Here’s a real estate owner’s nightmare: After a couple built a brand new sixth floor atop their five-story townhouse, the city ordered them to remove the whole thing.

The building, on Manhattan’s Upper West Side, was within a “landmark” district. The city’s Landmark Preservation Commission decided that the addition didn’t comply with the city’s landmark rules, and ordered it removed.

The addition included a kitchen, dining room and terrace, and was designed to turn the fifth-floor rental unit into a duplex.

The owners said they had no idea the addition ran afoul of the rules, and noted that the city’s building department had issued a permit for it. The building department said the architect should have mentioned in his application that the building was in a landmark area. But the architect said he checked the department’s landmark database and the building wasn’t listed there.

City preservationists argued that it was necessary that the entire addition be dismantled to avoid setting a precedent that landowners can get away with building in violation of the rules.

Many cities and towns have landmark or historic building rules, as well as complex zoning and other requirements. The moral of the story is that before you build, enlarge, or tear down anything, you should do your legal homework.