To some degree this depends on the size of the job you have in mind.
Unless the work is of a very minor nature some statutory approvals such as planning or building
regulation approval will be required. The development control department at your local council offices
will be able to help on this. They will also advise on whether or not grants are available. You can
then begin to assess the need to appoint a professional such as an architect or surveyor to carry out
design or if you are going direct a builder.
If the house is still under any scheme such as NHBC Buildmark
you must inform the company providing the warranty.
Your mortgage and insurer should also be advised of the work. You should inform neighbour's and you must
be sure of your land boundaries, any rights of way particularly close to boundaries and if any rights of
light from neighboring property have been acquired. Your builder cannot help here and if this is found
out after the agreement is signed and work is prevented you could be liable for delay costs.
If in any doubt seek professional advice, it could be cost be very cost effective.