Answer
Is a garden room under 15 square metres exempt from building regulations?
Usually yes, if it is under 15 square metres with no sleeping accommodation.
In short
Usually yes. A detached garden room with a floor area under 15 square metres and no sleeping accommodation is normally exempt from building regulations, with no restriction on how close it sits to a boundary. Between 15 and 30 square metres it stays exempt only if it is 1 metre from any boundary or built of non-combustible materials. The electrics must always comply with Part P.
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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