If City Hall sits idly by as judges keep rewriting the zoning code, Mayor Bill de Blasio can kiss all his affordable-housing plans goodbye. Indeed, the entire housing market will take a dive.

The latest, most outrageous ruling came from Justice W. Franc Perry this month: He effectively ordered the top 20 floors shaved off a new 52-story condo building at 200 Amsterdam Ave. — when the project is basically done.

The builder had the city’s OK — indeed, had stopped work for four months while the Department of Buildings resolved an earlier challenge.

But Perry ruled that it’s not kosher to use air rights from parts of nearby lots to build higher than the code would otherwise allow. That’s crazy: The city has allowed the practice for 40 years, during which the DOB has considered and OK’d this interpretation of the rules 34 times. Will Perry now order dozens of those buildings to chop off their top floors?

Based on the city’s sign-off, the developer sunk $325 million into the building and would have to shell out millions more to do this “deconstruction.” How is that even remotely fair?

And what builder is going to risk investing in any new project if a judge can rule after the fact that official approvals are irrelevant, simply because he disagrees with the city’s long-held interpretation of its own laws?

This is a growing judicial trend. In another case, on the Two Towers project downtown, Justice Arthur Engoron blocked a project claiming, “You can’t just do this because the zoning allows it.”

The developer at 200 Amsterdam vows to appeal Perry’s ruling, but the city Law Department needs to join in, too — and actively defend City Hall’s ability to interpret the zoning code.

Otherwise, the loony left will seize control. If city approvals become meaningless, the private sector will stop building, period.