Double-Storey Extension Cost Calculator
Two floors for roughly 1.5 times the single-storey cost — you reuse the same foundations and roof. It is the best value per square metre in domestic building.
Open the calculator →Price a double-storey in 30 seconds
5 steps. Covers depth, width, spec, roof type and region.
How this calculator works
The rate here is per square metre of footprint, not of finished floor area, because the second storey shares the foundations and the roof with the first. That is the whole economic argument for going up as well as out: you pay once for the expensive parts and get twice the floor.
The model reproduces the four sizes published on our cost guide — a 3m × 4m at around £29,000 and a 5m × 5m at around £53,000 — using £1,900 per m² of footprint plus a £6,000 fixed allowance for foundations, drainage and tying into the existing house. Those figures assume the ground is straightforward: a sloping plot, a high water table, nearby trees or a shared drain running under the footprint can each add thousands in groundworks that no per-metre rate can predict, which is why a site visit and trial holes matter before a quote is treated as firm. Going up rather than out carries one design cost worth flagging early: the loads from a second storey and the wider openings underneath almost always need a structural engineer to specify the steels and the foundations, and that calculation is what your building control sign-off hangs on. If you are still weighing a single storey against two, our house extension cost guide sets out single-storey costs on the same basis, so you can see what the second floor actually adds per square metre of usable space.
Double-storey costs by footprint (2026)
| Footprint | Typical 2026 cost | Notes |
|---|---|---|
| 3m × 4m | £28,000–£34,500 | Often permitted development |
| 4m × 4m | £33,000–£40,000 | |
| 4m × 5m | £39,000–£48,000 | The most common size |
| 5m × 5m | £47,000–£57,000 | Usually needs full planning |
| Pitched gable roof | +£2,500–£4,000 | Over a flat roof |
| Pitched hipped roof | +£4,000–£6,500 | Often required on a semi |
Ranges match our double-storey cost guide. Bathroom and kitchen fit-out are excluded.
Double-storey calculator FAQs
Roughly, yes. Foundations, groundworks, roof and the connection to the house are largely the same whether you build one floor or two, so the second storey only adds walls, a floor structure, glazing and internal finishes. Cost per square metre of finished space falls sharply. The catch is that the headline total is still a larger number, so a double storey only saves money if you actually need the upstairs rooms: building up for space you will not use is not a saving, however good the rate per square metre looks.
More often than for a single storey. Two-storey rear extensions have tighter permitted development limits — typically 3m depth, and they must not come within 7m of the rear boundary. Anything beyond that needs a householder application.
For a new extension you are building new foundations sized for two storeys from the start, so yes. Adding a storey on top of an EXISTING single-storey extension is a different question entirely — that needs the existing foundations exposed and assessed, and often underpinning. If that is your plan, get a structural engineer to assess the existing foundation before you fix a budget, because the difference between footings that can be built on and ones that need underpinning can swing the cost of the whole project by a five-figure sum.
Gable is cheaper and simpler. Hipped costs £1,500–£2,500 more but is frequently what planning will accept on a semi-detached house, where a gable would look out of keeping. Ask your designer which is likely before budgeting for the cheaper one.
Related cost guides
- Double-storey cost guide — full 2026 pricing by size
- Extension cost calculator — price a single-storey instead
- Two-storey planning rules — PD limits and when they apply
- What it adds to value — where the money comes back
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