Side extensions: the width and single storey limits

Side extension permitted development width is capped at half the width of the original house, and a side extension has to be single storey with a maximum height of 4 metres. Those two limits in paragraph A.1 of Class A, Part 1, Schedule 2 to the General Permitted Development Order 2015 are what stop most side extension ideas before anything else does. On designated land, side extensions are excluded from Class A altogether, so a house in a conservation area or a National Landscape needs a planning application whatever the width.

Everything else about Class A still applies on top: the 50 per cent curtilage test, the eaves limit near a boundary, the materials condition.

The half width limit and how it is measured

The comparison is with the original dwellinghouse, which means the house as it was first built, or as it stood on 1 July 1948 if it is older. Extensions added by you or by previous owners do not count towards the width you measure against. A 1930s semi that already has a 2 metre side addition from the 1970s is still measured against the width of the 1930s house.

Width means the widest part, taken across the elevation rather than along the diagonal, and the extension's width is measured at its widest point including external wall thickness. The government's technical guidance for householders works through the measuring conventions, and it is worth reading before you draw anything, because the convention on bay windows, chimney breasts and existing projections is not obvious from the Order itself.

A worked illustration. An original semi measuring 5.4 metres across its front elevation gives a half width of 2.7 metres. Deduct a 300 millimetre cavity wall and the usable internal width of the side extension is under 2.4 metres. That is enough for a utility room or a widened hall. It is not enough for a side kitchen with a run of units on both walls, which is what most people have in mind when they ask the question. The dimensions here are illustrative.

Why side extensions must be single storey

Paragraph A.1 bars a side extension of more than one storey. There is no prior approval route around it and no larger allowance of the kind that applies to single storey rear extensions.

The reasoning is about the character of the street. A two storey side extension on a semi changes the pair into something that reads as a terrace, and on a detached house it can close the gap to the boundary that gives a street its rhythm. Councils refuse these on exactly that basis, so the restriction reflects how applications are actually decided.

That has a practical consequence for loft rooms. A side extension built under Class A cannot carry a first floor, but the roof over it can be brought into a loft conversion under Class B or C if those classes are separately satisfied. The two allowances are independent and each has to work on its own terms.

The four metre height ceiling

Maximum height is 4 metres, measured from the highest part of the roof to the natural ground level at the point nearest the external wall. On a sloping plot that measurement is taken at the lowest point of the ground adjacent, which can eat into the allowance faster than people expect.

Two further height rules bite more often than the 4 metre figure. The extension must not be higher than the highest part of the roof of the existing house, and its eaves must not be higher than the existing eaves. And if any part of the extension comes within 2 metres of the curtilage boundary, the eaves height is capped at 3 metres, which on a typical narrow side strip is the limit that actually controls the design.

A 3 metre eaves height with a pitched roof over a 2.4 metre wide extension gives a shallow pitch or a lean-to against the flank wall. That is why so many side extensions end up with a flat or near-flat roof: the geometry leaves little else.

Wrap-around extensions and the two allowances

A rear extension and a side extension built as one continuous enlargement is where the rules collide. The technical guidance treats the whole addition as a single enlargement that has to satisfy the side limits and the rear limits at the same time, which in practice almost never works. The wrapped corner extends beyond both the side elevation and the rear wall, so it fails the half width test the moment it reaches the depth you wanted at the back.

Building the two parts at different times does not fix it. The test looks at the house as enlarged, not at the sequence of works.

So a wrap-around is normally a planning application. That is not bad news. A householder application costs comparatively little, is determined in eight weeks, and lets you design to the site rather than to the Order. A Planning Statement setting out the design response and the neighbour amenity position does more for a borderline wrap-around than any attempt to squeeze it under permitted development.

Side extensions on designated land

Article 2(3) land covers conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites. On that land, Class A does not permit any enlargement extending beyond a wall forming a side elevation of the original house.

The exclusion is absolute on width. It is not a tighter limit. A 1 metre side infill on a terraced house in a conservation area needs permission in the same way a 3 metre one does. Article 4 directions can go further still and remove householder rights outside designated areas, which is common in the suburbs of historic towns, so check the authority's own constraint mapping rather than assuming the designation layer tells the whole story.

Listed buildings are a separate matter again. Permitted development rights and listed building consent are different consents, and works to a listed building need consent whether or not the Order permits them.

Designing to the limit without a stepped plan

Where the half width rule leaves an awkward strip, four approaches do more than arguing about measurement:

1. Combine a narrow side extension with a single storey rear extension kept within the rear depth limits, accepting the corner is left open rather than wrapped.
2. Use the side strip for circulation, a utility or a shower room, and put the living space at the rear where the depth allowance is larger.
3. Take the full width you want and apply for permission, using the eight week householder route rather than designing around a limit that does not suit the plot.
4. Put the additional floorspace in an outbuilding under Class E, remembering the 50 per cent curtilage test applies to everything other than the original house.

Whichever route you take, the hard surfacing that goes with a side extension has its own rules, and the drainage condition attached to Class F hard surfaces catches people who have thought carefully about the building and not at all about the path beside it.

If you are relying on permitted development, the only formal way to confirm the works are lawful is a certificate of lawfulness of proposed use or development under section 192 of the Town and Country Planning Act 1990. A council officer's email is not that certificate, and a future buyer's solicitor will ask for one.

Enter your postcode on PlanScope to see which documents a side extension 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.