Garage Heating

First of all, in the 20 years I spent as a professional home inspector I have seen these types of conversions many times and they are rarely done well and often detract, rather than add, to a homes value.  So, if you are going to do this project, do it right!

That means that you need to treat this just like an addition.  Basics would include removing the overhead door and reframing the opening to add siding from the outside.  You'll also need to build up the foundation so that is is level across the front.

Next, the floor will have to be leveled as most garage floor slabs slope toward the overhead door so that water which drips off the car can run off.

Once that is done, you can think about the rough mechanicals, including heating the garage.  It is not likely that your home's existing heating system will be set up to easily extend to the garage so you'll most likely have to add a 2nd heating zone.  You might want to think about using a mini-split heat pump system that can heat the room in the winter and cool it in the summer.

As you can see, this project may be more difficult than you first expected.  But done well, it can provide long lasting comfort and value.