DC's Office of Planning recently released a roadmap for what they're calling the "Midcity East" area—essentially the next frontier for rising land values and gentrification in the District of Columbia. The plan contains a lot of worthy aspirations, but lurking near the heart of it is a basic contradiction. They want to do various things with reconnecting streets and parks that will make the neighborhood a better place to live. And they want to ensure that the neighborhood remains affordable. But they also want to "Strengthen the Zoning Code to preserve the availability of the current supply of single family housing stock" — i.e., make it harder than it currently is to replace existing structures with denser ones.
But this just doesn't work. If a neighborhood becomes a better place to live, then you inevitably get some combination of more homes are built and the price of homes rises. To the extent that you prevent homebuilding, you only intensify the affordability concerns.
I get that the worry is that new multifamily construction in DC tends not to feature the kind of large three-bedroom units that would be suitable for families with kids. But this is not an inherent property of multifamily housing. It's a response to developers' perception of what there's demand for. If you want a regulatory intervention that's likely to increase the amount of big, family-sized homes the answer is to upzone not downzone. When buildable square feet are at a premium, you get the most value with small units. When you can get permission to build many square feet of housing it makes more sense to serve other segments of the market. But trying to "preserve" single-family homes means a small, fixed number of lucky folks will live there and everyone else will be priced out.