At one time the rule of thumb was 100mm of concrete on the floor that is now 150mm of concrete. If putting in a two post and the vast majority of people do not have the head room or want a garage too out of contect with the house 200mm of concrete is best.
Also be serious about the electrical supply if you intened to use it as a workshop, no need if it is just lights and a couple of small power tools to be used.
A friend run a 2.5mm cable to his garage as he was only going to use it to store his car. He started spraying etc and a heavy duty compressor and heating soom started blowing mcb's.
