sq footage included in the overall sq ft (but having a 4 car garage would add value over a house with a 2 car garage).

I'd pay attention to permits too. You might be able to add a "shed" without permits - and in your area permits to add onto the house might be $20k - Obviously having this data would be important.

I have a friend who built a detached garage/man cave on his property. Parking on grade level then upstairs is his office/tv den. He can pull cars out and has a golf simulator. Something like that might work???

Perhaps adding garage on the bottom and a den on the top of an addition to your house would create enough value you in your house that it makes sense to add onto the house??? Who knows... good luck...