Flat-roof dormer Cost Calculator
A UK flat roof dormer in 2026 typically costs £18,000–£28,000 for a single rear dormer (5–7m²) and £30,000–£42,000 for a full-width or twin-front dormer (10–14m²) — roughly £2,200–£3,000 per m² of habitable space added. Price is driven by size and number of dormers, glazing (uPVC vs aluminium,…
Open the calculator →Price flat-roof dormer in 30 seconds
2 steps, using the figures from our cost guide.
How this calculator works
Every figure in this calculator is the one published in the cost table on our flat-roof dormer guide. Pick the option that matches your job and the calculator returns that published range, adjusted for your region - it does not model anything the guide does not already say.
That is deliberate. A calculator whose rates drift from the page it sits under is worse than no calculator, because two numbers on the same site disagree.
What it cannot know: access, the condition of what is already there, and anything structural. Those are what separate three real quotes from each other.
Published flat-roof dormer costs (2026)
| Option | Typical 2026 cost | Notes |
|---|---|---|
| London (inner) | £32,000–£42,000 | |
| London (outer) | £28,000–£36,000 | |
| South East | £24,000–£31,000 | |
| South West | £22,000–£28,000 | |
| Midlands | £19,500–£25,000 | |
| North West | £18,500–£24,000 | |
| Yorkshire | £18,000–£23,500 | |
| North East | £17,000–£22,500 |
These are the figures from our flat-roof dormer cost guide.
Flat-roof dormer calculator — FAQs
It uses the same published figures as the cost guide it sits under, adjusted for your region. It is a budgeting estimate, not a quote - only a site visit produces one of those.
Related cost guides
- Flat-roof dormer cost guide — The full price guide
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