How much do loft conversion drawings cost in London?

The price by type of loft

The roof you choose decides the drawings. A conversion that only adds rooflights is the smallest job: the roof stays as it is, so the drawings show new windows, a staircase and a floor. A rear dormer changes the roof shape and needs a volume calculation. A hip-to-gable swaps a sloping side for a vertical wall and adds floor area at the same time. A mansard rebuilds the whole slope and is often a planning application rather than permitted development.

Which type fits is a practical question as much as a legal one. A Victorian terrace with a narrow back addition suits a dormer set in from the party walls. A 1930s semi with a hipped roof is the classic hip-to-gable candidate. A house with little headroom at the ridge may only manage rooflights. We look at all of this on the measured visit.

What the planning drawings have to prove

If you rely on permitted development, the drawings are the evidence. Class B of the General Permitted Development Order allows a roof extension to add up to 40 cubic metres to a terraced house and 50 cubic metres to any other house, and the council checks each limit against your numbers. We therefore draw a roof plan, sections through the ridge and eaves, and a volume schedule.

• No part of the extension may rise above the highest part of the existing roof.
• Nothing may extend beyond the roof slope on the front elevation if it faces a road.
• The edge of the extension must sit at least 0.2 metres back from the eaves, measured along the slope, unless it is a hip-to-gable.
• Side windows must be obscure-glazed and fixed shut, unless the opening part is more than 1.7 metres above the floor.
• No balconies, verandas or raised platforms, so a Juliet balcony needs its own look.

A rooflight scheme sits under a different rule, Class C, which says a rooflight may not protrude more than 0.15 metres beyond the plane of the original slope. That is why the rooflight option is the smallest set to draw.

When the cost goes up

Photo: sludgegulper, BY-SA 2.0

The starting price assumes a house with permitted development rights and a roof that has not been altered. Three situations add work.

The first is a flat or maisonette. Householder rights do not apply to flats, so a loft conversion above one needs a full planning application, and often the freeholder's consent as well. The second is a conservation area or an Article 4 direction. Conservation areas are excluded from Class B altogether, which turns a certificate into a planning application. The third is an earlier roof alteration. Volume is counted from the original roof, so a previous dormer or hip-to-gable uses up part of your allowance and has to be measured and drawn.

We check your address for all three before we quote. If your loft cannot be done under permitted development, we say so and price the planning application instead.

The other drawings a loft conversion needs

Planning is half the story. A loft turns a storage space into habitable rooms, so building control wants to see the stair, the escape route, the insulation and the new floor. Our building regulations drawings are £825 inc survey. Structural calculations for the new floor and the steel at the ridge are £595.

On a full-plans building regulations application the council must decide within 5 weeks, or 2 months if you agree to it. That decision is separate from planning and neither depends on the other, so the two applications can run side by side.