Side extensions under Class A: width and height limits

A single storey side extension can be permitted development, but three limits apply at once. It must be single storey, no more than 4 metres high, and no wider than half the width of the original dwellinghouse. All three come from paragraph A.1(j) of Class A in Schedule 2 Part 1 to the Town and Country Planning (General Permitted Development) (England) Order 2015, and failing any one of them means a householder planning application.

On designated land, which covers conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites, side extensions are excluded from Class A altogether. If your house sits in a conservation area, stop reading the limits and start preparing an application.

Where the side extension limits sit in Class A

Class A permits the enlargement, improvement or other alteration of a dwellinghouse, then takes most of it back with a long list of exclusions. The side extension exclusions are grouped together, and they bite only when the enlargement extends beyond a wall forming a side elevation of the original dwellinghouse.

That phrase does a lot of work. Original dwellinghouse means the house as it was first built, or as it stood on 1 July 1948 if earlier. A side extension built by a previous owner in 1995 is not part of the original dwellinghouse, so you cannot measure your half width against it, and its footprint counts against you elsewhere in the Order.

Several other Class A limits apply to side extensions in parallel. The overall height cannot exceed the highest part of the existing roof. The eaves cannot exceed the eaves of the existing house. Where any part of the extension is within 2 metres of the curtilage boundary, the eaves height is capped at 3 metres, which on a side extension next to a boundary is usually the binding constraint rather than the 4 metre overall figure. And nothing may extend beyond a wall that fronts a highway and forms a side elevation of the original house, which is covered in more detail in the piece on the elevation fronting a highway.

The half width restriction and how it is measured

Half the width of the original dwellinghouse. Simple to state, argued over constantly.

The width is measured across the original house, not across the house as extended, and not including any later additions. For a plain rectangular plan that is straightforward. For a 1930s semi with a projecting front bay, a rear outrigger and a later garage, it is not, and the government's technical guidance on householder permitted development rights is the document officers reach for when deciding how to measure.

The width of the extension is measured at its widest point, including any projecting elements, and the reading that officers apply is the external face of the new wall. A 3.6 metre wide extension on a 7.2 metre house is at the limit, so a 100 mm render build-up added later takes it over.

Two practical traps. First, people measure the house across the front including the bay, which inflates the permitted width and produces an unlawful extension. Second, on a semi-detached pair people measure across both houses. The original dwellinghouse is your house.

Single storey only, and what that excludes

A.1(j) permits a side extension of a single storey only. There is no two storey side extension under Class A, at any width, anywhere.

That rules out the common aspiration of extending over an existing side garage to make a fourth bedroom. It also rules out a room in the roof of the side extension where that room makes the extension more than a single storey, which is a question about the building rather than about how the space is described on the drawings. A side extension with a habitable room above it is two storeys regardless of the label.

Rooflights in the side extension roof are a different matter and generally acceptable, provided the roof itself stays within the height limits. A dormer in that roof is not, because it takes you into the territory of a second storey and, for the main roof, into Class B.

Height limits at the eaves and overall

Three heights matter and they interact.

The overall height of the side extension cannot exceed 4 metres. Where the extension is within 2 metres of a boundary, the eaves cannot exceed 3 metres. And the extension cannot be taller than the highest part of the existing roof or have eaves higher than the existing eaves.

Heights are measured from the highest part of the surface of the ground next to the building. On a sloping site that reference point moves, and a flat roof that measures 3.9 metres at the front of a falling site can measure 4.2 metres at the back. Measure at the worst point and show the levels on the section drawing.

A pitched roof side extension within 2 metres of the boundary is the hardest combination to achieve. The 3 metre eaves cap forces a low wall plate, and a pitch steep enough to look right then pushes the ridge over 4 metres. Most of these end up with a shallow hipped roof, a flat roof behind a parapet, or a planning application.

Wrapping around to the rear

A side extension that continues round the back is treated as one enlargement, so it has to satisfy the side elevation limits and the rear extension limits that apply to its respective parts. In practice that combination is hard to achieve, because the half width restriction and the rear projection limits rarely line up with the shape anyone actually wants.

Most wrap-arounds go in as householder applications, and that is often the better outcome anyway. A single application for the whole thing avoids the risk of building half under permitted development, discovering a problem, and having to apply retrospectively for the rest. The design argument also gets easier, because you can justify the proportions on their merits rather than reverse-engineering them from a set of numbers.

Note that the 50 per cent curtilage limit in Class A counts all buildings in the curtilage other than the original house, so an existing outbuilding can eat the allowance. The piece on the 50 per cent curtilage rule sets out how that calculation runs.

Common measurement mistakes

The ones that produce enforcement problems, in rough order of frequency:

1. Measuring half width against the house as already extended rather than the original dwellinghouse.
2. Ignoring the 3 metre eaves cap where the extension sits within 2 metres of a boundary.
3. Measuring height from the lowest ground level rather than the highest ground next to the building.
4. Treating a room in the side extension roof as still single storey.
5. Forgetting that the side extension is excluded entirely on designated land.
6. Overlooking the materials condition, which requires the exterior to be of a similar appearance to the existing house.
7. Adding a side-facing window at first floor level without the obscure glazing and restricted opening the conditions require.

Any hard surfacing or drainage alteration that comes with the extension is a separate question, and front garden work sits under a different class, as set out in the article on front garden paving and permeable surfaces.

When a full application is the better route

If the scheme needs more than half the width, two storeys, a boundary position with a proper pitched roof, or sits on designated land, apply. A householder application is determined in eight weeks, the fee is modest against build cost, and a permission removes the risk entirely.

Where the works do sit within Class A and you want certainty, a lawful development certificate for proposed use or development under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that works are lawful. A planning officer's informal email is not a defence. For anything marginal on width or height, the certificate is worth the fee.

A Planning Statement explaining how the design responds to the street and the neighbouring house makes a householder application easier to recommend for approval.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.