Class B roofs: dormers, loft conversions and hip-to-gable rules

Class B can grant planning permission for enlargements to the roofs of qualifying houses in England. Common examples are a dormer and a hip-to-gable enlargement. Internal loft work is a separate question from changing the external roof.

Start with the original roof, count earlier enlargements, and check every restriction. A roof volume below 40 m³ or 50 m³ is only one of the tests.

How much roof volume can you add?

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These are increases above the original roof space, measured externally. Original generally means the house as it stood on 1st July 1948, or as first built if later. Existing means immediately before the proposed work. They are not interchangeable.

Count every previous enlargement of the original roof space, whether it used Class B or a planning application. Count all parts of the proposed work together, including a hip-to-gable enlargement and a dormer. Buying the house, changing the design or applying in stages does not reset the allowance.

For example, a terrace with 15 m³ of previous added roof space has 25 m³ left within its 40 m³ allowance. This calculation does not establish compliance with the other rules.

The required figure is the difference between the resulting external roof volume and the original external roof volume. For a replacement scheme, calculate the resulting roof space accurately rather than simply adding the figures on successive applications. It is not the room’s floor area or its usable internal volume. Dormers and hipped roofs have different shapes: have the drawings show the calculation rather than applying one simple formula to every design.

Where can the enlargement go?

• No part of the house may exceed the highest part of the existing roof as a result of the work
• The work must not extend beyond an existing roof slope which both forms the principal elevation and fronts a highway
• The external-wall and eaves requirements below must also be met

The highway-facing principal-roof restriction contains both tests. Do not replace it with a blanket statement that every front dormer is prohibited, or that every side dormer is allowed. Identify the principal elevation, its roof slopes and any highway first. A highway can include a private way or unadopted street.

The 200 mm eaves setback

For an ordinary dormer, the original eaves must be maintained or reinstated. So far as practicable, the edge of the enlargement closest to those eaves must be at least 0.2 m (200 mm) away, measured along the roof slope from the outside edge of the eaves. Do not measure horizontally, vertically or from the gutter.

Hip-to-gable and outrigger roof exceptions

There are two different exceptions in B.2(b). They must not be rolled into one general exemption.

For the external-wall rule, minor overhangs such as tiles, gutters, fascias and barge boards are disregarded. The qualifying rear or side extension can be an original projection, such as an outrigger, or a later extension from the principal part of the original house.

For the rear-wall junction, different roof levels and the distinction between Classes A and B, see our outrigger dormer guide with drawings and appeal decisions.

An L-shaped roof or an outrigger dormer is not automatically permitted. Establish whether the proposal actually joins the original roof to the roof of the relevant extension. The volume, height, principal-roof, materials, window and eligibility requirements remain. These roof exceptions do not give permission for a ground-floor wraparound: that proposal must separately meet Class A’s outrigger and side-width rules.

Materials and side windows

Materials used in exterior work must have a similar appearance to the exterior of the existing house. Similar appearance does not necessarily mean an identical product.

Any window inserted in a side-elevation wall or roof slope must be obscure-glazed. It must also be non-opening unless all parts which can open are more than 1.7 m above the room floor. Exactly 1.7 m is not enough for an opening part. A high opening does not remove the obscure-glazing requirement.

When is Class B unavailable?

• On article 2(3) land, including conservation areas, National Parks, the Broads, National Landscapes and World Heritage Sites
• For a flat, maisonette or building containing flats
• Where permission to use the house as a house arose only under Part 3 Classes G, M, MA, N, P, PA or Q
• For a house built under Part 20
• Where the existing house has already been enlarged under Class AA
• Where a relevant planning condition or Article 4 direction removes the right

Class B does not include a verandah, balcony or raised platform, or the installation, alteration or replacement of a chimney, flue or soil and vent pipe. Check the appropriate separate route for each part of a combined project.

Class B has no separate blanket listed-building exclusion, but listed building consent is a different control and may be required. The existing building and use must also be lawful for the GPDO permission to apply.

Do you need to apply before building?

Class B has no prior approval procedure. If the proposal meets the right and it has not been removed, the GPDO grants the planning permission. A certificate of lawfulness for proposed development can formally establish the planning position before work starts.

Keep evidence of the original roof, previous alterations, external volume calculations and dimensioned drawings. Building Regulations, structural design, party-wall matters and private rights remain separate.

Sources and Class B legislation

Checked against the current GPDO Part 1 and article 2 definitions. The government householder technical guidance provides explanations and examples, but must be read with subsequent amendments to the legislation.

Page Updated: 25th September 2026