What Is the 50% Rule for House Extension?

What is the 50% rule for house extension? The 50% rule is a permitted development limit set out in Schedule 2, Part 1, Class A of the Town and Country Planning (General Permitted Development) (England) Order 2015, as amended. It states that any extension built under permitted development must not result in the total area of all buildings within the curtilage of the original house, excluding the original house itself, exceeding 50% of the total area of the original curtilage.

In practice this means that if your garden, driveway, and other outbuildings already occupy more than half the total site area, you cannot extend further under permitted development and must apply for full planning permission instead. The 50% limit applies to outbuildings and extensions combined, so existing sheds, garages, home offices, and previous extensions all count against the allowance. The original curtilage is the site as it existed on 1 July 1948, or as it was first built if constructed after that date.

This rule catches out many homeowners who have already built a substantial outbuilding or garage. Even if the proposed extension itself is small, combining it with existing ancillary structures may take the total above 50%. A pre-application enquiry with the local planning authority, or a lawful development certificate application, is the safest way to confirm whether your project falls within permitted development before committing to design fees.

For projects that do require a full planning application because permitted development is unavailable, residential extension visualisation is a standard supporting document that helps officers and planning committees understand the scheme in its context. A clear CGI can reduce the risk of refusal on visual amenity grounds.

For combined extension and conversion projects, loft conversion visualisation can be provided alongside the extension CGI from the same 3D model, reducing overall visualisation cost.