Planning PermissionThe rules that apply if your loft is to fall within permitted development, can be summarised as follows:
(i) No part of the dwelling house would as a result of the works, exceed the height of the highest part of the existing roof.
(ii) No part of the dwelling house would, as a result of the works, exceed beyond the plane of any existing roof slope which fronts a highway. A highway is not defined but includes all public roads, most footpaths, bridleways and byways. Your local authority could give further advice as to the classification of any such access way.
(iii) Would as a result of the development in its own right exceed 50 cubic metres in the case of a terraced house, or 70 cubic metres in any other case.
(iii) Would as a result of the development when including other extensions to the dwelling house exceed 50 cubic metres or 10% of the original dwelling whichever is the greater in the case of a terraced house, or 70 cubic metres or 15% of the original dwelling whichever is the greater in any other case.
(iv) The maximum total of all developments is limited to 115 cubic metres; your dwelling would have to be unusually large to fall within this category.
If your proposals do not fall within the above categories this does not mean you cannot have your conversion but you would require Planning Permission. Your local authority may well have published guidelines as to what is and what is not acceptable when considering a loft conversion. It could be in your best interest to discuss your proposal informally with your planning department so as not to waste time and money upon unrealistic proposals. Just because other dwellings within your locality have loft conversions does not automatically mean you will be granted permission. It may be that they were constructed as permitted development but the design being contrary to council policy.