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Confused about planning permission for your rear extension? This guide explains permitted development rights, size limits, and when London homeowners need to apply.
One of the first questions London homeowners ask when considering a rear extension is whether they need planning permission. The answer depends on several factors, including the size of the proposed extension, the type of property, and whether any special designations apply to your area. In many cases, rear extensions can be built under permitted development (PD) rights, which means you do not need to submit a full planning application. However, PD rights come with strict conditions, and getting these wrong can result in enforcement action and the requirement to demolish unauthorised work.
Under current permitted development rules, single-storey rear extensions can extend up to 3 metres from the original rear wall of a semi-detached or terraced house, and up to 4 metres from the original rear wall of a detached house. These limits are measured from the original rear wall as it was on 1 July 1948, or as the house was originally built if constructed after that date. The maximum height is 4 metres, and the eaves height must not exceed the eaves of the existing house.
Important
The larger home extension scheme, which temporarily allowed single-storey extensions of up to 6 metres (semi-detached) or 8 metres (detached), has been made permanent. However, these larger extensions require a prior approval notification to your local council, and neighbours must be consulted. This is not the same as full planning permission, but it is not automatic either.
London has a higher proportion of conservation areas than almost any other part of the UK. Boroughs like Islington, Camden, Kensington and Chelsea, and Westminster have extensive conservation area coverage, and some boroughs have Article 4 directions that remove PD rights entirely for certain types of work. Before assuming you can build under PD, always check with your local planning authority or use the council's online planning portal to determine whether your property falls within a conservation area or is subject to any special designations.
Even if your extension falls within PD limits, it is strongly advisable to apply for a Lawful Development Certificate (LDC) before starting work. An LDC costs around £103 (as of 2026) and provides formal confirmation from the council that your proposed extension is lawful. This is invaluable when you come to sell your property, as solicitors and mortgage lenders will want to see evidence that any extensions were carried out lawfully. Without an LDC, you may face delays or complications during the conveyancing process.
Top Tip
Apply for a Lawful Development Certificate before starting any building work. It costs around £103 and typically takes 6–8 weeks to process. It provides peace of mind and protects you when you come to sell your home.
Regardless of whether you need planning permission, all rear extensions must comply with Building Regulations. This is a separate approval process that ensures your extension meets standards for structural integrity, fire safety, energy efficiency, drainage, and ventilation. You can apply for Building Regulations approval through your local council's building control department or through a private approved inspector. Your builder or architect should be able to guide you through this process, and inspections will be carried out at key stages during construction.
If your rear extension will be built on or near the boundary with a neighbouring property, you may need to serve a Party Wall Notice under the Party Wall etc. Act 1996. This applies if you are building on the boundary line, within 3 metres of a neighbouring building's foundations, or within 6 metres if your excavations go deeper than the neighbour's foundations. The party wall process must be completed before work begins and involves serving formal notice on your neighbours at least two months before the planned start date. If your neighbours do not agree, a party wall surveyor will need to be appointed.
Yes, in many cases. Under permitted development rights, you can build a single-storey rear extension of up to 3 metres (semi-detached or terraced) or 4 metres (detached) without planning permission. Larger extensions of up to 6 or 8 metres are possible through the prior approval process. However, flats, listed buildings, and some conservation area properties do not have these rights.
A standard householder planning application typically takes 8 weeks for a decision, though some London boroughs can take longer. A Lawful Development Certificate for a PD extension takes around 6–8 weeks. The prior approval process for larger extensions takes 42 days. Allow additional time for preparing drawings and submitting the application.
Yes, all rear extensions require Building Regulations approval regardless of whether planning permission is needed. This covers structural calculations, foundations, damp-proofing, insulation, drainage, ventilation, fire safety, and glazing specifications. Your builder or architect will typically manage this process on your behalf.
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