How much does a mansard loft conversion drawing cost in London?

Why the mansard fee is above a standard loft

A dormer sits on the existing roof. A mansard replaces it, with a steep lower slope, a flatter upper slope and small windows set into the steep face. Every part of that is a drawn detail: the profile, the cladding, the junction with the neighbour’s roof and the parapet.

Our fee is £615 drawings + £150 survey (£765 total), with the survey in the total. The roof has to be measured accurately, because the drawings fix the height of the ridge, the width of the party walls and the depth of the new floor.

Why a mansard needs planning permission

In much of London, an application is the only route.

Under Class B a roof addition may not extend beyond the plane of the roof slope that faces the highway, and may not exceed the highest part of the existing roof. It is also limited to 40 cubic metres on a terraced house. A full mansard breaks the first rule and usually the volume limit.

There is a second barrier in much of London. The Class B right does not apply on article 2(3) land, which includes conservation areas. So in most places where mansards are traditional, an application is the only route, and the fee is £548 for one house.

What a council looks for in the drawing

Councils publish their own tests for mansards. One example is Tower Hamlets’ draft mansard roof guidance note of November 2015, a consultation draft rather than adopted policy. It describes a steep lower pitch of about 70 degrees under an upper pitch of 30 to 36 degrees, and says a flat-top mansard may be considered only exceptionally, where neighbours already have one.

The same draft prefers keeping the original V-shaped back wall where the house had a butterfly roof, so that the older roof form still shows. Always check the current guidance of your own borough; ours is a style example, not a rule for every street.

We draw the profile at the borough’s own angles, set the slopes against your neighbours’ and show the parapet on the elevation, so the officer can compare it with the policy page by page.

Fire route, stair headroom and the party wall

On a typical two-storey house a mansard adds a storey above 4.5 metres, so Approved Document B asks for fire-resisting doors and partitions on the escape route, or the sprinkler alternative it describes in paragraph 2.23. The stair needs 2 metres of headroom, reducing to 1.9 metres at the centre and 1.8 metres at the edge in a loft conversion.

A mansard usually means raising or cutting into the party wall, so the Party Wall Act notice has to be served at least two months before work starts. Islington, for one, has removed permitted development rights in 40 of its 42 conservation areas, so check the street before you pick the date.