Many kinds of buildings and structures can be built in your garden without the need to apply for planning permission. These can include sheds, garages, greenhouses/potting sheds, accommodation for pets and domestic animals, summer houses, swimming pools, ponds, sauna cabins, enclosures (including tennis courts) and many other kinds of structure.
In the first instance we suggest you make a visit to your local planning department where they will be able to advise if you would be permitted to build a structure in your garden.
In a terraced house or any house in a Conservation Area, National Park, an Area of Outstanding Natural Beauty or the Broads, the volume of the original would be increased by more than 10% or 50 cubic metres whichever is the greater.
For any other kind of house, outside those areas, the volume of the original house would be increased by more than 15% or 70 cubic metres (whichever is the greater). In any case, the volume to the 'original house' would be increased by more than 115 cubic metres. Volume is calculated from external measurements.
If an extension to your house comes within 5m of another building belonging to your, the volume of that building counts against the allowance for additions and extensions.
Any building which has been added to your property and which is more than 10 cubic metres in volume and which is in 5m of your house is treated as an extension of the house and so reduces the allowance for further extensions without planning permission.
If you are concerned about a legal problem involving planning you should get professional advice or ask your local planning department.