A couple of years ago some art school students built an apartment in a Berlin subway tunnel.
Some years later another artist built a little penthouse unnoticed on the roof of an apartment building.
I went to visit it after I found it listed on real estate website. It turned out to be sort of a mock real estate agent pitch performance. Later the artist told me that all renters, neighbors and janitors though it must be legitimate and nobody suspected it to be built without permission from authorities or the owner.
https://www.bloomberg.com/news/articles/2016-02-11/photos-a-...