Do I need planning permission for a loft dormer?

Most rear dormer loft conversions are permitted development, provided the extra roof space stays within 40 cubic metres for a terraced house or 50 cubic metres for a semi-detached or detached house, and the dormer isn't on a front roof slope facing the road.

The short answer

• Up to 40 cubic metres of extra roof space for a terrace, 50 for a semi or detached house.
• Not on the front roof slope where it faces a road.
• No higher than the existing ridge, and no balconies.
• Not permitted development in conservation areas, National Landscapes, national parks or World Heritage Sites.
• Always needs building regulations approval, whether or not you need planning permission.

The limits

To be permitted development, a dormer or other roof extension must meet every one of these:

• For a terraced house, roof extensions can add no more than 40 cubic metres to the original roof space. Schedule 2, Part 1, Class B, Class B.1(d)(i)
• For a detached or semi-detached house, roof extensions can add no more than 50 cubic metres to the original roof space. Schedule 2, Part 1, Class B, Class B.1(d)(ii)
• A dormer can't go on the front roof slope where it faces a road. Schedule 2, Part 1, Class B, Class B.1(c)
• No part of a roof extension can be higher than the highest part of the existing roof. Schedule 2, Part 1, Class B, Class B.1(b)
• Balconies, verandahs and raised platforms aren't permitted development. Schedule 2, Part 1, Class B, Class B.1(e)(i)
• Materials must look similar to the existing house.
• The dormer should be set back at least 20cm from the eaves (measured along the roof), so far as practicable. Hip-to-gable conversions are exempt.
• Nothing can overhang the outside face of the original walls.
• Side-facing windows must be obscure-glazed and fixed shut below 1.7m above the floor.
• Rooflights alone, without a dormer, are a separate and simpler right (Class C): they can stick out no more than 15cm from the roof.

Working out the volume

The limit is on how much the roof extension adds to the original roof space, measured externally. Ask your designer or builder to calculate it from the drawings and to show the calculation, because the council will check it if you apply for a Lawful Development Certificate. A typical full-width rear dormer on a terrace adds 20 to 40 cubic metres.

Everything counts towards the total, including a hip-to-gable conversion (where a sloping side roof is built up into a vertical gable) and any roof extension a previous owner added. If your house already has a small dormer, the new one only gets what's left of the allowance.

Front and rear roof slopes

A dormer can't extend the roof slope on the front of the house where it faces a road. Rear dormers, and dormers on a side slope that doesn't face a road, are fine. Rooflights (windows set into the roof slope, sticking out no more than 15cm) are a separate right and can go on the front, unless an Article 4 direction removes it.

Balconies and Juliet balconies

A balcony or roof terrace isn't permitted development. A Juliet balcony, a railing across a full-height window or door with no platform to stand on, isn't a balcony and is generally fine.

Semis and terraces: the party wall

A loft conversion in a semi-detached or terraced house usually involves work on the shared wall, such as cutting in steel beams. That needs a party wall notice to your neighbour under the Party Wall etc. Act 1996, separately from planning and building regulations.

Where the rules are stricter

• Loft dormers in a conservation area
• Loft dormers in a National Landscape or national park
• Listed buildings
• Article 4 directions
• Flats and maisonettes

Planning permission and building regulations

Planning permission and building regulations are separate. Planning is about whether you can build it and how it looks; building regulations are about whether it is built safely (structure, fire safety, insulation, drainage). Permitted development only covers planning, so you may still need building regulations approval.

Converting a loft into a room always needs building regulations approval, whether or not you need planning permission. It covers the floor structure, fire safety and escape (often a protected staircase), stairs, insulation and ventilation. Work on a shared wall in a semi or terrace usually needs a party wall agreement.

Getting certainty

If you're confident your project is permitted development, you can apply to your council for a Lawful Development Certificate. It's a formal confirmation that the work is lawful without planning permission, it costs less than a planning application, and it's useful when you come to sell the house. If you're not sure, many councils offer pre-application advice.