How do I find out what I can do with the space above my double garage?

What do you hope to use the 'finished' garage space for once the project is completed? Do you want to use it mainly just to park a vehicle, as four-season storage, or extra living space?

Is the garage on slab or crawl space? Is the house attached to the garage? Is the house on a basement or crawl space? Where is the nearest water outlet/tap to the garage? Where is the nearest drain located? I am wondering what the reason is that you were told water could not be installed inside the garage.

Because you have electricity, you have a good start. As far as running water up there, running the water is pretty straight forward, the problem is usually getting rid of the waste water; drain system. I have the same problem in my shop. I use a composting toilet that utilizes sawdust and biodegradable bags. Since I make tons of sawdust in my shop this is an excellent choice for me. I won’t go into detail, but you can research it online. As far as cooling, I went with window a/c units. For heating I use a forced-air kerosene unit, but space heaters would also be a viable option. I didn’t have insulation for a long time, but I recently installed bat insulation which made a major difference. I used R-30 insulation so I could get the most insulation I could afford. I also did the installation one section at a time as I could afford it. I guess the bottom line is that you can do whatever set your mind to. You are only limited by your budget and imagination, and we are all here to help feed your imagination

Did this start out as legal unfinished square footage, sometimes called "bonus space" by the home builder? If not, how have you gone about making sure the updates made by the previous owner are safe and up to code?

If you want to know what kind of legal structure can be added over your garage space you need to check with your city or county building codes. Pulling the proper permits and getting inspections means your future self may be able to sell your property at a higher price because of your improvements. (You should be able to find out if the previous owner got permits for the work he already completed.)

If you just want to take your chances and hope you don't get caught and fined for non-permitted work....well, that's for you to decide.

If you don't add plumbing, there are still lots of options to make good use of the area.

If there is good access and you can get stuff up there, it could be a decent man cave space..... or a hobby room workshop if it's wired for it. If you're into toy trains just think of the layout you could put together.

It could be a rainy day play room for kids if the access stairs are safe,

For comfort, you could add a small fridge, portable AC or heaters....again assuming there is sufficient power.

I was thinking you could also use it as a work out room....and that made me think of the floor joists-what kind of load will they support?

There are lots of safety issues you should research before bringing friends and family up but it could be a great space to share.

Hi there, here are a few ideas that could inspire you for that space above your garage - https://www.hunker.com/13728691/bonus-room-above-the-garage-ideas-and-inspiration. WRT to the water, you can have a look at portable sinks. They're pretty easy to install - https://allportablesinks.com/blogs/news/what-are-portable-sinks-and-how-do-they-work