Blog · Costs and planning ·
How Much Does an Extension Cost in Cambridgeshire in 2026?
"How much will it cost?" is the first question everyone asks, and most of the answers online come from national averages. The figures below come from something different: our own detailed estimates for extensions and loft conversions around Cambridge, brought up to 2026 prices. They're the same figures our free cost calculator uses.
Single-storey extensions
These are typical price ranges for a single-storey extension with a flat roof and roof lantern, opening into the house with a steel beam. Prices include VAT and cover the full build: foundations, walls, roof, windows and doors, steelwork, electrics, heating, plastering and decorating.
| Floor area | Basic finish | Mid-range finish | High-end finish |
|---|---|---|---|
| 20 m² | £92,000–£107,000 | £101,000–£117,000 | £119,000–£138,000 |
| 30 m² | £126,000–£146,000 | £138,000–£160,000 | £163,000–£189,000 |
| 40 m² | £161,500–£186,500 | £177,000–£205,000 | £209,000–£242,000 |
A pitched, tiled roof usually comes in slightly cheaper than a flat roof with a lantern: about £132,000–£152,500 for a 30 m² mid-range extension.
Loft conversions
| Floor area | Rear dormer | Hip-to-gable with rear dormer |
|---|---|---|
| 30 m² | £79,000–£92,000 | £98,500–£113,500 |
| 40 m² | £95,500–£110,500 | £114,000–£132,000 |
These are for a mid-range finish and include the staircase, structural steel, insulation, electrics, heating and decorating.
Why smaller extensions cost more per square metre
Some costs are much the same whatever the size: setting up the site, scaffolding, steel beams to open up the house, and the work where the new build joins the old. On a small extension those costs are shared over fewer square metres, so the price per m² is higher. That's why price-per-m² figures you see online can be misleading.
What pushes the price up or down
- Opening up the house. Removing a wall needs steel beams, temporary propping and making good. It's often worth it, but it's a real cost.
- The roof. Flat roofs with roof lanterns and pitched roofs with roof windows cost different amounts, and tying a new roof into an existing one adds work.
- Glazing. Large aluminium bi-fold or sliding doors cost far more than a pair of French doors.
- The ground. Clay soil and nearby trees, common around Cambridge, can mean deeper foundations.
- Drainage. Building over drains or adding a soakaway adds cost.
- The finish. Brick choice, joinery, flooring and lighting all add up.
What isn't included
The figures above are for the build. On top, budget for your architect and structural engineer, planning and building control fees, any party wall costs, and the fit-out: kitchen, bathroom, flooring and furniture.
Getting a figure for your project
- In two minutes, for free: our cost calculator gives a guide price range from the size and type of your project.
- From your drawings: if you have plans, our estimate from your drawings gives a trade-by-trade breakdown, instant or checked by Paul.
- A fixed quote: get in touch and Paul will visit, talk it through and price it properly.
Every project is different, so treat these figures as a guide rather than a quote.