How is half the width measured?

Across the original house, wall to wall, as it stood in 1948 or as first built. A house 8 m wide can have a side extension up to 4 m wide.

Can I build a side extension on a corner plot?

Only if it doesn't sit forward of a side wall that faces the road. On most corner plots that rules it out, so a planning application is needed.

Can I build a side return extension on a terrace?

A side return infill on a Victorian terrace is usually treated as a side-and-rear extension, so it must meet both the half-width and the rear-depth limits. Many do — check yours with your address.

Side extension without planning permission

The limits

House type	Max width	Max height
Detached	Half the house	4 m
Semi-detached	Half the house	4 m
End of terrace	Half the house	4 m
Flat or maisonette	Not permitted	Not permitted

Other conditions

Single storey only.
No higher than 4 m, and eaves no higher than 3 m within 2 m of a boundary.
No wider than half the width of the original house.
Not forward of the front wall, or of a side wall that faces a road.
Materials similar in appearance to the existing house.
Together with other buildings, no more than half the garden covered.

When you can’t use permitted development

Flats & maisonettes
No householder permitted development rights at all.
Listed buildings
Listed building consent is needed for almost any change.
Conservation areas
Not permitted development — needs planning permission.
Article 4 directions
Your council may have removed this right on your street.
Planning conditions
An old permission may have removed permitted development rights. Not published anywhere.
New builds
Often have permitted development removed by condition. Check your deeds.

Your exact limits

House type, plot size, Article 4 and trees — worked out for your address.

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Wrap-around extensions

An extension that goes both to the side and the rear is judged as a whole. Because the side part must be single storey and half-width, and the rear part must meet the rear limits, most L-shaped wrap-arounds need planning permission — the rear-facing part of a side extension usually breaks the depth limit on its own.

Side extension rules by council

Some councils have Article 4 directions that change these limits. Pick yours to see what applies locally.

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Common questions

How is half the width measured?: Across the original house, wall to wall, as it stood in 1948 or as first built. A house 8 m wide can have a side extension up to 4 m wide.
Can I build a side extension on a corner plot?: Only if it doesn't sit forward of a side wall that faces the road. On most corner plots that rules it out, so a planning application is needed.
Can I build a side return extension on a terrace?: A side return infill on a Victorian terrace is usually treated as a side-and-rear extension, so it must meet both the half-width and the rear-depth limits. Many do — check yours with your address.