Quoted from Max_Badazz:keep in mind taxes ..... your town will classify the space differently and bump up your taxes quite a bit each year (the cost that keeps taking from you)
Occasionally! An existing garage shouldn’t add to existing taxes. DC you might need to pull permits but if you’re fast you can do it yourself and they’ll never know 
I don’t use my garage as a gameroom, I use it as a storage and workbench/room and to entertain for parties.
I dug a trench, kept all the grass clumps to the side, daisy-chained off an existing 20a circuit in the house that has very little load on it (rarely used wall outlets) smacked a new outlet on the side of the house and chained off that, and put in a decent shutoff in the garage so if ever it trips anything it will cut power in the garage and not affect the house circuit. My electrician friend helped me and by the end of the day I emptied a little dirt and patted all the grass back down and you’d never know I dug a 30ft long 2ft deep trench. It blended perfectly!
Also my garage is anchored by huge 4x4’s and technically doesn’t have a floor or foundation so if the city ever tries to bitch and claim it’s a taxable outbuilding or some nonsense, I can rip it out of the ground fairly simply if I take the right steps and the post I mounted the electrical too is not directly tied to the structure.
While not exactly ideal for an outdoor garagecade, you could use some of the same ideas I believe. The only issue is really the moisture/humidity and the cold. Those will destroy your nice games. Run a dehumidifier and go with the split system to get both heating and cooling.
