Roughly a rear dormer 5 m wide, 2.5 m high and 3 m deep, measured as the extra volume added to the roof. Your architect or loft company will calculate it from drawings.

Can I do a loft conversion in a conservation area?

Not with a dormer — Class B doesn't apply in conservation areas, National Parks or National Landscapes. Rooflights (Class C) are still permitted, with limits.

Do I need a party wall agreement?

Usually, on a semi or terrace. That's a separate legal requirement from planning — you must notify your neighbours under the Party Wall Act before starting work on a shared wall.

Loft conversion without planning permission

The limits

House type	Extra roof space	Front-facing dormer
Detached	50 m³	Not permitted
Semi-detached	50 m³	Not permitted
Terraced	40 m³	Not permitted
Flat or maisonette	Not permitted	Not permitted

Other conditions

No higher than the highest part of the existing roof.
No dormer on the roof slope that faces a road at the front.
Set back at least 20 cm from the eaves, as far as practicable.
Materials similar in appearance to the existing house.
Side-facing windows obscure-glazed and non-opening below 1.7 m.
No verandas, balconies or raised platforms.
Previous roof enlargements count towards the volume.

When you can’t use permitted development

Flats & maisonettes
No householder permitted development rights at all.
Listed buildings
Listed building consent is needed for almost any change.
Conservation areas
Not permitted development — needs planning permission.
Article 4 directions
Your council may have removed this right on your street.
Planning conditions
An old permission may have removed permitted development rights. Not published anywhere.
New builds
Often have permitted development removed by condition. Check your deeds.

Your exact limits

House type, plot size, Article 4 and trees — worked out for your address.

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Rooflights only: even simpler

A conversion that only adds rooflights (Velux-style windows) doesn't change the roof shape, so it falls under Class C, not Class B. Rooflights can project up to 15 cm from the roof slope, and side-facing ones must be obscure-glazed and 1.7 m above the floor. This is allowed in conservation areas too.

Hip-to-gable conversions

Turning a hipped (sloping) end into a vertical gable wall is a roof enlargement under Class B, so it counts towards the 50 m³. A hip-to-gable plus a rear dormer often comes close to the limit — get the volume calculated before you commit.

Loft conversion rules by council

Some councils have Article 4 directions that change these limits. Pick yours to see what applies locally.

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Common questions

How big is 40 m³?: Roughly a rear dormer 5 m wide, 2.5 m high and 3 m deep, measured as the extra volume added to the roof. Your architect or loft company will calculate it from drawings.
Can I do a loft conversion in a conservation area?: Not with a dormer — Class B doesn't apply in conservation areas, National Parks or National Landscapes. Rooflights (Class C) are still permitted, with limits.
Do I need a party wall agreement?: Usually, on a semi or terrace. That's a separate legal requirement from planning — you must notify your neighbours under the Party Wall Act before starting work on a shared wall.