You can perform certain types of work without needing to apply for planning permission. This is called 'permitted development'. All the information regarding your "permitted development rights" as well as a handy interactive house and guidance on common planning projects can be found on the Planning Portal.
If you do not need planning permission this is called "lawful development".
If you want to be certain that your proposal does not require planning permission you can apply for a certificate called a Lawful Development Certificate.
* It is not a requirement to apply for a lawful development certificate however it is advisable as it is formal confirmation that the works/changes have been lawfully completed. This would show up on any property searches