What drawings do I need for a loft conversion in London?

The lawful development certificate set for a permitted development loft

Permitted development does not need an application, but a buyer's solicitor or a lender may ask for proof when you sell or remortgage. That proof is a lawful development certificate. GOV.UK describes it as confirming that a proposed use of buildings or other land would be lawful under section 192 of the planning Act. The drawings have to show every permitted development limit has been met, so they carry more measurements than a normal planning set.

For a rear dormer or hip-to-gable we show the existing and proposed roof plans, the side and rear elevations, a section, and a volume calculation of the new roof space. The section proves the dormer is set back from the eaves: the guidance asks for at least 0.2 metres, measured up the roof slope, except on a hip-to-gable. The side elevation shows any window on the flank wall is obscure-glazed and does not open below 1.7 metres from the floor. Our post on what a lawful development certificate costs sets out the council fee.

The planning application set for a mansard or a conservation area loft

When the loft needs planning permission, the drawings change in purpose. They no longer prove a set of numbers. They have to persuade an officer the new roof suits the street. That means front and rear elevations of the whole house and often the neighbours, a roof plan showing how the new roof meets the next-door roofs, and a section showing the new ridge and party wall heights.

Many London councils have a roof-extension design guide, and some streets have an established pattern of mansards the officer will compare against. In a conservation area the set usually goes with a short heritage or design statement. We draw the mansard to the council's own guidance rather than to a standard profile, because the angle of the front slope and the window sizes are what officers comment on most.

For why a mansard almost always needs that application, see whether a mansard loft needs planning permission.

What the building regs drawings add

Whatever the planning route, every loft conversion that creates a habitable room needs building regulations approval. GOV.UK warns that without it you will not have the certificates of compliance you may need when you want to sell your home. The building regs set is where most of the detail in a loft goes.

It shows the new stair and its headroom, a protected escape route down through the house with fire doors where the regulations ask for them, the insulation in the roof slopes, walls and floor, the ventilation, and the new floor joists sitting on steels. Each steel is drawn with its bearing so the builder can order it. The calculations behind those steels are a structural engineer's job, and building control checks them with the drawings. Our post on what building control asks for explains how the officer reads a full plans set.

A London terraced house: the party wall drawings

In a terraced or semi-detached house, the new steels nearly always bear on the party wall you share with next door. GOV.UK lists cutting into a party wall among the work you must tell your neighbour about. The notice has to be served between 2 months and a year before the work starts.

The notice should go with drawings that show where each steel bears and whether a mansard raises the party wall. We mark those positions on the building regs plans so the same drawing can go with the party wall notice. That stops the two sets saying different things. Serving notices early matters on a terrace, because a neighbour's surveyor will want the structural detail before agreeing an award.

What it costs, and what is billed separately

Our loft conversion drawings start at £695, and the price for your loft is fixed before we start. A rear dormer is £695, an L-shaped dormer £745, a hip-to-gable £745 and a mansard £845. Structural calculations, the council's application fee and the building control charge are separate costs, and the chart below shows the council fees for each route.

If you are unsure which route your loft takes, send us the address. We check the conservation area and the roof slope facing the street before you pay for any drawings, and the loft conversion drawings page lists every loft type we draw.