Side extension width and the half the original house rule

A single-storey side extension under Class A of the General Permitted Development Order cannot be wider than half the width of the original dwellinghouse. That is the rule most householders trip over, and the trap is the word original: the comparison is with the house as first built, or as it stood on 1 July 1948 if it is older, not with the house as it stands today. Side extension permitted development width is therefore a question of history as much as of measurement.

A 1930s semi that gained a side garage in 1985 has already used the allowance. Nobody tells the current owner that until the certificate is refused.

What the half width restriction measures

Class A restricts any part of the enlarged dwellinghouse that extends beyond a wall forming a side elevation of the original dwellinghouse. Such an extension is permitted only where it has a single storey, a height of no more than 4 metres, and a width no greater than half the width of the original dwellinghouse.

All three limits apply together. Meeting the width and failing the height takes you outside Class A just as effectively as the reverse.

Width is measured across the widest part of the proposed extension and compared with the width of the original house. Where the original house is not a simple rectangle, measure across its widest point and say in the application how you did it. A projecting bay, a chimney breast or an original single-storey rear wing all affect what the width of the house is, and reasonable people disagree about them.

Why the original dwellinghouse is the baseline

The original dwellinghouse is the building as first built, or as it stood on 1 July 1948 for anything older. Every subsequent addition counts, regardless of who built it, when, or whether it needed permission at the time.

So the calculation runs in two steps. Establish the width of the original house. Then subtract any existing side addition from the half-width allowance before working out what is left.

Evidence is the practical problem. Historic Ordnance Survey editions, mid-century aerial photography, the deeds, old sale particulars and council tax or rating records all help. Where a house has been extended more than once, build a schedule with an approximate date and footprint for each addition and include it with the application. And demolishing an earlier side extension does not restore the allowance, because the baseline is fixed by history rather than by what stands today.

Height limits on a side extension

Two height figures matter.

The overall height of a side extension cannot exceed 4 metres, and it cannot have more than one storey. Separately, where any part of the extension is within 2 metres of a boundary of the curtilage, the eaves height cannot exceed 3 metres. Most side extensions on an ordinary suburban plot sit within 2 metres of the flank boundary, so the 3 metre eaves limit is usually the binding one.

Roof form follows from that. A shallow pitch or a flat roof with a parapet often works within the limits where a pitched roof matching the house does not, which is why so many permitted development side extensions look subordinate whether or not the designer intended it.

Wrapping a side and rear extension together

A wrap-around extension, running along the side and returning across the rear, is the design most householders want and the one Class A handles worst.

Each part has to satisfy the limits that apply to it. The side element is tested against the half-width, single-storey and height rules. The rear element is tested against the rear projection limits and the eaves rules. Where the two meet, the combined form usually breaches something, most often by producing a side element wider than half the original house or by exceeding the rear projection where the return sits.

Treat a wrap-around as a full planning application from the start unless the geometry is unusually forgiving. It is faster than applying for a certificate, being refused, and then applying anyway. Our note on rear extensions on a corner plot and the side boundary covers the related problem where a flank wall fronts a highway.

Designated land and the loss of side extension rights

On article 2(3) land the side extension right disappears entirely. That covers conservation areas, National Parks, the Broads, National Landscapes and World Heritage Sites.

In those areas no part of the enlarged dwellinghouse may extend beyond a wall forming a side elevation of the original dwellinghouse under Class A. Cladding is also excluded, and rear extensions of more than one storey are not permitted. A side extension in a conservation area is a planning application, full stop.

Check for an Article 4 direction on top of that. Directions withdrawing Class A rights over particular streets are common, and they are not always obvious from the council's mapping.

Designing a subordinate side addition

Where a full application is needed, councils apply a consistent set of design expectations to side extensions, usually set out in a householder design guide.

Set the front face back from the principal elevation, commonly by a metre or so, so the original frontage still reads as the front of the house. Drop the ridge or eaves below those of the host dwelling. Keep the width visibly less than the original house. Match materials and window proportions, and carry the original details such as cills, heads and verge treatment across. Retain a gap to the boundary where the street has a rhythm of gaps, because filling the last gap in a terrace of semis is the change officers resist most.

Parking is the other recurring issue. A side extension built over a driveway removes an off-street space, and councils with an adopted parking standard will ask where the displaced car goes. Where the street is already at capacity, that question can decide an otherwise acceptable design, so show the retained parking on the site plan with dimensions rather than leaving it to be inferred.

Where the house is listed or in a conservation area, the design case has to be made in a Heritage Statement proportionate to the significance of the building, and the gap between houses is often part of what makes the area special.

Evidence a council will want

For a lawful development certificate under section 192, submit a dimensioned existing site plan showing the original house outline and every later addition, a proposed plan and elevations with the width and height dimensions marked, the historic evidence establishing the original footprint, and a short written analysis applying each limit in turn.

For a full application, the drawings matter more than the words, but a short Planning Statement explaining the subordination of the design and the relationship to the street saves the officer guessing at your reasoning.

But get the original footprint pinned down before you draw anything. Everything else in Class A follows from it.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. A lawful development certificate is the only formal way to confirm that works are lawful.

You can see which supporting documents a householder application needs from your postcode. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.