You only need planning permission if there is a significant change of size, style or use. Permitted development allows you to make changes up to a certain size, it’s good to know the basic rules as an extra few centimetres can make a big difference.
The Planning Portal website homepage has a guide that shows what is allowed.
Anything affecting boundaries, available light, neighbouring structures and party walls have added restrictions. An architect will be able to help with all of this but is only really necessary if you are making a statement, want to use new technology, have a restricted space or other complications.
If you are replacing existing, without changes, then you don’t need permission but it still needs to be built to the current building regulations. A good builder will know how to achieve this without needing the specifications laid out for them.
Or you could just call Kev, I wouldn’t.