You do not  need planning permission for your garden room..if , 

 1. No plumbing , taps or toilet in the Room.

2. The garden Room is not used as a bedroom room.

3. Is not above 6 m's in height with pitched room or 3 ms if a flat room is used.

4. The building does not exceed more than 25 sq meters.

5. The building leaves 25m square left in the back garden.

Below is link to the local planning authorities webpage  with regards to planning .Note we ourselves will always check the requirements if there is any doubt over the legality of planning your garden room .