What counts as a side extension
The rules bite when the enlarged part "would extend beyond a wall forming a side elevation of the original dwellinghouse" (para A.1(j)). "Original" means the house as it stood on 1 July 1948, or as built if later (art 2(1)), so the walls that count are the original ones, not the side of a later extension.
The limits
| Rule | Limit | Source |
|---|---|---|
| Height | No more than 4 m | A.1(j)(i) |
| Storeys | Single storey only | A.1(j)(ii) |
| Width | No wider than half the width of the original house | A.1(j)(iii) |
| Road-facing side | Not beyond a side wall of the original house that fronts a highway | A.1(e)(ii) |
| Front | Not beyond the principal elevation | A.1(e)(i) |
| Within 2 m of a boundary | Eaves no higher than 3 m | A.1(i) |
| Roof and eaves | No higher than the highest part of the existing roof; eaves no higher than the existing eaves | A.1(c), (d) |
| Garden coverage | Buildings other than the original house cover no more than 50% of the curtilage, leaving out the original house's footprint | A.1(b) |
| Joined to an earlier extension | The old and new parts together must meet the limits | A.1(ja) |
Heights are measured from the ground next to the building, using the highest part of the ground where it slopes (art 2(2)). Exterior materials must look similar to the existing house, except for a conservatory (para A.3(a)).
The half-width rule
The extension must not have "a width greater than half the width of the original dwellinghouse" (A.1(j)(iii)). As a hypothetical example, if the original house is 7 m wide, a side extension may be up to 3.5 m wide. The Order does not say exactly where to take the width from, so on a house with an irregular plan, or one already widened by a later addition, agree the figure with the council before you rely on it.
Corner plots and road-facing sides
On a corner plot, a side extension on the road side would extend beyond a side wall that fronts a highway, which Class A does not allow (A.1(e)(ii)). For Part 1, "highway" includes an unadopted street or a private way (para I), so a private road counts too.
Wrap-around extensions under the current rules
The Order has no separate rule for wrap-arounds. An L-shaped extension that goes beyond both the rear wall and a side wall is an enlarged part that extends beyond a side elevation, so the side limits in A.1(j) apply to it, as well as the rear limits in A.1(f) or (h). Our check tests the width of the whole extension against half the width of the original house. An extension that runs across the whole back of the house and down the side is wider than the original house itself, so on that reading it is more than half its width and falls outside Class A. If your design wraps around, apply for a Lawful Development Certificate or a planning application rather than relying on this page.
A February 2024 government consultation proposed changes to householder rights, but GOV.UK still says "We are analysing your feedback", so the current limits stand.
Conservation areas and other article 2(3) land
On article 2(3) land, development is not permitted by Class A if the enlarged part would extend beyond a wall forming a side elevation of the original house (A.2(b)), and that applies to the total of any joined extensions too (A.2(d)). Article 2(3) land is a conservation area, an area of outstanding natural beauty (National Landscape), an area specified under section 41(3) of the Wildlife and Countryside Act 1981, the Broads, a National Park or a World Heritage Site (Sch 1 Pt 1). There, a side extension needs a planning application, at a council fee of £548 for a single house from 1 April 2026. See our conservation area check.
Other things that can remove the right
Flats have no Part 1 rights (art 2(1)). A condition on an earlier permission (art 3(4)) or an Article 4 direction can remove them, and a listed home needs listed building consent for works affecting its character (LBCA 1990 s7). For extensions at the back, see single-storey rear extensions and two-storey extensions.
What the free check tells you
The free check tests your plot outline against conservation areas and the other article 2(3) designations, so it can tell you whether a side extension is ruled out on the data we hold, and lists the limits to stay within. It does not measure your house. Where the council does not publish a dataset, we say so rather than guess.
For certainty: a Lawful Development Certificate
For certainty, apply to your council for a Lawful Development Certificate for proposed works (TCPA 1990 s192). The council fee for an extension is £274, half the £548 application fee, and the decision is due within 8 weeks of a valid application. Read more on Lawful Development Certificates.