You can perform certain types of work without needing to apply for planning permission. This is called 'permittted 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 also 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
Is your property privately owned?
You will need to look on your deeds. If you have a mortgage, your deeds are usually with your bank, building society or mortgage provider. Alternatively, you can contact the Land Registry on 01733 288288. There is a charge by the Land Registry for obtaining this information.
Is your property council owned?
If Yes - The Housing or Legal departments may be able to help.
If you have any concerns over your boundary being incorrect please contact Land Registry.