Sorry to get a little off topic but I've never heard of basements here in Florida being Illegal or anything to do with sea level, it's the water table that prevents basements. You can still have them but it's a LOT of work to set up and maintain.
I get the technicalities with making ac work in a garage though, if you ever want to open the garage door you're basically letting all of the air conditioning out and the air conditioner has to work really hard to remove all that heat from the garage.
Inside you shouldn't have issues with humidity, unless you have a good a/c running nonstop with a well sealed house you shouldn't have the humidity drop all that low. Mine is pretty much always 50% or so and we have the a/c going pretty strong all year. I still had trouble with tanks though, but in racks it's not a concern.









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