Do I need planning permission?

Usually not. Most outdoor buildings of this kind are permitted development, as long as the building is single-storey, sits behind the front wall of the house, covers no more than half the land around the original house, and stays inside the height limits. Those are 4m for a dual pitched roof, 3m for anything else, and 2.5m if any part is within 2m of a boundary. Conservation areas, listed buildings and flats follow different rules.