I tried by example, but I keep getting DEBUG messages. _north not found. Cycles for each direction, sometimes multiple errors per direction. I was adding to mapgen_functions.cpp and was basically using the abandoned storefront coding as a reference. I have to be missing something though… I just can’t figure out what.
Ah 
Nevermind. Just forgot to add it to overmap_terrain.json.
If it’s a 1x1 and not a variation of something already in game, you should be able to add it as a shop via regional_settings.json once #5134 lands. (Bigger buildings are next on my list)