A Seattle-based reader asked me what I thought of this Seattle Transit Blog post about the paucity of three-bedroom dwellings in transit-accessible neighborhoods, which floats a variety of regulatory requirements and incentives to encourage their construction. Having recently bought a three-bedroom dwelling in a transit-accessible neighborhood of a large and prosperous American city, I can confirm that this has, yes, become a prohibitively expensive undertaking. So I get the problem.
But this is actually a hard problem to solve. When I was in my mid-20s, I spent several years living in a very spacious rowhouse that had ample room to raise children in. But it was a five-income household, comprising myself and four roommates. It's difficult for 1.5 to two full-time workers to outbid three or four full-time workers for housing. That's especially true because even though people from all walks of life can enjoy city living (I was raised in a big city and would like to raise a family in one myself), childless people are more likely to want to pay a premium for the modern urban lifestyle. Cities crush suburbs in terms of nightlife amenities that parents don't have much time to take advantage of, for example, while shopping for a bunch of people is cheaper and easier at a suburban supermarket.
Meanwhile, the share of American households that consist of parents with multiple small children is plummeting, even though the overwhelming majority of our existing housing stock is suburban-style dwellings aimed at appealing to that market segment. Under the circumstances, it should come as no surprise that urban developers aren't all that interested in targeting this market. And while you can require people to build three-bedroom houses, you can't really force them to design dwellings that are "really" primarily appealing to families as opposed to roommates.
Now in any given city it's not like you'd be considering these regulations against the backdrop of a super-efficient ex ante regime. So it's easy to imagine family-friendly zoning changes that are good. But as with inclusionary zoning proposals, the important thing is to take a good hard look at whether your proposed rule change is going to mean more dwelling units or fewer. More units means more households of all kinds will be able to live in your city. Fewer units means fewer households of all kinds will be able to live in your city. There's no real way to micromanage around that basic dynamic. An affordable city is one that accommodates lots of people, not just one in which a handful of people get a sweetheart deal on a house.