Prior approval or permission for a large rear extension

A single storey rear extension of up to 6 metres on a semi-detached or terraced house, or up to 8 metres on a detached one, can be built without planning permission under the larger home extension arrangements, but only after you have been through the prior approval procedure with the council. Anything deeper than that, or outside the other limits in Class A, needs a householder planning application. The choice between the two routes is not about which is easier. It is about what the council is allowed to consider, and what an objecting neighbour can achieve.

Under prior approval, an objection puts amenity impact on the adjoining occupier in front of an officer. Under a full application, it puts everything in front of an officer.

The permitted development limits for a single storey rear extension

Class A of Part 1 of Schedule 2 to the General Permitted Development Order sets the limits. For a single storey rear extension the basic depth allowance is 3 metres for an attached house and 4 metres for a detached one. The larger allowances of 6 and 8 metres are available only through the prior approval route.

The other limits apply either way:

• Maximum height of 4 metres overall, and no more than 3 metres at the eaves where the extension is within 2 metres of a boundary.
• No more than half the area of land around the original house may be covered by buildings.
• Materials of similar appearance to the existing house.
• No verandas, balconies or raised platforms above 0.3 metres.
• Not permitted at all on designated land, which includes conservation areas, National Parks, National Landscapes and World Heritage Sites.
• Not available where permitted development rights have been removed by condition or by an Article 4 direction.

Listed buildings are a separate matter. The extension may still be permitted development in planning terms, but listed building consent is needed for the works, and that is a discretionary decision.

How the neighbour consultation scheme works

The procedure is set out in Class A itself and it is prescriptive. Before starting work, you give the council a written description of the proposal with the dimensions, a plan of the site showing the proposed development, the addresses of the adjoining premises, and your contact details.

The council then notifies the owners and occupiers of adjoining premises, meaning properties sharing a boundary, and gives them 21 days to object. If nobody objects, the council tells you prior approval is not required and you can build. If somebody does object, the council must consider the impact on the amenity of all adjoining premises, and it decides whether prior approval is granted.

Its assessment is limited to that question. Design, character, loss of garden and highway impact are all outside it.

Timescales for each route

The council has 42 days from receipt of the notification to determine the matter. If it does not respond within that period, and the proposal does fall within the limits, you may proceed. A householder planning application carries a target of eight weeks, with an extension of time agreed in some cases.

Fees are indexed, so check the current schedule rather than a figure quoted online.

What an objection does under each route

This is the difference that matters. Under prior approval, an objection from a neighbour triggers an officer assessment of amenity impact, usually meaning loss of light, overshadowing, outlook and any overbearing effect on the adjoining property. The officer cannot refuse because the extension is unattractive, because the street has no other extensions of that depth, or because the garden is being reduced.

Under a full application, all of those become available. So does policy: the local plan's design policies, any householder design guide adopted as a supplementary document, and the 45-degree or 25-degree tests some authorities apply to daylight and outlook.

Counter-intuitively, that can make the full application the safer route where the neighbour relationship is poor but the design is good, and the prior approval route the safer one where the design is unremarkable but the neighbours are content.

Design freedom compared

Prior approval offers no design flexibility at all. The dimensions are fixed by the Order, the materials must be similar to the existing house, and there is no scope for a flat-roofed glazed box with a parapet if that puts the height over 4 metres.

A full application allows a wraparound, a side-and-rear combination, a taller ceiling, a lantern above 4 metres, contemporary materials, or a deeper extension where the plot can absorb it. It also allows the design to be argued, which is the point of a Design and Access Statement where the authority asks for one.

On a house with an awkward plan, the extra fee usually buys a better building.

Evidence and drawings needed

For prior approval, the documents are minimal: a written description with the dimensions, a site plan showing the proposal, and the addresses of the adjoining premises. Many councils also accept or request elevations, and providing them reduces the chance of an argument about what was approved.

For a householder application, expect a location plan with the site outlined in red, existing and proposed floor plans and elevations at a stated scale, a site plan, and the completed forms with the ownership certificate. Where the property is in a conservation area or affects a listed building, a heritage statement is usually required as well.

Keep the prior approval paperwork. The council's letter confirming that prior approval is not required is the document a purchaser's solicitor will ask for in five years, along with the building regulations completion certificate.

When a full application is the better choice

Choose the application where any of the following applies:

1. The house is on designated land, where the larger extension route is not available.
2. Permitted development rights have been removed by an Article 4 direction or by a condition on an earlier permission.
3. The design needs more height, a different roof form or materials that are not similar to the existing house.
4. The extension wraps around the side, or combines with a loft conversion that has its own limits.
5. The half-curtilage coverage limit has already been reached by earlier outbuildings.
6. You want the certainty of a decision notice with conditions rather than a procedural clearance.

Where the extension is beyond the limits and has already been built, the routes change entirely, and the comparison between a certificate and a retrospective application applies instead. Appeal rights differ too, and the householder appeal route has a 12-week deadline that catches people who spend a month deciding.

A lawful development certificate is the only formal way to confirm that works are lawful, so if certainty matters, that is the route rather than an assumption based on the dimensions. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Enter your postcode on PlanScope to see which documents your extension needs.