The limits at a glance
| Rule | Limit | Source |
|---|---|---|
| Depth beyond the original rear wall | 3 m, or 4 m for a detached house | A.1(f)(i) |
| Height | 4 m | A.1(f)(ii) |
| Within 2 m of a boundary | Eaves no higher than 3 m | A.1(i) |
| Roof and eaves | No higher than the existing roof or the existing eaves | A.1(c), (d) |
| Front and side | Not beyond the principal elevation, or a side wall that fronts a highway | A.1(e) |
| Garden coverage | No more than 50% of the curtilage covered (see below) | A.1(b) |
| Not included | Verandahs, balconies, raised platforms, antennas, chimneys, flues, soil and vent pipes, roof changes to the house | A.1(k) |
Heights are measured from the ground next to the building, using the highest part where it slopes (art 2(2)). The Order does not say whether exactly 2 m from a boundary counts as "within 2 metres", so keep a margin.
What "original" and "rear wall" mean
"Original" means the house as it stood on 1 July 1948, or as built if later (art 2(1)). An extension added by a previous owner does not move the rear wall back, and if the new work joins it, the two together must stay within the limits (A.1(ja)).
The Order does not define "rear wall" or "principal elevation": neither article 2 nor the Part 1 interpretation paragraph (para I) does. The GOV.UK householder technical guidance gives the government's interpretation, but it was last updated on 10 September 2019 and predates Class AA, so our rules come from the Order as in force from 27 August 2026. If the back of your house steps in and out, agree with the council which wall counts.
The 50% rule
Buildings within the curtilage other than the original house, including earlier extensions, sheds and garages, must not cover more than half of the curtilage, leaving the original house's footprint out of the sum (A.1(b)). As a hypothetical example, on a 300 m² plot where the original house covers 60 m², extensions and outbuildings together may cover no more than 120 m², half of the other 240 m².
Going further: the larger home extension route
Para A.1(g) allows up to 8 m (detached) or 6 m, still single storey and no higher than 4 m, off article 2(3) land and SSSIs, if you first notify the council under para A.4. Neighbours get at least 21 days to comment; work may begin once the council says prior approval is not needed or gives it, or after 42 days with no decision. The fee is £249. See our larger home extension guide.
Conservation areas and other article 2(3) land
Article 2(3) land is a conservation area, a National Landscape (area of outstanding natural beauty), the Broads, a National Park, a World Heritage Site or an area specified under section 41(3) of the Wildlife and Countryside Act 1981 (Sch 1 Pt 1). There, a single-storey rear extension within 3 m or 4 m can still fall under Class A, but with no cladding in stone, artificial stone, pebble dash, render, timber, plastic or tiles, nothing beyond a side wall, and nothing of more than one storey beyond the rear wall (A.2). The larger route is not available. See our conservation area check.
Platforms, materials and conservatories
A raised platform is one more than 0.3 m high (para I), so raised decking or a terrace outside new doors above that height falls outside Class A. Exterior materials must look similar to the existing house, except for the materials of a conservatory (A.3(a)); a conservatory must still meet every size limit.
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)), an Article 4 direction or listed status can change the answer. 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.
What the free check tells you
On the free check, tap your house on the map. We test your plot outline against conservation areas, National Parks, National Landscapes, World Heritage Sites, SSSIs, Article 4 directions, listed buildings and Tree Preservation Orders, then show whether the larger route is open and which limits apply. It does not take measurements; where a council does not publish its data, we say so (how we check).
For certainty: a Lawful Development Certificate
For certainty, apply to your council for a Lawful Development Certificate for proposed works. Once issued, the works are conclusively presumed lawful unless something relevant changes before you start (TCPA 1990 s192(4)). The fee for an extension is £274, half the £548 application fee, with a decision due within 8 weeks of a valid application. See Lawful Development Certificates and planning fees 2026.