Class A side extensions: single storey and half the width

Under Class A of Part 1, Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015, a side extension has to be single storey, no more than 4 metres high, and no wider than half the width of the original dwellinghouse. That is the class A side extension two storey rule in short: there is no two storey side extension under permitted development, at any width, on any plot. The restriction sits in the paragraph of A.1 dealing with extensions beyond a side elevation, and the lettering has shifted with amendments, so cite it from the consolidated order on legislation.gov.uk rather than from memory.

Architects hit this clause late, usually when a client has already seen a sketch with bedrooms over the new ground floor.

What the clause says and why designs fall foul of it

The paragraph bites when the enlarged part extends beyond a wall forming a side elevation of the original dwellinghouse. If it does, three limits apply at once, and failing any one of them takes the whole extension out of Class A:

• more than a single storey
• a height exceeding 4 metres
• a width greater than half the width of the original dwellinghouse

There is no balancing exercise and no fallback. A 1.9 metre wide side extension that happens to be two storey is as far outside Class A as a full width one.

A separate restriction applies on article 2(3) land, which covers conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites. On that land, extensions beyond a side wall of the original dwellinghouse are excluded from Class A altogether, so the width and storey test never arises. Check the designation before you measure anything.

Width measured against the original dwellinghouse

The comparator is the original dwellinghouse, which means the building as it stood on 1 July 1948, or as built if later. It is not the house as it stands today.

That distinction decides cases. A 1960s semi with a 1990s rear extension and a converted garage still has to be measured against the 1948 footprint, so the useful question for a client is not how wide the house is but how wide it was. Evidence matters here: historic mapping, the deeds, earlier planning permissions on the authority's online register, and aerial photography.

Two further points catch people out. Width is measured across the elevation, not along the depth of the extension, and a projecting original bay or chimney breast can change the figure you are halving. Where the original house was not a simple rectangle, measure the width of the wall forming the side elevation that the extension goes beyond, and record how you measured it in a note on the drawing.

Height limits on a side extension

Two height controls run together. The side extension paragraph caps the overall height of the enlarged part at 4 metres. Separately, a paragraph earlier in A.1 caps the height of the enlarged part at the height of the highest part of the roof of the existing dwellinghouse, and the eaves height at the eaves height of the existing dwellinghouse.

Then there is the boundary rule. Where any part of the enlarged part is within 2 metres of the boundary of the curtilage, the eaves height of that part must not exceed 3 metres. Side extensions are almost always within 2 metres of a flank boundary, so in practice 3 metres at the eaves is the figure that governs the design, not 4 metres overall.

That combination is why so many side extensions end up with a low eaves and a pitched roof sweeping up to a ridge below 4 metres. It is not an aesthetic choice. It is the only shape the clause allows on a tight plot.

Wrap-around extensions and the combined effect

A wrap-around, extending beyond both the rear wall and a side elevation as one enlarged part, is generally not permitted development. The side element has to meet the side paragraph, including the half-width test, and the rear element has to meet the rear depth limits, and the enlarged part has to satisfy both simultaneously as a single piece of development.

In most terraced and semi-detached cases it cannot. The side return is wider than half the original house once it wraps across the back, or the rear projection exceeds the depth allowed for a single storey rear extension, and the whole thing needs a householder application.

Where a client wants the wrap, the honest advice is to stop testing it against Class A and design it properly against the local plan and any householder design guide. You will usually get a better building and a cleaner consent.

Designing a compliant side and rear combination

It can be done, but only where the two elements read as separate pieces of development and each stays within its own limits. The practical approach is:

1. Establish and evidence the original dwellinghouse footprint and width
2. Confirm the site is not on article 2(3) land and that no Article 4 direction removes Class A locally
3. Size the side element at half the original width or less, single storey, eaves at or below 3 metres within 2 metres of the boundary
4. Size the rear element within the single storey depth limit for the house type, with the roof no higher than 4 metres
5. Check the 50 per cent curtilage coverage test across all additions since 1948
6. Check materials, which must be of similar appearance to the existing house, and avoid balconies, verandas and raised platforms above 300 millimetres

Step two is the one people skip. An Article 4 direction can remove householder permitted development rights across a whole area, and the direction's own schedule defines what has gone. Read it, do not assume.

When to abandon the right and apply

Permitted development is a fallback position, not a prize. Where the design the client wants needs two storeys on the flank, or more than half the original width, or sits on designated land, a householder application is the route, and the policy tests are about character, amenity and overlooking rather than arithmetic.

Two things make that application easier. A short Planning Statement that sets out the local plan policies and addresses the obvious objections, and a careful treatment of flank windows, since side facing openings are where neighbour objections concentrate. The condition that usually follows is dealt with in upper floor side windows and the obscure glazing condition. Garage conversions often sit alongside a side extension scheme, and the parking consequence is set out in garage conversions and the parking condition that follows. Window and door changes within the existing walls raise their own questions, covered in replacement windows and doors: when they are development.

If you want certainty that a side extension falls within Class A, the only formal route is a lawful development certificate under section 192 of the Town and Country Planning Act 1990 for proposed works. A planning officer's email is helpful, and it binds nobody.

For a householder application, PlanScope drafts the Planning Statement and Design and Access Statement from a postcode and a short set of answers, with a TO BE CONFIRMED flag wherever a site fact is missing. Enter your postcode on PlanScope to see what the application needs.

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.