How much will a loft extension cost in London in 2026?

The type of conversion is what sets the price, not the floor area. A Velux conversion keeps the roof structure and adds rooflights, so it is the cheapest but only works where headroom already exists. A rear dormer builds a flat-roofed box out from the back slope, creating full-height usable space — this is the standard Fulham terrace solution at £55,000 to £85,000 including an en-suite and the staircase.

A mansard rebuilds the rear roof at a steep angle, close to vertical, and gives substantially more floor area than a dormer. It is the most expensive at £80,000 to £110,000 and almost always requires planning permission rather than falling under permitted development. On a Fulham terrace with marginal headroom, a mansard is sometimes the only way a conversion works at all.

Costs have risen materially since 2021, mostly in labour and structural steel rather than finishes, and the increase has not reversed. Figures published before 2023 should be treated as roughly 20 to 30 percent low. Be particularly cautious of cost calculators that have not been updated — several ranking high in search results still quote pre-pandemic rates.

What is usually excluded from a headline figure: VAT at 20 percent, architect and structural engineer fees at 8 to 12 percent, party wall surveyor fees of £1,500 to £3,000 per neighbour, and the planning or lawful development certificate application. Add roughly 30 percent to a quoted build cost to reach a realistic all-in budget.

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