Prior Approval for Larger Home Extensions: The Class AA Procedure Explained

Introduction

Since 2013, the "prior approval" procedure has allowed larger single-storey rear extensions — beyond the standard permitted development limits — without full planning permission, subject to a neighbour consultation process. Class AA of the General Permitted Development Order allows extensions of up to 6m for terraced and semi-detached houses (compared to the standard 3m) and up to 8m for detached houses (compared to 4m). For homeowners in north London seeking to add significant rear space, understanding when prior approval is available — and when it is not — is important. This guide explains the procedure. For related guidance, see our permitted development guide, rear extension guide and LDC guide.

What Class AA Permits

Under Class AA of the GPDO, a single-storey rear extension is permitted development (subject to prior approval) where:

• The extension is single-storey only (no Class AA for two-storey extensions)
• The extension does not exceed 6m projection from the original rear wall (for terraced or semi-detached houses) or 8m (for detached houses)
• The eaves height of the extension does not exceed 3m
• The maximum height of the extension does not exceed 4m
• The extension does not result in more than 50% of the curtilage being covered by buildings
• The extension materials are of similar appearance to the materials used in the construction of the original house

Class AA does not apply to:

• Properties in conservation areas (where standard PD Class A rear extensions remain available within the 3/4m limits, but the Class AA extended limits do not apply)
• Listed buildings
• Flats and maisonettes (only houses benefit from Class AA)

The Prior Approval Process

Class AA is not automatic permitted development — it requires "prior approval" from the planning authority. The prior approval process is:

1. Application to the LPA: The homeowner (or their architect) submits a prior approval application to the local planning authority, including the application form, a description of the proposed development, drawings showing the proposed extension, contact details of adjoining owners, and the fee.
2. Neighbour notification: The planning authority must notify all adjoining owner-occupiers of the proposal. Adjoining owners have 21 days to make representations about the impact of the proposed extension on their amenity (daylight, outlook, privacy).
3. Prior approval decision: The planning authority must determine whether prior approval is required (i.e. whether the impact on the amenity of adjoining properties is acceptable). If no objections are received, or if the authority is satisfied with the impact, it must grant prior approval. If objections are raised, the authority assesses the impact and can refuse if the effect on neighbours' amenity is unacceptable.
4. Timeframe: The authority has 42 days to make its decision. If it fails to do so, prior approval is deemed granted.

Practical Application in NW3

The Class AA prior approval procedure is not available in NW3's conservation areas — which cover most of the area's Victorian and Edwardian housing stock. This is an important constraint. For NW3 homeowners who want a rear extension deeper than 3m (on a terrace or semi), a full planning application is required, not prior approval. Camden's planning policies will then assess the proposal against the character of the conservation area, impact on neighbours, and the specific design of the extension.

However, for properties in non-conservation area parts of north London — NW2 Cricklewood, N2 East Finchley, NW4 Hendon, and outer areas of NW5 — the Class AA prior approval route is available and can deliver extensions significantly larger than standard PD would permit.

What "Impact on Amenity" Means

When assessing prior approval, the planning authority can only consider the impact of the extension on the amenity of adjoining properties. It cannot consider:

• The design quality or appearance of the extension
• The impact on the character of the area generally
• Other planning policy considerations (in contrast to a full planning application)

The narrow scope of the assessment means that prior approval applications are generally approved unless there is a specific and demonstrable impact on a neighbour's daylight, outlook or privacy. See our daylight and neighbour impact guide.

Conclusion

The Class AA prior approval procedure provides a faster and lower-cost route for larger single-storey rear extensions than a full planning application — but it does not apply in NW3 conservation areas. For homeowners in non-conservation area parts of north London, prior approval for extensions of up to 6m or 8m is a worthwhile route worth exploring before committing to a full planning application. An architect who is familiar with the procedure will advise quickly on whether it is available for your specific property and project. Use our free matching service to connect with an experienced north London architect. For cost guidance, visit hampsteadrenovationcosts.co.uk.