Larger Home Extension Prior Approval in London (2026 Guide)

What is the larger home extension prior approval route?

The larger home extension route is a permitted development right that allows a single-storey rear extension deeper than the standard 3m (attached house) or 4m (detached house) limit — up to 6m on a terraced or semi-detached house and up to 8m on a detached house. Because these deeper extensions could affect neighbours, the right is conditional on a prior approval step: before you build, you notify the council, which consults the adjoining owners. If no neighbour objects, or if the council judges any objection does not warrant refusal, you receive prior approval and can proceed.

The critical thing to grasp is what prior approval is not. It is not a planning application, and the council has no power to assess the design, appearance or acceptability of your extension in principle — Parliament has already granted the right. The council's only job is to determine whether the impact on the amenity of adjoining properties is acceptable. That narrow remit is exactly why the route is faster and more certain than full planning: there is no officer weighing subjective design merit, only a defined consultation with your immediate neighbours.

What are the size limits for a larger rear extension?

The headline depths are the reason people use this route, but they come bundled with height limits that are just as binding. A single-storey rear extension can extend up to 6m from the original rear wall of a terraced or semi-detached house, or up to 8m from a detached house. The extension must be single-storey throughout, must not exceed 4m in height to the ridge, and — where it comes within 2m of a boundary — the eaves height must not exceed 3m. Miss any one of these and the right is lost.

Two further points catch London homeowners out. First, the depth is measured from the original rear wall of the house as it stood in 1948 or as first built, not from any later extension you have already added — so a previous rear addition eats into your allowance. Second, the extension must not cover more than half the area of land around the original house, and materials should be similar in appearance to the existing dwelling. These are permitted development conditions in the round, and a design that clears the depth limit but fails on curtilage or height still needs full planning permission.

• Terraced or semi-detached house — single-storey rear extension up to 6m deep
• Detached house — single-storey rear extension up to 8m deep
• Maximum height 4m to the ridge; single-storey only, no balconies or raised platforms
• Eaves no higher than 3m where the extension is within 2m of a boundary
• Depth measured from the original rear wall, so earlier extensions reduce your allowance
• Must not cover more than half the land around the original house

How does the neighbour consultation scheme work?

The neighbour consultation scheme is the procedural heart of this route and the reason it is called prior approval. You submit details of the proposed extension — a written description, a plan of the site, and drawings showing the depth, height and position — to the council before any work starts. The council then notifies the owners or occupiers of every adjoining property, describing your extension and giving them 21 days to object on the ground of impact on their amenity, such as loss of light or an overbearing effect.

What happens next depends on the response. If no adjoining neighbour objects within the 21 days, the council must grant prior approval and you can build. If one or more neighbours do object, the council must consider whether the impact on their amenity is acceptable and decide whether to grant or refuse prior approval on that basis alone. Either way, the council has 42 days from receipt of your submission to notify you of its decision — and if it fails to respond within that period, approval is deemed granted and you may proceed. You must not start the extension until the process has concluded.

What evidence and drawings do you need?

Although prior approval asks for less than a full application, accuracy matters more, because the whole right depends on staying within the numeric limits. You need a measured survey confirming the position of the original rear wall, so the depth is proved from the correct baseline, and drawings that clearly annotate the projection, the ridge height and the eaves height where the extension runs close to a boundary. Getting the baseline wrong is fatal: an extension measured from a later addition rather than the original wall can breach the 6m or 8m limit without anyone noticing until enforcement raises it.

Because prior approval is a notification rather than a permission that certifies lawfulness for all time, many London homeowners follow it with a Lawful Development Certificate. The certificate is the council's formal confirmation that the completed works are indeed permitted development, and it is invaluable when you come to sell — a buyer's solicitor will ask for proof that a large rear extension was lawful, and prior approval correspondence alone can be thinner evidence than a certificate. Commissioning accurate drawings once, for both the prior approval and the certificate, is the efficient way to cover the whole process.

When does prior approval beat a full planning application?

The route wins when your project fits squarely inside the permitted development box and you value speed and certainty. A single-storey rear extension on a house — not a flat — in an area that is not a conservation area or other designated land, staying within the depth and height limits, is the classic candidate. Here prior approval delivers a decision in at most 42 days with no design judgement to argue, against the eight weeks or more and the subjective assessment of a full householder application. For a homeowner who wants a large kitchen-diner opening onto the garden and nothing more complex, it is usually the better tool.

It stops being the right tool the moment the project steps outside the box. If you want two storeys, a side extension, a change to the roof, or anything on a flat or in a conservation area, permitted development does not apply and you need full planning permission. The same is true if a previous extension has already used up your depth allowance, or if you want a design that a full application would let you argue for on its merits but that breaches a permitted development condition. Choosing between the two routes is really a question of whether your ambition fits the fixed limits — and reading that correctly at the outset saves the most time of all.