The first thing to do is to get the work stopped (even if that is not your ultimate intention). You can do that by calling council and telling them that there is unauthorised work going on, including an unapproved change of floor plan.
At the same time you can seek orders at Fair Trading, both through normal channels and, at the same time, by seeking an interim order to have the work stopped immediately.
That would then put the committee in a very strong bargaining position for the next stage of how you want to proceed.
However, if you go in at this stage talking about compensation for unauthorised work, you will muddy the water and may well end up getting nowhere