Side Extension Planning Rules

The essential position

A single-storey side extension can be permitted development if it meets Class A limits, including a maximum width of half the original house and a maximum height of 4 metres. Two-storey side extensions normally require planning permission and are assessed for street character, spacing and terracing effect.

Key points

• Check whether the side elevation fronts a highway.
• Measure width against the widest part of the original house.
• Conservation-area and Article 4 restrictions can remove the route.
• Retain visual separation and subordination on planning applications.

1. Single-storey permitted development limits

A qualifying side extension must be single storey, no more than 4 metres high and no wider than half the width of the original dwellinghouse. Where it is within 2 metres of a boundary, the 3-metre eaves limit also applies.

If the extension also projects beyond the original rear wall, rear-extension rules apply as well.

• Dimension original and proposed widths.
• Show eaves and overall height.
• Check earlier side additions.

2. Highways, corner plots and designated land

Class A does not permit an enlargement forward of the principal elevation or forward of a side elevation that fronts a highway. Corner plots therefore need careful analysis of which elevation has the required relationship with the road.

Side extensions are not permitted development on Article 2(3) land under Class A, and Article 4 Directions or conditions can remove further rights.

• Confirm the legal highway boundary.
• Review conservation and Article 4 maps.
• Do not rely only on the postal front of the house.

3. Two-storey side additions

Planning authorities often seek a setback from the front wall, a lower ridge and a gap to the side boundary to avoid a cramped or terraced appearance. Local guidance varies.

The extension should normally remain subordinate to the host house. Matching brick alone will not resolve excessive width or mass.

• Use roof hierarchy to reduce dominance.
• Align window proportions thoughtfully.
• Retain access and maintenance where needed.

4. Internal planning and future flexibility

Side additions are often used for utility, garage replacement, accessible bedroom or enlarged upper-floor accommodation. The design should avoid long corridors and residual rooms in the existing house.

Consider whether the ground-floor layout and structure allow future adaptation without undermining the street-facing composition.

• Coordinate stair and circulation.
• Protect daylight to retained rooms.
• Plan services and drainage efficiently.

Questions homeowners commonly ask

Official guidance and further reading

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