Building codes will be a concern, here there's supposed to be a
firewall between the garage and living space for example. Otherwise if
you want space that you can keep cool in the summer and warm in the
winter, I'd recommend building a simple frame structure that you can
tear out if and when you need to. It's sort of a 2-edged sword, if you
build it real nice and dodge the local building inspectors, you may
have to throw it all out when you sell.