Glossary
Infill development on tight and awkward London plots
Building in a gap between existing buildings, typically the side passage.
Building in a gap between existing buildings, typically the side passage.
An infill extension fills the gap left by the original side passage of a Victorian or Edwardian terrace, between the back addition and the boundary. It is the most common London ground-floor extension, typically 1.5 to 2.5 m wide, and it usually falls within permitted development when built alone.
It fills the side return alongside the original rear projection. Alone it can be permitted development at up to half the original house width; combined with a rear extension it becomes one enlargement under Class A and the width test then fails.
| Side infill alone | Infill plus rear extension | |
|---|---|---|
| Permitted development | possible under Class A | no — treated as one enlargement |
| Width limit | half the original house | combined width fails the test |
| Height limit | 4m, single storey | 4m |
| Eaves within 2m of a boundary | 3m | 3m |
| Inside a conservation area | not permitted development | not permitted development |
| Application fee if needed | £548 | £548 |
Source: the Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class A, as amended, read at 1 September 2026; fees from the Fees for Applications Regulations 2012 as amended, 2026 rates. A side infill on a Victorian terrace is the commonest London scheme that starts as permitted development and becomes an application the moment the rear extension is added to it.
The back addition — the narrower rear wing containing the original scullery and outside WC — was built narrower than the main house so that both it and the neighbour's could have windows. That leaves a strip of dead external space, usually damp, north-facing and used for bins.
Filling it converts an unusable strip into kitchen or dining space and, more importantly, allows the rear of the house to be reorganised into a single wide room rather than two narrow ones.
An infill alone is an extension beyond a side wall, so it must be single storey, no wider than half the width of the original house, no higher than 4 m, and — crucially — the whole rear extension must not exceed 3 m depth for an attached house or 4 m for a detached one, measured from the original rear wall.
Because most infills are combined with a rear extension into a wraparound, the combined footprint frequently exceeds these limits and a householder application at £548 becomes the route.
Rooflights are almost always needed, because the infill is bounded by two walls and gains little side light. A steel goalpost frame usually replaces the back addition flank wall. The boundary wall is normally a party wall, so a party structure notice is served with one month's notice, and any new foundation within 3 m of the neighbour's triggers a section 6 excavation notice at two months.
Infill extension in a Ealing context: London Borough of Ealing (Ealing Council) runs 30 conservation areas including the Ealing Broadway Conservation Area and the Pitshanger Conservation Area, and an Article 4 direction is in force there, so permitted development cannot be assumed. That decides how infill extension is handled: £548 and 8 weeks for a Ealing householder application, or £274 for the Lawful Development Certificate that records infill extension as lawful. The law behind infill extension sits in the Town and Country Planning Act 1990, the GPDO 2015 and the Building Regulations 2010.
| Typical width | 1.5–2.5 m |
|---|---|
| Depth limit, attached house | 3 m from original rear wall |
| Depth limit, detached house | 4 m |
| Maximum height | 4 m, or 3 m within 2 m of a boundary |
| Party structure notice | One month |
| Householder application fee | £548 |
Alone, usually yes. Combined with a rear extension it often exceeds the depth limits.
Not if rooflights are designed in. Glazing to the roof does far more than a rear elevation window.
Almost always, because the boundary wall and new foundations engage the Act.
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