Loft Conversion Cost Calculator
Get an instant build-cost estimate for your London loft conversion. Choose your loft type, floor area, spec level and borough — and get a realistic cost range with a full breakdown in under two minutes.
What does a loft conversion cost in London in 2026?
Loft conversions are one of the most cost-effective ways to add space and value to a London home — typically adding 15–25% to the property value for a project costing considerably less than a ground-floor extension of the same area. But costs vary widely depending on the type of conversion, the condition of the existing roof structure, and the spec of finish required.
The figures in this calculator reflect our experience pricing and delivering loft conversions across London in 2025–2026. They are build-contract costs only — the amount paid to your main contractor. On top of this, budget for:
- Professional fees: Architect and structural engineer — typically 10–15% of the build cost. A party wall surveyor if the conversion affects a shared wall (£800–£2,000).
- Planning fees: Most loft conversions are permitted development, but mansards and conservation area properties usually need a full planning application (currently £258).
- Building regulations: All loft conversions require building regs approval — typically included within the architect's fee or charged separately (£500–£1,500).
- VAT (20%): Standard on building work. Some projects may qualify for reduced rates.
- Contingency: Allow 10% of the build cost — older properties with unknown roof conditions can spring surprises.
Which loft conversion type is right for me?
| Type | Best for | Planning needed? | Typical total project cost |
|---|---|---|---|
| Velux / roof light | High-ridge roofs with good existing headroom | Usually no (PD) | £28,000–£50,000 |
| Dormer | Most London house types; great space gain | Usually no (PD) | £50,000–£90,000 |
| Hip-to-gable | Semi-detached and end-of-terrace houses | Usually no (PD) | £55,000–£95,000 |
| L-shaped dormer | Victorian terraces with rear outrigger | Usually no (PD) | £65,000–£110,000 |
| Mansard | Maximum space; prime London areas | Almost always yes | £85,000–£150,000+ |
Figures above include professional fees and VAT at 20%. They are indicative ranges — use the calculator above for a personalised estimate based on your borough and spec level.
Does my loft have enough height for a conversion?
The minimum usable headroom for a loft conversion is generally accepted as 2.2 metres from floor joist to ridge. You can measure this yourself by going into the loft with a tape measure. If your existing headroom is between 2.2m and 2.5m, a Velux or simple dormer conversion should be possible. Below 2.2m, a dormer or mansard is needed to create sufficient space — but that means higher structural cost.
Most London Victorian and Edwardian terraces have ridge heights of 2.4–2.8m, making them well suited to dormer or L-shaped dormer conversions. Bungalows and 1960s–70s houses often have lower pitches and require more extensive structural work.
Ready for a proper quote?
Our estimate gives you a realistic budget range. For a detailed, itemised quote based on your actual plans — contact us today. We'll visit the site, carry out a roof survey and give you a transparent, fixed-price tender.
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By My Local London Builder Team · Last updated May 2026 · All figures ex-VAT unless stated · Guidance only — obtain formal quotes before committing.