How much do loft conversion drawings cost in London?

What the fee covers

From £845 inc survey you get the measured survey of the house and loft, and drawings for the roof alteration: existing and proposed plans, a section through the new room and stair, and elevations of the front, back and sides. A Velux scheme and a rear dormer sit at the same starting price; a larger dormer or an L-shaped one takes more drawing time, and the fixed fee we send reflects that.

Hip-to-gable and mansard conversions are different projects. Our mansard design cost is set out in a separate guide.

Planning permission or permitted development?

Permitted development under Class B lets you add roof volume without applying, within limits: 40 cubic metres for a terrace house and 50 for any other house, nothing higher than the existing ridge, and no enlargement on a roof slope that fronts the highway. The new work must also stay set back from the eaves.

Class B does not apply on article 2(3) land, so a house in a conservation area needs full planning permission for a dormer. Check the street before you design, because the same dormer can be lawful on one road and need an application on the next.

Where permission is needed, the council's fee for a householder scheme is £548 from 1 April 2026.

Velux or dormer: how the choice affects the drawings

A rooflight conversion leaves the roof shape alone, and under Class C the glazing may not protrude more than 0.15 metres beyond the plane of the roof. The drawings concentrate on headroom, because a loft with a shallow pitch can lose usable floor to the slope.

A dormer adds headroom and volume, so the drawings carry more: the box, its cladding, side windows that are obscure-glazed and fixed shut below 1.7 metres, and the volume calculation that shows you are inside the limit.

What building control asks of the new room

Whatever the planning route, the conversion needs building regulations approval. The stair has to be safe: a headroom of 2.0 metres, or 1.9 at the centre and 1.8 at the side where the roof prevents it. The room needs an escape window of at least 0.33 square metres, with the sill no more than 1.1 metres above the floor.

The roof structure must be designed, and new roof elements must reach a U-value of 0.15 W/m²K. We draw this in our building regulations drawings, and structural calculations for the steel start at £435.

Get a fixed fee for your loft

Send the address and a photo of the roof, and we check the street for conservation area or Article 4 limits first, free. More about what we draw is on our loft conversion drawings page.

The figures, with sources

1. 40 m³ / 50 m³

   Additional roof volume allowed as permitted development on a terrace house and on any other house.

   Source: General Permitted Development Order 2015, Schedule 2, Part 1, Class B, checked 15 Oct 2026

2. Art. 2(3)

   Class B roof enlargements are not permitted development in conservation areas and other article 2(3) land.

   Source: General Permitted Development Order 2015, Schedule 2, Part 1, Class B.1(f), checked 15 Oct 2026

3. 0.15 m

   Greatest distance a roof window may project beyond the plane of the roof slope under Class C.

   Source: General Permitted Development Order 2015, Schedule 2, Part 1, Class C, checked 15 Oct 2026

4. £548

   Cost of a householder planning application to alter one house, from 1 April 2026.

   Source: Ribble Valley Borough Council, national fee table from 1 April 2026, checked 17 Oct 2026

5. 1.9 m

   Stair headroom at the centre line accepted where 2.0 m cannot be achieved in a loft conversion.

   Source: Approved Document K, paragraph 1.13, checked 15 Oct 2026

6. 0.33 m²

   Smallest unobstructed opening for an escape window, at least 450 mm high and 450 mm wide.

   Source: Approved Document B Volume 1, paragraph 2.10, checked 15 Oct 2026

7. 0.15 W/m²K

   Limiting U-value for a new roof in an existing dwelling.

   Source: Approved Document L Volume 1, Table 4.2, checked 15 Oct 2026

Questions