Non-functional objects like decorative buildings, sit on the floor layer of empty tiles. They are non-functional when used exclusively for decorative purposes.
The citizens of Duckietown love their cities colorful and fun, and encourage all efforts at adding non-functional components to the empty space.
A great starting point for a more visually pleasing Duckietown are colored felts to simulate grass or bodies of water. Any other contraption that would please the population is welcome, too.
Make sure the placement of your decorations does not interfere with the tilemap and signals layers.
Choose colors that Duckiebots do not give meaning to. E.g., avoid red, yellow and white.
Choose materials that do not reflect light, and you will save many hours debugging algorithms.
Moreover, although Duckiebot drivers all have their driving licenses and know to focus on the road, the background, i.e., whatever is in the room the Duckietown was assembled, matters too.