Concrete slab over tarmac for garage floor?

[millfield]

Hi

I have a garage which was attached to the side of my house as an afterthought 25 years ago (long before I moved in) and has the tarmac driveway for a floor - there is no damp proof membrane obviously so everything is getting damp and rusty in the winter.

The tarmac is solid and stable, so my question is can I lay concrete on top of the tarmac (with a damp proof membrane on top of the tarmac). I have asked locally and been told that I can and that I cannot. Obviously this leaves me a bit confused. It does seem a bit odd to dig up a perfectly sound base only to put hardcore back down.

A second part to this is would there be any problems with the edge of the concrete slab laying over where the footing for the house foundations would be?

Any advise would be greatly appreciated.

Thanks.

Rob