And actually you do not even really need to decide what particular use you want to put it to as most modern garages are designed to do it all, allowing for cars, general storage, and a home office or workshop all in the same space.
The cost of building a garage addition is relatively low compared to other home or room addition projects, as garages typically do not require extensive plumbing, insulation and sheet rocking.
If you do intend to use your new garage as liveable space for humans as well as auto-mobiles it will, of course, cost a little more to achieve that goal but the project is still an affordable alternative to basement or attic remodeling.
If you already have a garage the possibilities for remodeling are extensive as well.
Some people choose to add a second story to create extra living space or to install a new storage and shelving system to organize all the clutter that accumulates in the average garage.
As a new garage is a physical standalone building, it is essential that you work with a good contractor who knows what they are doing.
There are books and videos out there that claim to be able to show you how to build your own garage but this is definitely a project best left to the pros.
Construction issues aside (will it actually stay up?), building a garage addition will require permits and local ordinances will have to be observed; a good builder or general contractor will know exactly what those are.