Any building with three or more apartments must inform the City whether smoking is permitted anywhere in the building.

Mr. Bloomberg said at a news conference on Wednesday that he was proposing the bill because New Yorkers wanted to be protected from secondhand smoke. He insisted that the disclosure requirement would be strictly informational, and not a backdoor attempt to pressure landlords and buildings to ban smoking.

“We protect people from hurting themselves if they’re trying to jump off a bridge, we restrain them,” Mr. Bloomberg said. “Should you really do it with smoking? We’re not going to do it with smoking, but we — this is purely an informational thing.”

He means we are not going to do yet.

So what happens if a building says smoking is prohibited, and someone smokes. Can the management company be fined?

“If somebody in management says to the person, ‘You’re not allowed to smoke here,’ and the person ignores it, what do you do?” Mr. Spinola wondered. “What is the role of the management of the building, or the owner or co-op board or condo association?”

Mr. Spinola said that preventing people from smoking in their own apartments would be particularly fraught. “I’m told there’s a gray legal issue as to whether or not you can impose that, and you probably can’t impose it on somebody who was already living in that apartment,” he said. “I’m told there’s never really been a precedent by a judge as to what you can and cannot do. There’s always been a legal settlement.”