Oh boy. Here we go. To be clear, an "old garage" floor does not have the same type of "build" as the concrete slab in a home. It is usually sloped (which needs to be corrected) and it rarely has a vapour barrier incorporated underneath it. Both of these are problems for flooring.

Once the slab has been levelled and prepped, one of the easiest solutions to this is a standard vapour barrier (6mil polyethylene sheeting + moisture resistant tape to seal the seams) and cork underlay. At this point you can float a floor over the cork underlay and have the warm floor you want that is protected against moisture.

Some of the most successful garage conversions have used the cork underlay underneath a cork floating floor. Double the warmth underfoot and INSTANT insulation against heat loss in the winter. Quiet (keeps noise from exiting the space) and easy to live with as well.

Cork can be a carpet replacement product. No matter how you slice it, the slab has to be dealt with first. Don't be surprised if it takes a CHUNK OF CHANGE to get it ready for flooring. Many people have been floored (pun intended) by the cost just to get the garage concrete ready for new flooring. It is NOT cheap. But then again, a garage conversion can be quite costly. It needs to be upgraded to "live space" with all the permits required of an addition.

Wow, thank you that is super helpful!

You're welcome...just to let you know, I try hard not to "sell" cork on this forum. It isn't my intention. In this case it would work very well.

Please be prepared to see a BIG price tag for that slab ($3-$5/sf just for the leveling/preparation). It is a big part of the flooring budget for garage conversions. To find out your "total budget" for the flooring part, please bring in a WELL versed flooring expert who has all the big tools to do this properly. Someone who preps COMMERCIAL spaces will have all the knowledge necessary to get you a nice smooth even floor inside of a week (may be two). The concrete then has to dry (another week or two) and then they can come back and install the floor for you.

Get them to break down the costs: floor prep (can include labor and materials) + floor installation (ask them to include the cost of laying vapour barrier and underlay to that quote). Your cost of flooring will be the smallest price tag out of the three categories (prep; labor; flooring).

Good luck.