You will probably get permission to restore the outbuilding if you mention no more.

Wait until the snoops have finished inspecting - then do the interior without telling them. Deepshit if you get caught, but you are likely to get away with it. Never let them in to inspect unless they have court order and bailiff. That's what the former owner of Murrells used to do and he got away with it for decades. And of course the council will have no resources for the next five years.

Then put "temporary" roof over the intervening space in triple-wall polycarbonate - "logstore" or similar if challenged.

Wait until they are used to that then infill the rear walls - get the drift?

Risky, but might work.