Scott
Since you are building a garage with your new house, consider making it a two story garage and AC/heat it. The cost of the upstairs hobby/train room for trains will be lower than an extra room on the house. When it comes to garages, basements and storage folks up north have an advantage.
Many houses down south have car ports and if lucky a small storage or utility room for their tools, lawnmowers, bicycles etc. and not room for train layouts. Maybe you could add a second story to your two car garage as it is being built. Now is the time to swing a train or multi purpose room if you can afford it even if you have to have it only roughed in and finished later.
Charlie
