Glossary
Eaves setback and the 20cm rule on London loft conversions
The horizontal distance a roof extension must be set back from the eaves to qualify as Permitted Development. Typically 20cm.
The horizontal distance a roof extension must be set back from the eaves to qualify as Permitted Development. Typically 20cm.
A setback is the distance a building element is held back from a boundary, a building line or an existing face. Setbacks appear throughout permitted development — the 200 mm dormer set-back from the eaves, the 20 cm inset from party walls — and throughout conservation area design guidance.
Class B requires a roof extension to be set back at least 200 mm from the original eaves, measured along the roof slope. Extensions must not extend beyond a wall forming the principal elevation and fronting a highway. Outbuildings must not sit forward of the principal elevation.
Two-storey rear extensions must be at least 7 m from the rear boundary, which is the setback that most often defeats a two-storey scheme on a short London garden.
Most boroughs expect a side extension to be set back from the front building line — commonly 1 m — and set down from the main ridge, so that the extension reads as subordinate. Roof extensions in conservation areas are often required to be set back from the front roof slope entirely, and dormers set in from party walls and up from eaves by more than the statutory minimum.
It is the organising principle of London householder design guidance: an addition should be legible as an addition. Setback, set-down and a change of plane are the three devices that achieve it, and a scheme that uses them is far easier to approve than one that continues the main elevation flush.
A worked Tower Hamlets example of setback: with 58 conservation areas — the Spitalfields Conservation Area and the Whitechapel Market Conservation Area among the best known — London Borough of Tower Hamlets (Tower Hamlets Council) sees setback constantly, and an Article 4 direction is in force there, so permitted development cannot be assumed. Where setback pushes a Tower Hamlets scheme out of permitted development, the route becomes a £548 application decided in 8 weeks rather than a £249 prior approval in 6 weeks.
| Dormer set-back from eaves | 200 mm minimum |
|---|---|
| Two-storey rear from boundary | 7 m minimum |
| Side extension front setback | Typically 1 m by guidance |
| Outbuildings | Not forward of the principal elevation |
| Purpose | Subordination of the addition |
| Source | GPDO Class A and B, plus borough design guides |
Class B requires it, measured along the roof slope from the original eaves.
Not by statute, but almost every London borough's design guidance expects it.
Holding the extension roof below the main ridge so it reads as subordinate.
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