Side extensions and side-return infills
Reviewed October 2026 · Not legal, planning or financial advice
The side passage is often the least useful part of a plot. Building on it can add a utility, office, ground-floor bedroom — or, in a terrace, transform a narrow galley kitchen.
Side extensions
A side extension sits beside the house rather than behind it. The Class 1 exemption covers rear extensions, so side extensions generally need planning permission. Councils commonly look for an upper floor to be set back from the front and stepped down from the main roof, to avoid a terracing effect with the neighbour.
Side-return infills
On many terraced and older semi-detached houses, a narrow “return” runs along one side at the back. Infilling it widens the kitchen. Whether an infill counts as an extension “to the rear” for exemption purposes should be confirmed for your house.
Expect a glazed roof or rooflights (it would otherwise be dark), a party wall on one side, and drainage running along the passage — all of which push cost per m² above a simple rear extension.
Access
Building across the side passage can remove the only route from the street to the back garden. Plan for bins, bikes and maintenance access before committing.
Regulations and their interpretation change. This guide explains the position as we understand it in October 2026. Confirm what applies to your house with an architect, planning consultant or your local authority before relying on it.