Class A side extensions: the half-width rule explained
The half-width restriction in Class A catches far more householder schemes than the rear projection limits, and it is the reason most wrap-around extensions need permission.
Under Class A of Part 1 of Schedule 2 to the General Permitted Development Order 2015, an extension that extends beyond a wall forming a side elevation of the original dwellinghouse has to satisfy three limits at once. It must be single storey. It must not exceed 4 metres in height. And it must not be wider than half the width of the original dwellinghouse.
That third limit is the one that ends most wrap-around schemes, and it does so for a reason that surprises people: the rear part and the side part are treated as a single enlarged part.
What the side elevation restriction covers
Paragraph A.1 of Class A sets out the circumstances in which development is excluded. Most householders know the rear projection rules, which allow 3 metres for a terraced or semi-detached house and 4 metres for a detached one, extended to 6 and 8 metres for single-storey extensions under the prior approval procedure.
Far fewer know that a separate set of limits bites as soon as anything goes past a side wall of the original house. Those limits are absolute. There is no prior approval route that relaxes them and no neighbour consultation scheme that buys extra width.
The practical reading is that a side extension under Class A is a modest, single-storey addition, roofed low, occupying no more than half the original frontage. Anything more ambitious is a planning application.
Half the width of the original dwellinghouse
Width is measured across the original dwellinghouse, perpendicular to the side elevation in question, and the government's technical guidance for householders sets out how it is taken. Two points do the damage.
The first is that "original" means the house as first built, or as it stood on 1 July 1948 for anything older. An extension your predecessor added in 1992 does not widen the baseline. It reduces your remaining allowance while contributing nothing to the measurement.
The second is that half of a narrow house is a very small number. A terraced house 5 metres wide gives 2.5 metres of permitted side extension, which after wall thicknesses leaves an internal width of a little over 2 metres. That is a utility room or a wider hallway, and not the side infill people usually have in mind.
| House width (original) | Maximum side extension width |
|---|---|
| 5 m | 2.5 m |
| 6 m | 3 m |
| 8 m | 4 m |
| 10 m | 5 m |
Why a wrap-around usually fails
A wrap-around extension fills the rear and the side return in one L-shaped addition. Because it is one enlarged part, and because part of it extends beyond the side elevation, the side limits apply to the whole enlarged part rather than to the side leg alone.
Illustrative example: a 1930s semi 7 metres wide, with a 3-metre rear extension and a 2.5-metre side infill built as one structure. The side leg on its own would be under half the 7-metre width. Measured across the whole enlarged part, the addition is 5.5 metres wide, comfortably over the 3.5-metre limit, and the works fall outside Class A.
Some designers try to separate the two elements physically, leaving a gap or a change in roof and wall alignment. Whether that produces two separate operations rather than one is a question of fact and degree for the local planning authority, and it is exactly the kind of question a certificate is designed to settle before anyone digs.
Measuring width on an irregular plan
Real houses are rarely rectangles. Bay windows, original outriggers, projecting porches and staggered terraces all complicate the measurement, and the technical guidance addresses some of them without covering every case.
Working rules that hold up in practice:
- Measure the original house, ignoring later additions, and check the measurement against the earliest plan you can find.
- An original rear outrigger on a Victorian terrace is part of the original dwellinghouse, so the side elevation you are measuring against may be the outrigger's flank rather than the main house's.
- Chimney breasts, gutters and eaves overhangs are not usually taken as part of the width, but a projecting original bay may be.
- Where the plan is ambiguous, the answer is a certificate rather than an opinion.
Height, eaves and the 2-metre boundary rule
The 4-metre height cap applies to the whole of the side extension, and a separate restriction takes over near boundaries. Where any part of the enlarged part sits within 2 metres of the boundary of the curtilage, the eaves height cannot exceed 3 metres.
Most side extensions are within 2 metres of a boundary, since that is the point of them. So the working envelope is eaves at 3 metres, a shallow pitched or hipped roof to a ridge below 4 metres, and no dormers or roof lights that alter the roof plane beyond what Class C allows.
Materials have to be of a similar appearance to the exterior of the existing house, and on article 2(3) land the side extension right disappears altogether. Article 2(3) land covers conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites, and in those areas any extension beyond a side elevation needs planning permission whatever its width.
Splitting the works into two applications
Where the design cannot be squeezed into Class A, there are two honest routes.
The first is a single householder planning application for the whole scheme. It costs a fee, takes around eight weeks, and lets you design the wrap-around properly rather than around a measurement. A short Planning Statement addressing the council's amenity policies, with a Design and Access Statement where one is required, is usually enough for a householder scheme.
The second is to build the compliant rear extension under permitted development and apply separately for the side element. This is legitimate where the two are separate operations in fact, and it is risky where they are not, because a council can take the view that a single scheme has been artificially divided. Get a certificate for the permitted development part first.
The order also matters for the 50 per cent curtilage test. Once the rear extension is built, the ground it covers counts towards the half of the curtilage that may be occupied by buildings other than the original house, and a large garage or outbuilding may already have used much of that allowance. Measure the curtilage before you commit to either route.
Evidencing the original dwellinghouse
The measurement is only as good as your evidence for the original footprint. Useful sources include historic Ordnance Survey mapping through a library or archive service, title deeds and conveyance plans, aerial photography, and the council's own planning history for the address, which will show any earlier permissions.
Then apply for a lawful development certificate for proposed use or development under section 192 of the Town and Country Planning Act 1990. It is the only formal way to confirm that works are lawful, it produces a document lenders and buyers accept, and it is far cheaper than removing an extension after an enforcement investigation.
To see which supporting documents a householder scheme at your address is likely to need, browse the report types or enter your postcode on PlanScope.
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.
Frequently asked questions
How wide can a permitted development side extension be?
No more than half the width of the original dwellinghouse, measured across the original house rather than the house as extended. It must also be single storey and no more than 4 metres in height. If the extension sits within 2 metres of a boundary, the eaves cannot exceed 3 metres either, which usually settles the roof design.
Why does a wrap-around extension need planning permission?
Because the rear and side elements form a single enlarged part of the dwellinghouse. Once any part of that enlarged part extends beyond a side elevation of the original house, the side restrictions apply to the whole of it, and a wrap-around is almost always wider than half the original house. The usual answer is a householder application.
Is the half-width measured from the original house or the current house?
The original dwellinghouse, meaning the house as first built or as it stood on 1 July 1948 if it is older. Extensions added by previous owners do not increase your allowance. Where a house was extended sideways in the 1980s, the width you measure against is the pre-1980s width, which catches people out regularly.
Can I build the rear extension under permitted development and apply for the side part later?
Sometimes, and the sequencing matters. If the rear extension complies with Class A on its own and is a separate operation in fact, it can be built under permitted development, with the side element applied for as a householder application. Take advice before starting, because a council may treat closely connected works as one scheme.
Does a side extension count towards the 50 per cent curtilage limit?
Yes. Class A also prevents development where the total area of ground covered by buildings within the curtilage, other than the original dwellinghouse, would exceed 50 per cent of the curtilage of the original dwellinghouse. Outbuildings, garages and previous extensions all count towards that figure.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.