“How much will an extension cost?” is the first thing almost every homeowner asks, and the honest answer is that it depends on more variables than a single number can carry. But “it depends” is useless when you’re trying to work out whether a project is even viable, so here are the real 2026 figures — what they include, what they don’t, and where Essex sits against the national picture.
The Short Answer
Across the UK, a single-storey extension typically costs £1,800 to £3,000 per square metre for a finished, ready-to-use space. In London and the South East that rises to roughly £2,300 to £3,400 per square metre, because labour runs up to 20% above the national average.
Essex sits across both. Towards the London side and the commuter belt, expect the upper half of that range. Further into north Essex and towards the Suffolk border, figures tend to land closer to the national average.
If you only want the shell — foundations, walls, roof and windows with no internal finishes — you’re looking at £1,200 to £1,700 per square metre. That figure gets quoted a lot, and it’s one of the main reasons quotes appear wildly different: they aren’t always describing the same job.
What That Means for a Real Extension
Per-square-metre rates are hard to picture, so here is roughly what different sizes work out at for a completed single-storey extension.
| Extension size | UK average | Essex & South East |
|---|---|---|
| Small — 3m × 4m (12m²) | £21,600 – £36,000 | £27,600 – £40,800 |
| Medium — 4m × 5m (20m²) | £36,000 – £60,000 | £46,000 – £68,000 |
| Large — 5m × 6m (30m²) | £54,000 – £90,000 | £69,000 – £102,000 |
| Very large — 5m × 8m (40m²) | £72,000 – £120,000 | £92,000 – £136,000 |
These are typical market ranges for 2026, not a quote. Every property is different — ground conditions, access and the spec you choose can move the final figure significantly in either direction.
A Worked Example: Single-Storey Rear Extension Cost Breakdown
Most enquiries this guide covers are for a single-storey rear extension, so it’s worth breaking one down rather than leaving it as a single headline figure. Take a mid-sized 20m² single-storey rear extension — roughly £46,000–£68,000 in Essex, as above. Here’s typically how that splits:
- Groundwork and structure (roughly 35–40%) — foundations, drainage, brickwork, steels and the roof.
- Glazing (roughly 10–20%) — standard windows sit at the low end; bi-folds, sliding doors or a roof lantern push this toward the top.
- First and second fix electrics and plumbing (roughly 10–15%) — wiring, pipework, sockets, switches and any radiators or underfloor heating.
- Plastering, flooring and decoration (roughly 10–15%) — the finishing trades that turn a shell into a usable room.
- Professional fees and contingency (roughly 10–15%) — architect or designer, structural engineer, building control, and the contingency covered above.
Two projects at the same square footage can still land at different ends of that range depending on glazing choice alone — a wall of bi-fold doors costs meaningfully more than two standard windows and a single door, even though the groundwork underneath is identical.
Why Small Extensions Cost More Per Square Metre
People are often surprised that a 12m² extension doesn’t cost half of a 24m² one. The reason is that a large share of the cost doesn’t scale with floor area.
Scaffolding, site setup, skips, foundations, drainage connections, structural steels and the professional fees behind them are broadly the same whether the room is small or generous. Spread those fixed costs over fewer square metres and the rate per m² climbs. It’s why, if you’re on the fence about size, going slightly bigger is often better value per square metre than the headline rate suggests.
Single-Storey vs Double-Storey
A common assumption is that adding a second storey doubles the price. It usually doesn’t. The expensive groundwork — foundations, drainage, the roof structure — is already being paid for. Adding a floor above typically adds somewhere in the region of 50% to 60% rather than 100%.
That makes double-storey extensions one of the better value-per-square-metre options, provided the existing foundations can take the load and the design gets planning approval. Both are worth establishing early rather than late.
What’s Usually Included — and What Isn’t
This is where quotes diverge most, and where budgets get caught out. A typical full build quote covers:
- Groundwork, foundations and drainage
- Structure — brickwork, steels, roof
- Windows, external doors and glazing
- First and second fix electrics and plumbing
- Plastering, flooring and decoration
- Building control sign-off
What often sits outside the headline figure:
- Architect and structural engineer fees — commonly 5–15% of the build cost
- Planning application fees, where permission is required
- Party wall agreements if you’re building near a shared boundary
- Kitchen or bathroom fit-out where the extension houses one — frequently the largest omission
- Landscaping and making good to the garden afterwards
- VAT, which isn’t always shown in headline figures
When two quotes are thousands apart, the gap is usually here rather than in the building work itself.
What Pushes the Price Up
- Difficult access. If materials can’t reach the rear of the property easily, labour time rises.
- Ground conditions. Clay soil, high water tables, tree roots or nearby drains can all mean deeper or reinforced foundations.
- Structural openings. Large steels to open up to the existing house add both material and engineering cost.
- Glazing. Bi-folds, sliding doors and roof lanterns are among the fastest ways to move a budget.
- Spec. The difference between standard and premium finishes across a whole extension is substantial.
What Brings It Down
- Staying within permitted development, where possible, avoids planning fees and delay.
- A simple roofline. Complex roof geometry costs more than it looks like it should.
- Keeping the plumbing where it is. Moving soil pipes and drainage is more expensive than most people expect.
- Deciding the spec before work starts. Mid-build changes are the most expensive kind.
How to Compare Quotes Properly
The cheapest quote is not automatically the best value, and the most expensive isn’t automatically the most thorough. What matters is whether they describe the same scope.
Ask each builder the same questions: is this shell or turnkey? Are professional fees included? Is VAT included? What’s the payment schedule? What happens if the ground conditions differ from the assumption? A builder who answers those clearly and in writing is usually the one whose final invoice matches their quote.
Frequently Asked Questions
How long does an extension take?
A straightforward single-storey extension typically takes 10–16 weeks on site once work begins. Design, planning and building regulations approval beforehand can add two to four months depending on whether permission is needed.
Will an extension add value to my home?
Usually, though rarely pound for pound. A well-executed extension in Essex commonly adds value, but the bigger gain for most homeowners is getting a house that actually works for them rather than moving. Over-extending relative to the street can produce diminishing returns.
Do I need planning permission?
Many single-storey rear extensions fall under permitted development, but the limits on depth, height and proportion of garden covered are specific — and different for terraced, semi-detached and detached homes, or if you’re in a conservation area.
Should I budget a contingency?
Yes. Ten per cent is a sensible minimum on an extension, and 15% is wiser on older properties where what’s behind the walls is less predictable.
Getting a Figure for Your Project
Every number here is a market range. The only figure that means anything for your home is one based on your property, your ground conditions and the spec you actually want — which is what a proper site visit and written quote are for.
Planning a project? See our house extensions in Essex & Suffolk service, our guides to single storey extensions and planning permission, or get a free quote.
