Do I need planning permission for a garden room?
When a garden room, shed or summerhouse counts as permitted development, the key size limits, and when to check with your Oxfordshire council.
Often not — if it meets permitted development rules
Outbuildings such as garden rooms, sheds and summerhouses are usually 'permitted development' and don't need planning permission, as long as they meet a set of limits published by the Planning Portal.
The main limits
It must be single storey, with a maximum eaves height of 2.5 metres.
Maximum overall height of 4 metres with a dual-pitched roof, or 3 metres for any other roof. Within 2 metres of a boundary, the whole building must be no higher than 2.5 metres.
It can't be in front of the principal elevation facing a road, and outbuildings together can't cover more than half the land around the original house.
It can't include a balcony, veranda or raised platform over 0.3 metres high, and can't be used as separate living accommodation (a bedroom you sleep in).
When the rules are stricter
Listed buildings, conservation areas and Areas of Outstanding Natural Beauty (the Cotswolds and Chilterns cover parts of Oxfordshire) have tighter limits. If you're unsure, contact your district council: Oxford City, Cherwell, West Oxfordshire, South Oxfordshire or Vale of White Horse.
Building Regulations are separate from planning and may still apply, especially for larger rooms or ones with sleeping accommodation.
Sources
General guidance only — check official sources for your situation.