Side extensions are governed twice: by the national permitted development limits, which are about dimensions, and by local design guidance, which is about how the street looks. A proposal can satisfy the first and still be refused under the second, so both need reading before a design is fixed.
Permitted development side extensions: the national limits
Work beyond a side elevation is restricted in width relative to the original house, restricted in height, restricted to a single storey, and is not permitted at all on designated land or where it would extend beyond a principal elevation. Those are the dimensional tests. Where any one of them is failed, the route is a planning application rather than a certificate, and that is a normal outcome rather than a refusal.
2 storey side extension rules: two storey work to the side
A two storey side addition is outside the side extension rights and needs permission, and it is the shape councils scrutinise hardest, because a pair of semis extended to both boundaries turns a street into a terrace. The design answers that usually work are a setback from the front elevation, a roof that steps down below the main ridge, and a gap retained at first floor level toward the boundary.
Side extension planning permission: what local guidance adds
Most councils publish supplementary guidance for householder extensions, and it is far more specific than the national rules: how far to set back, what roof forms are acceptable, what happens to the front building line, and whether a terracing effect will be resisted. It is the document your application will actually be assessed against, so read it before drawing rather than after a refusal.
The boundary itself
Building to the boundary raises three separate questions. Whether the planning rules allow it, which is the guidance above. Whether the party wall procedure applies, which it usually does once excavation approaches a neighbour's foundations. And whether you will be able to maintain the new wall and its gutter without entering the neighbour's land, which is a practical problem to solve on the drawing rather than a legal one to argue later.
Questions people ask about side extension rules
How wide can a side extension be?
Permitted development limits the width in proportion to the original house. Anything wider is a planning application, which is routinely granted where the design suits the street.
Can a side extension be two storeys?
Not under the side extension rights. It needs permission, and the setback, roof form and boundary gap are what the case turns on.
Does the council have its own rules?
Almost always, as supplementary design guidance for householder extensions. It is what the officer assesses against.
Can gutters overhang the boundary?
Oversailing a neighbour's land is a private law matter and can be objected to. Design the eaves and drainage to stay within your own boundary.