Cost of a single-storey extension in Essex (2026 guide)
What a rear extension actually costs in 2026 — broken down by size, finish and location across Colchester, Chelmsford and the rest of Essex.
What a single-storey extension costs in Essex (2026)
A typical single-storey rear extension in Essex in 2026 costs £2,200 – £3,200 per m² for a builder's-finish project (plastered, watertight, first fix done — you supply kitchen, flooring and decoration).
That gives:
- 20m² extension: £44,000 – £64,000
- 30m² extension: £66,000 – £96,000
- 40m² wraparound: £88,000 – £128,000
What drives the price
- Groundworks — if you're on clay (most of Essex is), foundations go deeper and cost more. Trees nearby push depth further still.
- Roof type — a flat roof with rooflights is cheaper than a pitched roof with vaulted internal ceiling.
- Glazing — bi-fold doors are £4–8k; sliders £6–12k; a structural slimline corner glass detail can be £15k+.
- Finish — builder's finish vs. turnkey (kitchen, flooring, decoration included) is typically a 25–35% uplift.
Where Essex prices sit vs. London
Essex is broadly 15–25% cheaper than inner London but on a par with most of the home counties. Colchester, Chelmsford and Braintree are similar; Southend and Leigh-on-Sea trend slightly higher because of access constraints.
Planning permission
Most single-storey rear extensions fall under permitted development if they're under 4m deep on a detached house (3m on terraces/semis) and don't extend beyond half the original garden. Anything bigger needs full planning — budget 8–12 weeks for approval.
How to keep cost down without cutting corners
- Don't move services — keep the soil pipe, gas meter and electrical intake where they are.
- Standard glazing sizes — bespoke costs 30%+ more than off-the-shelf bi-folds.
- Simple roof — a flat warm-roof with 2–3 large rooflights gives you a vaulted feel for far less than a vaulted pitched roof.
- Get fixed-price quotes — never day-rate. A day-rate quote almost always overruns.
What to ask a builder
- Are you VAT-registered? (Most reputable extension builders are.)
- Can I see two finished extensions of similar scope?
- Who's on site every day — the owner, a foreman, or subbies?
- What's the payment schedule? (Stage payments only — never large up-front deposits.)
- What's covered by the workmanship guarantee, and for how long?
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