Planning Permission for Extensions: A Complete Homeowner's Guide
One of the first questions homeowners ask when planning an extension is whether they need planning permission. The answer depends on your property, its location, the size and type of extension you're proposing, and whether your Permitted Development rights are intact. This guide covers what you need to know — for London homeowners in particular.
Permitted Development Rights
In England, most homeowners have Permitted Development (PD) rights — a set of allowances that permit certain works without formal planning permission. Under current PD rights, a single-storey rear extension can be built without planning permission if it meets specific conditions:
For detached houses: up to 8 metres depth (or 4 metres on prior approval basis above 4 metres up to 8 metres). For semi-detached and terraced houses: up to 6 metres depth (or 3 metres on prior approval basis up to 6 metres).
Additional PD conditions apply: the extension must not cover more than 50% of the curtilage of the original house; the height must not exceed the existing eaves or ridge of the original roof; materials should match or be similar in appearance to the existing house; and the extension cannot project beyond the building line on the principal elevation.
When You Will Need Planning Permission
Planning permission will be required in the following circumstances:
The extension exceeds PD allowances. Any rear extension beyond the limits above, or any side extension, will require planning permission.
The property is in a conservation area. In designated conservation areas, PD rights are more restricted. Side extensions typically require permission, and any work visible from the street is subject to additional controls.
The property is listed. Listed buildings require Listed Building Consent for any works affecting the character of the building, in addition to planning permission for extensions.
PD rights have been removed. Many London properties are subject to Article 4 Directions that remove PD rights, either to the whole property or to specific types of development. Check with your local planning authority before assuming PD applies.
The proposed extension is double-storey. Double-storey rear extensions require planning permission — they are not covered by PD rights.
The Prior Approval Process (Larger Home Extension)
For single-storey rear extensions between 4–8m (detached) or 3–6m (semi/terrace), a prior approval application is required. This is a lighter-touch process than full planning permission — the council notifies neighbours, who have 21 days to raise objections, and the council then assesses the impact on amenity. Most prior approval applications are approved where the design is reasonable.
How Long Does Planning Permission Take?
A standard householder planning application takes 8 weeks from validated submission to decision in most cases, though complex or contentious applications may take longer. Extensions to this timeline are common where further information is requested or where objections are received.
What Improves Your Chances of Success
Good quality, well-drawn plans showing the design in context. A design that responds to the character of the area and the existing building. Pre-application advice from the local planning authority, where available. Proactive neighbour communication before submission. Materials and appearance that are appropriate to the street scene.
RCB works with experienced architectural designers and can help coordinate the planning process as part of our design-and-build service. 07359 872594 | contact@rcbgroup.co.uk | www.rcbgroup.co.uk