You are right that many people will do this type of work without formal permission, and the question is .... if it is done correctly then what is the problem?
None except that the onus will be totally on him to prove it has!
Which then leads on to ..... lets say you withdrew the application, then done the work, making sure it is done correctly and with photos at various stages (especially where work is concealed ie drains). So no need for plans, and no hassle with the bureaucracy.
Now, if need be at some future date, then you could either make an application for Building Regulation Regularisation (ie retrospective permission) - you have the pictures if required, and no plans required, and so it gets approved.
Sorry Woody, I think your being for too simplistic; if it was as easy as that everybody would be doing it, including me! LABC can get pretty ****ed if you try to circumnavigate them & as far as I’m aware they are under no obligation to accept photos as any sort of proof in lieu of inspection.
Or at sale time, if this is picked up, then you pay the £100 or so for an indemnity insurance policy for the new buyers. It's much less stress, and less expensive.
From what I’ve read about the new 6th April HIP regulations, I believe that may no longer be an option.
Sure the OP is having problems & I agree with most of what’s been said but I wouldn’t advise he go down that route. I suspect the reason his LABC may be getting sniffy is because he already has one ‘open’ submission which may have been a little inactive recently. I personally had my permitted development rights withdrawn because (quote) “I had submitted an extensive planning application & Building Notice submission” &, presumably, they wanted to keep an eye on what was going on. They didn’t give me any prior warning & I only found out because I wanted to change our conservatory roof height; which was outside BR’s anyway. But it does show that Planning & LABC appear (sometimes) to work together.
I suspect they may be suspicious that, rather than just installing a loo in the garage, it’s being converted to full habitable status; or, at least, will be intended to be used as such. To the OP, I would say just make sure you have all the information available for the meeting on Tuesday; exactly what you intend, drain runs, spec. etc & have quotes/costing information available & £3k is more than adequate. If you make them aware you know what your doing, they will be more likely to trust you. Don’t get stroppy, it’s far less hassle to work with them than agin them; a bit like “DIYnot” mods, they have a delete button &, unfortunately, we don’t have an accept button!