Do I Need Planning Permission for a Side Extension in 2026? (UK)
A single-storey side extension in 2026 UK is usually permitted development (no full planning needed) — provided width is under 50% of the original house, height is under 4m to the ridge, materials match, and you're not in a conservation area, listed building, Article 4 zone or designated land. Two-storey side extensions are NOT covered by PD and need full planning. This guide walks the exact PD limits, the 6 things that kick you out of PD into full planning, and the Lawful Development Certificate that protects you at sale.
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The 6 Conditions That Keep a Side Extension Under Permitted Development
1. Width: under 50% of original house
Side extension must be no wider than half the original house width (as it was on 1 July 1948 or when first built). For a 7m-wide semi, that's a 3.5m maximum side extension.
2. Single-storey only
PD covers single-storey side extensions only. Two-storey side extensions, dormers above, and rooms-in-roof linked to the side return all need full planning.
3. Height: max 4m to ridge
Maximum 4m to ridge and 3m to eaves. Within 2m of a boundary, eaves drop to 3m. Pitched roofs that tie into the existing roofline count, but only at PD-compliant heights.
4. Materials must match
Brick, render, roof tile and window style must match (or be similar to) the existing house. Pre-order matching brick samples — 8-week lead is normal.
5. No designated land
Conservation areas, AONBs, National Parks, World Heritage Sites and the Broads all remove most side-extension PD rights. Check your address on the council planning map first.
6. No Article 4 direction
Article 4 directions remove PD rights in certain streets — often Edwardian terraces, mining villages and characterful estates. A 30-second check on the council site saves a £5k retrospective application.
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