Owning a property on a freehold basis means you own not just the building itself, but the land it stands on. You are responsible for looking after everything to do with the property, from your possessions inside, to the walls and roof of the building’s outside structure.
As a result, if you want to make any changes to the structure of the property ‒ such as adding an extension ‒ then you are free to do so, as long as you have any necessary planning permission from the local council.