New York City has very little housing available. It’s not an issue of empty properties.

You would be surprised at how much commercial real estate is vacant in NYC. If that could be converted to housing, it would alleviate a lot of problems.

Also, the way commercial buildings are mortgaged is crazy - major landlords often have 0% down payments (claiming that their ability to attract rental tenants will add 20% equity overnight), and keep the building vacant rather than lowering rents to maintain the "valuation" of the building. Empty units can have their cost tacked on to the end of the mortgage. At some point, the bill comes due, but it takes a long time.

Converting a commercial space into residential (assuming it’s zoned that way to begin with) isn’t trivial. We’re talking massive plumbing and HVAC system changes, for example. I do agree if empty commercial buildings can be gutted and renovated as residential it is one way to introduce more housing into the market; I don’t think it’ll be much faster or cheaper than building from scratch though.

I agree, but the way those mortgages are handled, they incentivize buildings to have a lot of long-term (5- and 10-year) vacancies, which means that it doesn't have to be particularly fast to convert them.