I agree that vacant units, and units that are unsafe due to lead, asbestos, etc... are a very real problem and a good target for public policy. I think the idea that I often hear, that we can build new homes until rents are so low it decreases homelessness, to be a poorly thought out idea. To see a good example of how it turns out, look at San Jose’s new google village. Homelessness increases while the developers and google get rich, and the public get a few hundred “low income” studio apartments that if they don’t sit vacant will end up renting to the googlers who are the only ones who can actually afford the $3000 “low income” rent, for example.