I think likely the number of people living in existing housing stock is going down. I read somewhere that more than half of the rent controlled 3 bedroom apartments in San Francisco are occupied by a single older person (why leave your $1500 per month 3 bedroom apartment?). I've noticed that the single family home neighborhoods I've lived in have gone from 1-2 parents and 2-3 kids in the house to older couples or single people living in the house. And it's not just people that stay due to prop 13 -- when houses go on the market they often get bought by single people.