Planning · Updated May 2026

Do I Need Planning Permission for a Side Extension in 2026? (UK)

Q: What is a Lawful Development Certificate?

A council-issued confirmation that your build is permitted development. Costs £103, takes 8 weeks, strongly recommended for resale.

Q: Does the side extension need to leave a side passage?

PD does not require a side passage, but building control may ask for a 600mm cavity if the wall sits on the boundary.

Q: Do I need to tell my neighbours?

For PD: no formal notification, but Party Wall Act notices are legally required within 3m of a boundary structure.

Q: How long does a planning application take in 2026?

Standard householder applications take 8 weeks by law, 10–14 in practice. Pre-application advice helps avoid query bounce-back.

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.

PD limits explained 6 PD-out triggers Lawful Dev Certificate

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PD Rules

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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Frequently Asked

Common Questions

Can a side extension be 2-storey under permitted development?+

No. Two-storey side extensions are explicitly excluded from PD — they always need full planning permission, regardless of width.

What's a Lawful Development Certificate and do I need one?+

An LDC is a council-issued confirmation that your build is permitted development. Costs £103, takes 8 weeks. Strongly recommended — buyers' solicitors ask for it at sale.

Does the side extension need to leave a side passage?+

PD itself doesn't require a side passage, but many councils ask via building control for a 600mm cavity if the wall sits on the boundary. Party Wall Act notices apply regardless.

What if my house is on a corner plot?+

PD limits the side extension on the side facing a highway. Most corner plots need full planning for any side extension visible from the road.

Do I need to tell my neighbours?+

For PD: no formal neighbour notification, but Party Wall Act notices are legally required if you build within 3m of a boundary structure. For full planning: the council notifies neighbours.

How long does a planning application take in 2026?+

Standard householder applications: 8 weeks by law, often 10–14 weeks in practice. Pre-application advice (£300–£900) can cut down on bounce-back queries.

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