The following is a brief guide and it applies to England and Wales, for Scotland and Ireland it is advisable to contact your local authority.

You will need to apply for planning permission if:

1a For a Terraced House (including end of terrace) - the volume of the original house would be increased by more than 10% or 50 cubic metres (whichever is the greater).

1b For any other kind of house, the volume of the original house would be increased by more than 15% or 70 cubic metres (whichever is the greater).

1c Or in any case, the volume of the original house would be increased by more than 115 cubic metres. (The wording 'original house' means the house as it was first built or as it stood on 1st July 1948 if it was built before then. If extensions have been built since, then those extensions count against the allowance.)

2 The conservatory would be nearer to any highway than the nearest part of the original house, unless there is 20 metres or more between your house (including the conservatory) and the highway. (The term 'highway' includes roads, footpaths, bridleways and byways if there are public rights of way.)

3 More than half the area of land around the original house would be covered by additions or other buildings.

4 The conservatory is higher than the highest point of the roofline of the original house.

5 Any part of the conservatory comes within 2 metres of your boundary if it is more than 4 metres above the ground.

6 In the following cases, the volume of other buildings which belong to your house (such as garage or shed) will count against the volume allowances. In some cases, this can include buildings which were built at the same time as the house or existed on 1st July 1948:
(i) If an extension to your house comes within 5 metres of another building belonging to your house, the volume of that building counts against the allowance for additions and extensions.
(ii) Any building which has been added to your property and which is more than 10 cubic metres in volume and which is within 5 metres of your house is treated as an extension of the house and reduces the allowance for further extensions.

7 If your house is a listed building or is in a Conservation Area, National Park, an area of Outstanding Natural Beauty or the Broads then permission is likely to be required.