Councils should have on a file with the original development application (depending upon the age of the building) these plans.
Unfortunately sometimes people “borrow” the plans out of the folder at the council and don’t return them. This happened to us, so we then followed up with the original architects who were very helpful but did not in the end have the plans either.
Remember though that even if you track down the plans you can’t completely trust them. They show what was intended to be done, not necessarily what was done in reality, especially so with services that are hidden in the walls.
I believe there are sensors which can pick up plastic pipes filled with water etc, as our plumber uses them to track pipes in our building on occasion.