The height rules
An outbuilding within 2 metres of any boundary must be no more than 2.5 metres high overall. Further from the boundary, the limit is 4 metres for a dual-pitched roof and 3 metres otherwise. Eaves height is capped at 2.5 metres either way.
The 50% rule
All outbuildings, extensions and other structures together must not cover more than half of the land around the original house. On a small terrace garden this can bite.
'Incidental' use
A home office, gym, studio or store is incidental to the enjoyment of the house. A self-contained annexe with sleeping, cooking and bathing facilities used as separate accommodation is not, and needs planning permission.
Get the certificate
A lawful development certificate confirms in writing that the building did not need permission. Solicitors ask for it when you sell, and it costs a fraction of a planning application.