Before starting to add the costs, you need to know how you’ll be using the space.
Contractor vs DIY (Do It Yourself)
Even if you will try and do the job all by yourself you will still have to obtain a permit and also the project needs to be approved. Doing most of the job by yourself will save you a lot of money and you may spend no more than $8-9000 on the project.
You can hire a contractor to get over the preparation process, like acquiring the permit and doing the plans. Also the contractor can work on the plumbing and on the electrical system. The rest of the work you can do it all by yourself. By hiring a contractor you may spend up to $15-18.000 or more, depending on how big the project is.
Some updates that you may need to do on your garage is install some security systems, like fire sprinklers, ventilation, insulation etc.