Like so many "state of the" speeches, the easy part is making big promises that sound good to so many. The hard part is the details.

Mayor Bill de Blasio focused on affordable housing during his State of the City address Tuesday. He's right on the facts -- and on defining the problem. More than half of New Yorkers spend more than 30% of their incomes on rent, and soaring housing costs are at the root of many of the other economic issues the city faces.

His solutions, such as adding density and cutting bureaucratic red tape, are fraught with complications and difficult realities. But the centerpiece of the solution de Blasio proposes may be even tougher. How do you induce residential developers to include affordable housing in their projects? Big questions remain, and the reality may be far less gleaming than the promises.

Is 20% the right balance of affordable housing, as was originally proposed in the Astoria Cove project highlighted by the mayor Tuesday as a success story? Is it 27%, the proportion eventually used in Astoria Cove after hard-fought negotiations? Is it more? Any of these figures still mean that a developer will be building far more market-rate units than affordable units.

Then there are these questions: What's affordable, and are New Yorkers with the biggest needs being helped? Area median income stands at $83,900, according to the New York City Housing Development Corp. Most "affordable units" are geared to those who make 60% or 80% of that figure. At the low end, that's $65,000. But what about the many NYC families that earn so much less? Where does this leave them? The answer, of course, is it really doesn't help them at all.

Will developers agree without big concessions? Will neighborhoods want yet more development and increased density? Finally, how indeed will the mayor "make sure that affordable housing . . . stays that way."

De Blasio makes the promise of affordable housing sound, well, promising. But the reality is that affordable housing has been a favorite go-to issue for New York City mayors for a long time, and yet we're still talking about the need for it.

The hard part is yet to come.