When deciding between attaching an addition to your actual residence check with your local taxing body and your insurance provider for the Yin Yang.
I was told that if the train room is attached to your home in Delaware County, PA and has an HVAC system where I last built, that will be considered an addition to your home and the overall property tax assessment will be higher based on added square footage than if it was free standing utility building. Also the home owners policy would be increased. A free standing garage backed out of the propose foot print will result in a lesser assessment increase in Del Co PA.
Also many codes will call for a depressed garage floor and a fire door for gasoline fumes when there is an adjoining area for human occupation.
Of course there are architectural concerns in complementing the over all property.
I went through a living 4377 when I built my free standing train room.
Again, I side stepped all this in my good sized ranch basement. I would be embarrassed if I mentioned what I spent in my free standing train room.
