To be a permitted development your building needs to:
have less than 30m 2 of internal floor area
have a height limitation of 2.5m for a flat roof and 4m for a pitched roof, if your building is situated within 2m of any boundary (we can increase the height of your building to 3m if more than 2m inside the boundary)
take up no more than 50% of the garden (we have to take into account other garden structures, sheds, pool rooms, etc. to ensure compliance)