The law on householder permitted development rights changed in September 2013. The Welsh Government has produced guidance to help householders interpret the changes, see Planning: A Guide for Householders and External Solid Wall Insulation: A Planning Guide for Householders
The best way to find out is to use the Planning Portal Interactive House tool. This visual guide takes into account permitted development rights and explains the various limitations which apply (Make sure that you select the Welsh option as different rules apply in England). These rights do not apply to flats and maisonettes. Full planning permission is always needed for new dwellings.
You may need planning permission for a dropped kerb (e.g. access from a class ‘C’ road or above).
If the area between the dwelling and the road needs to be altered you’ll need permission from our Highways Department whether or not planning permission is required.