Height between layout levels

What is the minimum/standard/ideal height between the levels of a multi-level HO layout? I know that some would say there should be only one level, but I’m trying to optimize the small space I have available. Much of the layout will be an urban setting, so how much height is needed for industry and commercial buildings so that everything still looks comfortable in its surroundings?
Thanks.

Google mushroom layouts. Lots of answers in them pages. I like 18 inches. Fred

Mines 18 inches too, theres a few compromises, grade, a viewers height, building height. I took all those things into consideration and came out with 18".

Ken.

Thanks. Do either of you have any pictures to give me an idea of how an 18" height difference looks?