I turned my garage into a habitable room a couple of years back. This required some new circuits and a CU change. The building work was done under a Building Notice, so I had to ask my LBC what to do about notifying the electrical work, as I was no longer in a Scheme.
In short, they didn't know what to do. They initially accepted my qualifications, then did a U turn and said any work I did, how to have a retrospective EICR to an EIC. I did try the 3rd party certification route, and made contact with two companies, with one agreeing to cert it. But they never materialised into anything. Guess its more hassle than its worth.
In the end, it was simpler to get a company in to do the work etc.
As said, if you do any electrical work, and subsequently sell you property, you will have to provide appropriate certification, which will include a Completion Certificate from your LBC