Single-Storey Extension Planning Rules

The essential position

A single-storey extension may be permitted development where it stays within detailed limits. The most common errors are measuring from the wrong rear wall, overlooking previous additions, exceeding the 3-metre eaves limit near a boundary, or assuming a wrap-around extension is automatically permitted.

Key points

• Measure from the original dwellinghouse rather than the current rear wall.
• Rear, side and wrap-around extensions can be subject to different tests at the same time.
• The 6-metre and 8-metre allowances use the larger-home-extension prior-approval process.
• Permitted development does not remove Building Regulations or Party Wall requirements.

1. Rear-extension depth and the correct measuring point

For a straightforward single-storey rear extension, the familiar permitted development projection is up to 3 metres from the original rear wall for an attached house and 4 metres for a detached house. Historic extensions do not reset the measuring point.

Where the original rear elevation is stepped, each part of the proposal must be tested against the wall it extends beyond. A small projection measured from the newest wall can still breach the legal limit when measured from the original house.

• Obtain or reconstruct the original footprint.
• Dimension every rear projection on the proposed plan.
• Include earlier extensions in the 50% curtilage calculation.

2. Height, eaves and boundaries

A single-storey extension under Class A must not exceed 4 metres in overall height. Where any part is within 2 metres of a boundary, the eaves height must not exceed 3 metres.

Eaves are measured at the point where the external wall meets the roof plane, not simply to the gutter. Parapets, raised ground levels and sloping sites need careful interpretation because an apparently low extension can still exceed the permitted height.

• Show existing and proposed levels.
• Dimension eaves and ridge or parapet heights on elevations.
• Check every boundary, including angled rear boundaries.

3. Side and wrap-around extensions

A side extension relying on permitted development must normally be single storey, no more than 4 metres high and no wider than half the width of the original house. On Article 2(3) land, including conservation areas, side extensions are not permitted development under Class A.

A wrap-around extension is tested as both a rear and side enlargement. It may fail permitted development even when its rear projection and side width each appear acceptable in isolation, particularly where it joins an earlier addition.

• Check whether the side wall fronts a highway.
• Avoid assuming that an infill between existing additions is exempt.
• Use a planning application where a continuous wrap-around form gives a better design.

4. Larger Home Extension prior approval

The prior-approval route can increase the rear projection to a maximum of 6 metres for an attached house and 8 metres for a detached house. The council notifies adjoining owners and considers the impact on their amenity if an objection is received.

Work must not start until the authority confirms prior approval is not required, grants prior approval, or the statutory process has concluded in the applicant’s favour. All other Class A limits and conditions continue to apply.

• Submit accurate plans and a clear description.
• Allow for neighbour consultation before programming works.
• Do not treat prior approval as retrospective permission.

Questions homeowners commonly ask

Official guidance and further reading

• GOV.UK householder permitted development guidance
• Planning Portal: extensions