How to Get Planning Permission for a Side Extension in London

What Is a Side Extension?

A side extension is an addition to the side of a house, filling in (or extending beyond) the space between the main house and the side boundary. In London's semi-detached and detached houses, this space is sometimes available for development.

Side extensions in terraced houses are generally not possible — there is no side gap to build into.

When Does a Side Extension Need Planning Permission?

Under permitted development rights, a side extension can be built without planning permission if it is a single storey and:

• Its width does not exceed half the width of the original house
• It does not exceed 4 metres in height
• It does not protrude beyond the front wall of the original house

However, even if these conditions are met, permitted development does not apply in:

• Conservation Areas (for any side extension)
• World Heritage Sites
• National Parks or AONBs
• Where an Article 4 Direction has removed PD rights

In practice, the majority of London side extensions require planning permission — either because the house is in a sensitive designation, because the extension exceeds the PD limits, or because it is a two-storey or larger scheme.

Planning Policy for Side Extensions

Impact on the Street Scene

Planning authorities in London assess side extensions against the character of the street. Common concerns include:

• Terracing effect: A side extension that fills the gap between semi-detached houses can erode the characteristic spacing of the street, creating an unbroken terrace appearance. Many boroughs require that a gap of at least 1 metre is maintained at ground level.
• Subservience: A successful side extension should be subservient to the main house — set back from the front elevation, lower in height or pitch, or otherwise visually secondary to the host building.
• Materials: Materials that match or complement the host building are generally preferred.

Impact on Neighbouring Properties

The planning authority will consider:

• Daylight and sunlight: A side extension on the boundary can block daylight to neighbouring windows, particularly ground-floor windows of the adjacent property. The BRE daylight/sunlight methodology may be applied.
• Overlooking: Side extensions with side-facing windows can create overlooking issues for neighbours.
• Outlook: A long side extension running close to the boundary can create a sense of enclosure for the neighbouring property.

Two-Storey Side Extensions

A two-storey side extension adds first-floor accommodation alongside the existing house. These are more impactful than single-storey extensions and are assessed more carefully:

• Must comply with 45° code requirements for adjacent windows
• The upper floor must be at least 1 metre from the boundary in most circumstances
• Roof must not appear dominant relative to the main house
• Typically must be set back from the front elevation of the main house by at least 1–2 metres

Design Strategy for Approval

To maximise the chance of planning approval for a side extension:

1. Pre-application advice: Discuss the scheme with the planning officer before submitting. This is particularly valuable for side extensions, where the design nuances matter.
2. Set back from the front: A side extension that aligns flush with the front of the house is likely to be refused in many areas. A setback of 1–2 metres reads as subservient.
3. Maintain a visual gap: Even if building to the boundary, leaving a visual gap (lower parapet or gap in material) at street level helps reduce the terracing effect.
4. Match materials: Using brick to match the existing house reduces the visual impact of the extension.
5. Avoid large side windows: Windows facing the neighbouring property create overlooking concerns and are often conditioned out.

Combining Side and Rear Extensions

A side return extension (filling the side return alongside a Victorian terrace) is slightly different from a true side extension — it fills a narrow gap that already exists rather than adding to the width of the building at the frontage. Side return extensions are more commonly permitted development or approved at planning, as they do not create a terracing effect.

RCB Design & Build

RCB advises clients on the planning prospects for side extensions before any design investment is made. We work with experienced planning architects to develop schemes that maximise the space gained while meeting the design principles that planning authorities in London require.