Most house extensions in England never need a planning application — they are permitted development under Class A. But the limits are precise, and missing one by 20 cm turns a lawful build into an enforcement case. This checker applies the real numbers from the government's official technical guidance to your project.
Check your extension
The checker needs JavaScript. The limits it applies are in the official technical guidance.
The headline limits (houses in England)
| Project | Permitted development limit |
|---|---|
| Single-storey rear extension | 4 m deep (detached) / 3 m (any other house); max 4 m high; eaves max 3 m within 2 m of a boundary |
| Larger single-storey rear (prior approval) | Up to 8 m / 6 m via the neighbour consultation scheme (£249) — not on designated land or SSSIs |
| Two-storey rear extension | Max 3 m beyond the rear wall, and no closer than 7 m to the rear boundary |
| Side extension | Single storey, max 4 m high, no wider than half the original house — not PD on designated land |
| Outbuildings (Class E) | Max 4 m (dual-pitched roof) / 3 m (other) / 2.5 m within 2 m of a boundary |
| Everything combined | Buildings must not cover more than 50% of the land around the original house |
The traps people fall into
- "Original house" means as first built (or as it stood in 1948) — previous extensions eat your allowance.
- Designated land changes the rules — in conservation areas, AONBs, national parks and WHS land there are no PD side extensions, no multi-storey rear extensions, no cladding, and no larger-home scheme.
- Article 4 directions can remove PD rights street by street — check with our site constraints appraisal.
- Flats have no householder PD — Class A applies to houses only.
- PD is not a design permit — neighbour disputes over light continue regardless; screen yours with the 45-degree checker.
Make it certain: the lawful development certificate
A green result here is screening, not proof. For certainty — especially before selling — apply for a lawful development certificate: half the full application fee (currently £274 for a proposed householder LDC; see the fee calculator), and the council formally confirms the build is lawful.
How Fortress Associates can help
Extensions still need Building Regulations approval even when planning is not required — our Building Regulations drawing packages cover Approved Documents A–S. And if a neighbour objects over light, our BRE daylight and sunlight reports settle it with numbers (4–5 days, no advance payment). All the free tools live at /free-tools; talk to us about your project.
Sources & further reading
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