There’s a concrete reason for Mayor de Blasio’s notion to ban New York city’s “classic glass and steel skyscrapers” — he’s got rocks in his head.

Has any officeholder ever turned so viciously on his own town?

De Blasio’s poisonous City Hall reign is distinguished by corruption, laziness and incompetence — but not by rank stupidity. Until today.

He wants New York to be “first of any major city on the Earth” to make building owners “clean up your act . . . to save energy.”

De Blasio knows nothing about New York City because he has no love for it. Unlike his every predecessor, he doesn’t go to museums, concerts or restaurants.

His gas-guzzling van ride between Gracie Mansion and his Brooklyn gym is the extent of his immersion in Big Apple life.

Get “woke,” Bill! The five boroughs already suffer from the most expensive construction costs in the world. Office buildings cost $575 per square foot compared to $468 in London and half of that most everywhere else, according to the New York Building Congress.

So, haha! Let’s pile on another $100 or more per square foot to make them completely unaffordable to build, just to reduce carbon emissions that already don’t add up to beans!

Today’s new skyscrapers are the most energy-efficient in history — so much that developers such as Larry Silverstein, Douglas Durst and the Related Companies proudly, and justifiably, tout their energy-conserving features to appeal to tenants.

The crazy thing is that nobody’s done more to promote construction of “classic glass and steel” towers than de Blasio. He pushed his own agencies to rezone East Midtown to allow super-tall One Vanderbilt to rise next to Grand Central Terminal — and to pave the way for more.

So what made him switch overnight to making it prohibitively expensive to put up large new buildings of the kind the city needs to compete on the world stage?

Clearly he inhaled too much “green” air from Rep. Alexandria Ocasio-Cortez, who proposed making air travel obsolete.

De Blasio’s criticized for spending too much time in places like Iowa, where cornfields do without glass or steel. But he’d do us all a favor to stay there and not come back.