Class A: house extension rules, windows and doors

A house extension does not always need a planning application. In England, Class A can grant planning permission for an extension, new windows and doors, and other alterations to a house. It can also be relevant to a new basement beneath a house, although excavation and engineering operations need separate consideration. But the proposal must meet every relevant limit and condition.

We explain the main checks below. Start with the shape of the original house, not just the dimensions of the extension you would like to build. An existing extension, an outrigger or a road beside the house can change the answer.

The main Class A measurements

These are starting points, not a complete permission checklist. A proposal can meet the rear depth limit and still fail the side-width, height or other rules.

Find the original house before measuring

The original house is the house as it existed on 1st July 1948, or as first built if constructed later. Your purchase date makes no difference. A previous owner’s extension still counts as an extension.

Old plans, planning records, historic photographs and the building itself can help establish the original footprint. An original outrigger is part of the original house. A similar-looking addition built later is not. That difference can affect both the starting point for measurements and the joined-extension rules.

The existing house is a different reference point: the house immediately before the development. Class A uses the original house for several footprint and width tests, but the existing house for its overall roof and eaves height limits.

An outrigger can make your rear extension a side extension too

An outrigger is a projecting part at the back of a house, often the narrower back addition on a Victorian terrace. Its lengthwise wall can be an original side-elevation wall even though it stands well inside the overall width of the main house.

If your extension goes beyond that side wall, the side-extension rules can apply as well as the rear-extension rules. Being in the back garden, staying behind the front of the house or staying within the main house’s overall width does not avoid this test.

For example, an 8 m wide original house has a 4 m side-width allowance. A single connected wraparound enlargement measuring 8 m across fails that allowance. We do not measure just the narrow strip beside the outrigger and ignore the connected rear part.

Some narrower side-and-rear combinations can comply. But many familiar full-width wraparound designs cannot. Check the whole geometry before commissioning a scheme on the assumption that it is permitted development. Our side extension guide explains the side-wall test and our rear extension guide covers stepped rear walls.

Joining an existing extension changes the calculation

Under A.1(ja), where new work joins an existing enlargement of the original house, the relevant limits in A.1(e) to (j) apply to the total enlargement. That means the new work together with the existing enlargement it will join.

A lawful side extension can therefore cause a later joined rear extension to fail the half-width rule. Building in stages, changing owners or obtaining separate certificates does not reset the allowance. Physically separate proposals need their own assessment; a token gap should not be treated as a guaranteed solution.

On article 2(3) land, A.2(d) also applies the additional side and multi-storey rear restrictions to joined enlargements.

The 50% rule applies to the curtilage

The curtilage is the land closely associated with the house for its domestic enjoyment. It is not necessarily every piece of land you own, and it is not just the back garden.

Subtract the ground area of the original house from the curtilage area. Buildings other than the original house must not cover more than half the remaining area after the work. Count existing and proposed extensions and detached buildings, including buildings with planning permission. An original detached garage is not part of the original house merely because it was built at the same time.

This calculation is about ground coverage, not the total floor area of every storey. Passing it does not override any other Class A limit.

Height, eaves and nearby boundaries

The altered or enlarged part must not exceed the highest part of the existing house’s roof, and its eaves must not exceed the existing house’s eaves. The separate 4 m single-storey limit and, where applicable, the 3 m boundary-related eaves limit can be more restrictive.

Eaves are measured where the external wall, projected upwards if necessary, meets the upper surface of the roof. Do not measure to the gutter. On a flat roof, a parapet can count towards the overall height even though it is not included in the eaves measurement.

If any part of the enlargement is within 2 m of a curtilage boundary, its eaves must not exceed 3 m. This does not create a general requirement to leave a 2 m gap beside an extension.

Article 2(2) measures height from the immediately adjacent ground. Where the ground is not uniform, its definition refers to the highest adjacent ground surface. Sloping sites, changes in levels and different existing eaves heights need particular care; raising a deck or adding fill is not a safe shortcut.

Front walls, side walls and highways

Class A does not allow an enlargement beyond a wall forming the principal elevation of the original house. That restriction does not depend on the principal elevation facing a road. The principal elevation is usually the main front, identified from the building’s design and relationship with its surroundings.

An enlargement is also excluded where it extends beyond an original side-elevation wall which fronts a highway. A highway can include a public footpath, not just a road used by cars. Corner plots need particular care.

