Part of making maps is using assets like brushes, textures and stamps that depict buildings, terrain and other things on a map. My criteria for whether something is allowed is basically - would a reasonable percentage of the membership here want to read / look at this post and material being posted. Some things that get posted are pretty niche since this site does host some peoples maps that are pretty niche themselves.
So making icons of buildings including 3D ones used for making maps and 3D terrain is absolutely within the scope of the guild. We have a lot of posts about using sketchup, blender and even Unreal by the site owner and this thread's OP Robbie. A 3D model of a building is a representation of space which is what a map is. The representation can take a wide range of forms from line graphs made with felt tip pens like an underground railway system to fully realistic textured 3D virtual worlds using latest game engines. We do fantasy and sci-fi as well as real world mapping and historical and con-world type of mapping. We show from galactic / planetary maps to battle maps to treasure maps, all of them are welcome posts. If you think you might be ok then you probably are. Its usually pretty obvious when a post is off topic as its often so off topic its unbelievable. Even then we do have an off topic board for those kinds of posts with some limits.
Where anyone will most definitely be out of order is when the posts come down to attacking another member or disrupting the board to a degree that it gets flagged and for anyone who sees a post like that then use the "triangle !" icon at the bottom of the post and report it for attention to the moderators. Reporting a post does not necessarily mean it will be taken down but it will be monitored by the mods. Do report spammy posts too. So far we have been a board and forum that has bucked the trend of the modern web and we enjoy a cordial and enthusiastic membership - long may it continue.