Ridge height and the limit on London loft conversions

Why does ridge height decide a loft conversion?

Because Class B forbids any enlargement above the highest part of the existing roof. The ridge sets the ceiling for the whole scheme, and the dormer must also sit at least 200mm back from the original eaves. Inside a conservation area Class B does not apply at all, so both tests give way to a full application.

Source: the Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class B, as amended, read at 1 September 2026. ‘Originally built’ means the house as first constructed or as it stood on 1 July 1948, so a previous owner’s dormer counts against the volume allowance and any earlier ridge alteration counts against the height test.

Where does ridge height bite?

Class B roof extensions must not exceed the highest part of the existing roof — so a dormer or hip-to-gable can fill the roof but not raise it. A mansard almost always exceeds it, which is one reason mansards need a full application.

Outbuildings are capped at 4 m for a dual-pitched roof, 3 m otherwise, and 2.5 m total within 2 m of a boundary. Single-storey rear extensions are capped at 4 m overall and 3 m within 2 m of a boundary.

How is it measured?

From the highest part of the ground immediately adjacent to the building, not from the lowest, and not from a finished internal floor level. On a sloping London site this distinction routinely changes whether a scheme is permitted development.

What if I want to raise the ridge?

It is a full planning application, and the assessment turns on the effect on the roofscape of the terrace or pair. Where a house is one of an unbroken pair, raising one ridge is very difficult; where the roofline is already varied, it is often achievable. Party wall notices follow, because the party wall has to be raised.

What does ridge height mean for a London project?

A worked Bexley example of ridge height: with 16 conservation areas — the Bexley Village Conservation Area and the Old Bexley Conservation Area among the best known — London Borough of Bexley (Bexley Council) sees ridge height constantly, and there is no borough-wide Article 4 direction, so permitted development generally survives outside conservation areas. Where ridge height pushes a Bexley scheme out of permitted development, the route becomes a £548 application decided in 8 weeks rather than a £249 prior approval in 6 weeks.

Ridge height at a glance

Common questions about ridge height

• Mansard roof
• Dormer
• Hip-to-gable loft conversion
• Rooflight (Velux)
• Permitted development