200mm Is that awkward height between to high for a floating floor, yet too low for a suspended floor.
I would go with a 50 x 150mm C24 Timber frame with strutting and insulation, this will leave you with a 50mm gap above the concrete floor. I would then try to find a way of venting the...
I've been out to a few of these concrete sectional garages where the homeowner want's to convert it to an office / bedroom, and I have yet to find one that would be cost effective to do.
I would consider the option of starting again and having a new extension built from blockwork (cheaper...
Sounds a little unreasonable to me.
If the job was done 20+ years ago then even the L.A. Building Control wouldn't have a record for that any more.
Are they trying to get a small bargaining chip to reduce the price a little?
Self levelling compound from Wikes will normally do the job.
Straight on to the solid floor to level it.
New DPM on the "new" floor then the timbers and insulation on the DPM.
I would say you are probably best to think you'll be starting again. The local BCO will agree on site once you have exposed the foundations, but I fear he'll take the safety root first.
You won't actually get a Planner check if you have completed your extension to the Planning Approval. The only time they will check is if a neighbour complains about a breach of the Planning Approval.
My advice would be to call into the Planning department for a chat and take advice from a...