If you are intending to extend your home, the first priority is to ensure that the work you do (if under permitted development) adheres strictly to the current legislation and more than that, the interpretation of the legislation by the local planning authority.
Permitted Development Rights (PD) are authorised in the format of General Development Planning Orders (GDPO's) which differ relevant to your location in the UK - England, Wales, Scotland or Northern Ireland.
A GDPO gives implied planning consent to carry out certain classes of development.
It is advisable therefore to ensure that plans are submitted to the local authority who will confirm as to whether the proposed alterations are within the relevant criteria or, are drawn up by a fully qualified chartered surveyor.