Its not that hard.
If you build something you either need planning permission or you don't because its exempt.
If you need planning permission, you must build what you have permission for ( you would be amazed at the number of people who build something else). If you are ever selling, you will need an Opinion on Compliance with the planning permission from a suitably qualified person ( architect, engineer,etc.,etc) as proof of what you have done. They should also confirm that the works have been done in compliance with the Building Regulations.
If the works did not need planning permission, you will still ( if you are ever selling) need a similarly qualified person to certify that the works did not need p.p. because they were exempt and also that works were done in compliance with Building Regulations.
A lot of people do not understand this and have works done by builders with zero regard for relevant legislation. When they need to sell they then look for an Opinion on Compliance from someone anyone. Personally, I do not understand why anyone would offer such an Opinion if they had not been involved in the works.
Having just gone through the process and having paid a lot of money for professional fees ( steep but very well worth it in the end) I can see the reasons why you would and why you wouldn't use a professional during the process. And grand if you can get someone to sign off months or years later. But what happens when no-one will sign off on the Cert/Opinion?
mf