Loft conversion cost calculator

What moves the number?

• Loft type. A rooflight-only conversion needs no structural roof alteration; a rear dormer adds a new floor and a weathered box; a hip-to-gable adds a gable wall; a mansard removes and rebuilds the roof entirely.
• Staircase position, because taking a new flight off a small landing often means reconfiguring the floor below.
• Whether existing trussed rafters must be replaced with a steel and purlin frame.
• Bathroom count, and whether the soil stack can be reached without rerouting.
• Roof covering, since a conservation condition requiring natural slate costs more than the alternatives.

What sits outside the build cost?

Three costs sit outside the builder’s figure. The statutory fee is set nationally: £548 for a householder planning application, £274 for a lawful development certificate, £249 for larger home extension prior approval. Building control is set locally and runs roughly £500 to £1,200 across London; on a loft it earns that money, because the new floor structure, the protected stair and the escape window are all inspected items rather than paperwork. Professional fees are the third set: drawings, a structural engineer for the beams and the purlin frame, and a party wall surveyor for every neighbour whose wall the new steel bears on — which on a mid-terrace means two of them, not one.

One route is worth ruling out early rather than late. Class AA of the permitted development order allows an additional storey on some houses, and in practice it almost never lands: London councils determined just 280 upward-extension prior approvals in the twelve quarters to March 2026 and refused 160 of them, granting 120 (MHCLG planning application statistics, PS1, 33 boroughs pooled, retrieved 1 September 2026). Across the whole capital, over three years, that is a handful a month. If you want a genuine extra storey, a mansard through a full application is the realistic route and should be budgeted as one.

How should you use the figure?

As an envelope to design inside rather than a target to reach afterwards. On a loft the sequence matters more than on most jobs, because the governing dimension is discovered rather than chosen. Measure from the underside of the ridge to the top of the ceiling joists before you settle on a type: the new floor build-up and the insulation each take a slice out of that, and somewhere around 2.4 metres is where a rooflight conversion stops being viable and a dormer becomes necessary. Fix the range, choose the conversion the roof will actually carry, then put one defined drawing package in front of two or three contractors so the quotes are comparable.

Leave room in the programme for the paperwork even where no permission is needed. London boroughs recorded 36,657 certificate of lawful development applications in the twelve quarters to March 2026 (MHCLG planning application statistics, PS1, retrieved 1 September 2026). The certificate is how a permitted development loft gets evidenced for a future buyer, it costs £274, and it takes eight weeks of its own that run before the scaffold rather than alongside it.

The build ranges above are our own reading of London loft tenders in 2026, not a published statistic and not a quotation. Treat them as wider than they look: on a loft the largest single unknown is what the existing roof structure turns out to be, and nobody knows that until the plasterboard is off. Statutory application fees are the England rates in force in 2026 under the Town and Country Planning (Fees for Applications) (England) Regulations 2012 as amended. Building control charges are set by each local authority or by a registered approver, and differ across London.