A stepped frontage may have more than one relevant wall. Government guidance treats projecting beyond a wall as including the area beyond an imaginary continuation of that wall, not only the space immediately in front of it. Whether a side elevation fronts a highway depends on the actual relationship, not simply the presence of a road somewhere nearby.

Can Class A allow a two-storey extension?

Potentially, but an enlargement with more than one storey must project no more than 3 m beyond the original rear wall and must not be within 7 m of a curtilage boundary opposite the rear wall. An enlargement beyond an original side wall cannot have more than one storey under Class A.

The roof pitch of an enlargement with more than one storey, or an upper storey added to an existing enlargement, must match the original house’s roof pitch so far as practicable. Adding an upper floor to a deep existing ground-floor extension can fail because the total enlargement is assessed together.

These rules concern an extension to the house. Adding storeys above the main house is the separate Class AA route.

Materials, windows, doors and conservatories

Materials used in exterior work must have a similar appearance to the exterior of the existing house. They need not be the identical product, but the visual result matters. The materials condition has an exception for materials used in constructing a conservatory; the other applicable extension limits still apply.

Class A can cover new or replacement windows and doors and alterations such as infilling a garage-door opening. Some work which does not materially affect the external appearance may not be development at all. See our windows and doors guide and garage conversion guide.

An upper-floor window in a side wall or side-facing roof slope must be obscure-glazed. It must also be non-opening unless the parts that open are more than 1.7 m above the floor of the room. A high opening does not remove the obscure-glazing condition.

Government guidance treats external solid-wall insulation as an improvement, so restrictions aimed specifically at an enlargement may not apply in the same way. Check appearance, protected-land restrictions and the precise work proposed.

Does the house have Class A rights?

• These are English rules for a dwellinghouse, not a flat, maisonette or building containing flats
• Class A excludes a house whose permission to be used as a house was granted only through Part 3 Classes G, M, MA, N, P, PA or Q
• A house built under Part 20 is excluded
• Check the planning history for conditions removing or restricting permitted development and check any relevant Article 4 direction
• Article 3 prevents reliance on permission in connection with an unlawfully constructed building or an unlawful use, and does not permit development contrary to a planning condition

Do not decide whether a building is a dwellinghouse solely from a C3 or C4 label. An HMO or mixed-use property can require a closer look at the actual planning unit and use.

Conservation areas and other article 2(3) land

Article 2(3) land includes conservation areas, National Parks, the Broads, National Landscapes (legally areas of outstanding natural beauty) and World Heritage Sites. Here Class A does not permit:

• An enlargement beyond an original side-elevation wall
• A rear enlargement with more than one storey
• Cladding any part of the exterior with stone, artificial stone, pebble dash, render, timber, plastic or tiles

The larger 6 m and 8 m rear-extension route is unavailable on article 2(3) land or a Site of Special Scientific Interest. Ordinary rear extensions may still qualify, subject to the remaining rules.

Listed buildings

Class A does not impose a blanket listed-building exclusion. However, works affecting the character of a listed building as a building of special architectural or historic interest require listed building consent. Planning permission and listed building consent are separate checks.

What Class A does not cover

Class A excludes work consisting of or including a verandah, balcony or raised platform. Part 1 defines a raised platform as one higher than 0.3 m. Government guidance says a Juliet balcony with no platform would normally avoid the balcony restriction, but the actual design and the other rules still matter.

Alterations to the existing roof, microwave antennas, chimneys, flues and soil and vent pipes are excluded from Class A. Check the relevant roof, antenna or chimney class separately. The roof forming part of a Class A extension should not be confused with altering the existing house’s roof.

What should you do before building?

1. Establish the original house, its planning history, curtilage and any restrictions
2. Draw the whole proposal and every existing enlargement it will join, then test all relevant walls, dimensions and conditions
3. Use the larger home extension procedure before starting where the rear depth exceeds the ordinary 3 m or 4 m allowance
4. Consider a Lawful Development Certificate to obtain a formal decision on a precisely described proposal
5. Check Building Regulations, any Party Wall etc. Act 1996 requirements, other property rights and any CIL extension requirements before work begins

If all applicable rules are met, Class A grants planning permission. For ordinary Class A development a separate planning application is not normally required. A certificate records the lawfulness of the proposal; it does not waive a limit or approve a different scheme.

Sources and Class A legislation

Current GPDO Part 1 and article 2 definitions provide the statutory wording. The government householder technical guidance explains measurements and examples, but must be read alongside later legislative amendments.

Page Updated: 25th September 2026