These are England's householder rules, not automatic allowances for every residential property. [1] [2]
Measure from the correct starting point
Measure the rear projection from the original rear wall, not the end of a later conservatory or extension. “Original” means the house as it existed on 1 July 1948, or as first built if later. Where the rear is stepped, more than one wall can form part of the original rear elevation. Each relevant projection must be assessed. [1]
Joined earlier extensions matter. You cannot necessarily add 3m behind an existing 3m addition, and changing ownership does not renew an allowance. The guidance assesses the total enlargement where a new extension joins an existing one. [1]
Depth is only one limit
For a single-storey rear extension, check the following alongside depth:
- The extension must not exceed 4m overall height.
- Where any part is within 2m of a boundary, the eaves must not exceed 3m.
- The applicable existing-house roof and eaves limits must also be respected.
- Buildings other than the original house must not cover more than 50% of the relevant curtilage area, excluding the original house's ground area.
Other conditions address matters such as materials and prohibited features. A proposal meeting every measurement on a short checklist can still fail if rights have been removed or another condition applies. [1]
Side and upper-floor additions are different
A side extension under Class A must be single-storey, no more than 4m high and no wider than half the original house's width. An enlargement with both rear and side elements must satisfy both sets of tests. [2]
An extension of more than one storey has a 3m rear projection limit and must not be within 7m of a boundary opposite the rear wall. Roof-pitch and upper-floor side-window conditions also apply. Side extensions of more than one storey are not permitted under Class A. [1]
When can 6m or 8m be considered?
The larger single-storey rear route can reach 6m for attached houses and 8m for detached houses, but requires the neighbour-consultation/prior-approval procedure before starting. It is unavailable on article 2(3) land, including conservation areas, and on Sites of Special Scientific Interest. [3] [1]
Where a preferred design exceeds permitted development limits, a planning application may still be possible. Do not confuse a national permitted development limit with a promise of approval or a universal planning refusal threshold.
Do not build to a consultation proposal
The government's 2024 consultation proposed wider householder changes. A consultation is not an operative permission. Check the current limits, relevant legislation and commencement before relying on any larger dimension reported in the news. [4]