Planning permision for loft conversions

Building Contractors Hampshire: "What permission is Required?

If the building is listed or situated in a conservation area listed building consent and planning permission may be required.

Houses not listed or located in a conservation area can be extended up to a particular volumetric limit without planning permission being required. However, if a dormer is or dormers are being formed whose volume, once the volume of any previous extensions to the property is added, is likely to exceed that permitted, planning permission is required. Dormers must not project above the ridge."