Many rear extensions can be built without planning permission under permitted development rights. Here's exactly when you need permission and when you don't.
Under permitted development rights, you can extend a detached house by up to 4 metres to the rear, or a terraced or semi-detached house by up to 3 metres, without a formal planning application — provided the extension meets a set of specific criteria set out in Schedule 2, Part 1, Class A of the Town and Country Planning (General Permitted Development) (England) Order 2015. These criteria include that the extension must not exceed the height of the eaves of the original dwelling, must not be higher than 4 metres at the ridge, and must use materials similar in appearance to the existing house.
These limits apply to the addition since the house was first built (or 1 July 1948, whichever is later), not just the extension you are currently proposing. If previous owners have already extended the house, that volume counts against your permitted development allowance.
Under the larger home extension scheme — sometimes called prior approval or the neighbour consultation scheme — you can extend a detached house by up to 8 metres to the rear, or a terraced or semi-detached house by up to 6 metres, subject to a prior approval process with your council. Neighbours are notified and can raise objections, but the council can only refuse on specific grounds: impact on the amenity of adjoining properties.
The prior approval process costs around £120 and typically takes 8 weeks. It is substantially faster and cheaper than a full planning application, and the grounds for refusal are narrower. If your neighbours are unlikely to object (or their objection relates to something other than amenity impact), this is often the best route for larger extensions.
Several circumstances always require a full planning application regardless of the extension's size. Listed buildings require listed building consent for any works affecting their character. Extensions in conservation areas that would be visible from a highway require a full application. If the extension would cover more than 50% of the original curtilage (garden and other land around the house), permitted development is not available. If your original planning permission included a condition removing permitted development rights — which is common on newer developments — you will need a full application even for a small extension.
Properties in National Parks, Areas of Outstanding Natural Beauty, and other designated areas have more restrictive permitted development limits. In these areas, rear extensions are typically limited to 3 metres for all house types.
The most reliable way to confirm that your proposed extension falls within permitted development is to apply for a Lawful Development Certificate (LDC) from your local planning authority. An LDC costs approximately £103 and formally confirms in writing that your works are lawful. This is not strictly required — you can build under permitted development without an LDC — but it provides legal certainty and is valuable evidence when selling the property. Many mortgage lenders and buyers' solicitors now request an LDC for extensions built without a full planning application.
You can also check whether an Article 4 Direction has removed your permitted development rights. Article 4 Directions are applied by councils in conservation areas and other designated zones, and can remove Class A permitted development rights entirely. Your council's planning portal will show Article 4 Directions affecting your area.
If you submitted a full planning application for a rear extension and it was refused, the most common refusal reasons are that the extension would harm the character and appearance of the area (Policy DM1 or equivalent), that the extension would cause unacceptable harm to neighbouring amenity (daylight, outlook, or privacy), or that the materials are not in keeping with the existing building. Planning Decoder decodes your specific refusal reasons and tells you whether an appeal is likely to succeed or whether a revised application would be a better route.
Before you commission architect drawings, know what your council has approved nearby. A Site Intelligence Report maps comparable rear-extension decisions at neighbouring properties, Article 4 Directions and any enforcement history on your street. Delivered in 48 hours.
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