To find out whether you need planning permission you can check with the Planning Portal.
The information on the planning portal will advise on works which can be done without planning permission however this has no formal status in law and cannot be relied on at a later stage.
If formal confirmation is required, apply for a lawful development certificate for proposed use or development. If the decision of the lawful development certificate is that planning permission is required, an application for planning permission will need to be submitted and processed in the normal way.
Search maps online for further information on your property/site by entering your postcode and then tick the constraints box.
If your property pre-dates 1974, you may need to do a historic planning search.
If permission is not required you may still require Building Regulations approval and you may still be liable to pay the Community Infrastructure Levy (CIL) if the development is for a new dwelling or a residential or retail extension over 100sqm in gross internal floor area.
It is also worth checking your property deeds or transfer of title as there may be restrictive covenants in place which will restrict what you able to do regardless of planning legislation.