It depends what its for, rather than what it is made of. If it is a shed, a summer house, a sauna etc. then its OK unless ANY of the following apply. It would be nearer to any highway than the nearest part of the "original house", unless there would be at least 20 metres between the new building and the highway. The term "highway" includes public roads, footpaths, bridleways and byways. More than half the area of land around the "original house" would be covered by additions or other buildings. It is for non-domestic purposes, for example, for parking a commercial vehicle, running a business or for storing commercial goods. It would be more than 3 metres high (or 4 metres high if it has a ridged roof). If your house is a listed building, or you live in a Conservation Area, a National Park, an Area of Outstanding Natural Beauty, or the Broads, and you want to put up a building or structure with a volume of more than 10 cubic metres.