You may need planning consent but for Building regulation purposes a garden shed may be exempt.
If it is located at least 1m from any boundary of your property then it is exempt if it is:
- under 30m² in internal floor area,
- single storey and detached,
- contains no sleeping accommodation
- If it is within 1m of your boundaries then it may still be exempt if, in addition to the above:
- it is constructed substantially of materials of limited combustibility
If a shed is under 15m² in internal floor area and contains no sleeping accommodation it is exempt.