote is 30% low you can’t tell whether it’s a cowboy or a fair deal. Six-plus signals you’re a tyre-kicker, reputable builders won’t bid, and you’re left with the bottom of the market. Below: how to recognise the 30%-low cowboy without leaving money on the table on the high side.
How many extension quotes is the right number?
Across 1,200+ extension projects matched on BestBuilders in 2025, here’s how outcome quality correlated with quote count:
- 1 quote: 38% of projects went over budget by >15%. You have no idea whether the price is fair.
- 2 quotes: 21% over budget. Better, but still vulnerable: if one is 30% low you can’t tell whether it’s a fair price or a cowboy.
- 3 quotes (sweet spot): 9% over budget. The median is now visible — outliers either way are obvious.
- 4–5 quotes: 8% over budget. Marginal improvement on 3, but better triangulation on complex jobs.
- 6+ quotes: 22% over budget. Reputable builders drop out when they see they’re one of many. You end up choosing from the bottom of the market.
The 7 red flags in a 30%-low quote
One quote noticeably below the others isn’t automatically a cowboy — it might just be a builder with spare capacity. But these 7 red flags appear in 90% of the disaster jobs we’ve seen:
- 1. No itemised breakdown — just “extension build: £X”. A fair quote breaks down groundworks, brickwork, roof, fenestration, M&E, plastering, kitchen, finishes.
- 2. No payment schedule — or asks for >20% upfront. Standard 2026 schedule: 10% deposit, stage payments tied to milestones (DPC, roof on, plastered, snagged).
- 3. No VAT line — means they’re sub-VAT-threshold (turnover <£90k) or operating cash. Either way, your statutory rights and warranty cover are weaker.
- 4. No insurance details — ask for the public liability certificate (£5m minimum) and the contract works policy. Won’t share = won’t cover damage to your house.
- 5. No structural engineer specified — on any extension touching a wall, you NEED a SE for the steel beam calcs and building control sign-off.
- 6. No formal contract — ask for a JCT Minor Works or FMB Domestic Building Contract. “Handshake” or “invoice-only” jobs leave you with no enforcement when things slip.
- 7. No references from the last 12 months — ask to phone two previous clients from this year. A new client list is fine; refusing to share one is not.
2026 UK extension cost ranges
Use these as the median bracket to anchor your quotes against:
- Single-storey rear extension (20 m²): £40,000–£70,000
- Single-storey wraparound (30 m²): £65,000–£110,000
- Double-storey rear (40 m²): £75,000–£135,000
- Side-return infill (Victorian terrace): £38,000–£65,000
- Loft conversion dormer: £45,000–£80,000
London adds 20–30%. South-East adds 10–15%. North England / Scotland tracks the bottom of the range.
How to make quotes genuinely comparable
1. Share a single brief with all 3 builders
Same drawings (or same sketch + dimensions), same finish spec, same kitchen budget allowance, same flooring spec. Otherwise you’re comparing apples and oranges.
2. Specify the structural engineer separately
The SE works for you, not the builder. Get the calcs and Building Control submission costed independently — about £800–£1,800 for a typical extension.
3. Pin the kitchen budget
Most extension quotes include a “kitchen allowance” figure. Make all three builders use the same allowance (say, £12,000) so the build-only difference is visible.
4. Ask the same 7 red-flag questions of each
If a builder bristles at the questions, that’s your answer.
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