Here in the Mid Atlantic most people have basements, so that is were most of the other modelers I know have their layouts.
But I live in a 110 year old house, and the basement is not really suitable for model trains - even after we did a lot of work to it. It has a low ceiling, lots of ductwork, heating pipes, boiler, oil tanks, well pump and tank, water conditioner, A/C air handler, support posts, electric panels, etc, etc.
So my layout space is in a room above my detached garage. The garage is 32’ x 40’ with a 12/12 pitch roof to match the house.
This provides a 22’ x 40’ room, with angled upper walls like an attic, for my train room. It is fully insulated, it has osb sheathing on the walls rather than drywall to allow easy attchment of the layout structure. It is fully heated and cooled by a propane furness and A/C unit in the “attic” space above the the layout room.
I designed the building specifily for all the purposes it serves and supervised its constructiion.
It only has two windows, one front, one back, set pretty high off the floor, nearly 5 feet.
The lower level of the garage houses our two cars, garden tractor and my workshop.
My model train workshop area is in the house, in the basement.
My layout was originally screwed in place, built attached to the walls/floor. But currently I am rebuilding it into moveable modules because we do not plan to retire here - so the layout needs to be able to go with me. I have past the point in this hobby were I have any desire to start over - I will not tear down and throw out any more layouts.
So, I may “complete” a new version before we move, or I might not - but in any case whatever modeling I do get completed will travel to the new retirement house and it will be selected or custom built to have a suitable space for the layout.
That may be a basement, or another custom building such as I have now.
Sheldon