Answer
Do I need planning permission for a summerhouse?
Normally no, within the outbuilding limits (2.5 metres within 2 metres of a boundary).
In short
A summerhouse is treated as an outbuilding, so it is normally permitted development within the same limits as a garden room: single storey, behind the principal elevation of the house, and no more than 2.5 metres high within 2 metres of a boundary. Designated land and listed property are tighter.
A note
General guidance, not advice. Rules vary by site, and Article 4 directions or conditions can change what applies. Check with your local authority and the Planning Portal.
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