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A lot of American cities have fucked up in a very important way: they haven’t been building enough housing. And just turning the shed in the back yard into a rental ain’t gonna cut it.

In California, lawmakers are set to pass laws that would make it easier for homeowners to build “granny flats” on their existing property—either converting garages or other buildings into new apartments, or building tiny new apartments in their yards. In San Francisco, home to one of America’s worst affordable housing crises, “city officials are expected to pass legislation allowing landlords to carve fresh apartments out of underutilized spaces, including storage areas and utility rooms.”

Fine. We need the apartments. But this approach is akin to telling children that turning off the lights when they leave the room will prevent global warming. It’s gonna take more than that. In San Francisco, planners estimate the new rule could eventually “add some 14,000 units to the city’s housing stock.” That’s nice. San Francisco is a city that issued less than 13,000 new building permits last year, which was a significant increase from the prior year. Meanwhile, experts estimate that the city needs 30-40,000 new units per year just to keep pace with demand. The entire amount of new housing that this rule might create is not even enough to plug the hole in a single year of demand. The situation is so bad that even highly paid tech employees are getting zero money down loans to buy multimillion-dollar homes. (If we have another tech crash, perhaps the zeroing-out of the value of their collateral, shares in tech companies, will open up some new housing for people with more pedestrian jobs).

There’s another problem. We need housing that is affordable for middle class and poor people. Without a vast increase in the housing supply of in-demand cities, the free market simply will not create apartments with rents that low. And even though, in theory, building a ton of unaffordable apartments will eventually have a positive effect on rents at the bottom of the market, the nature of housing is that we need it now. Today. It is a human need, like food and health care. It is not something we are in a position to allow the free market decades to maybe sort out.

Here is a good explanation of why it is extremely hard to rely on private developers to build affordable housing: because it is extremely hard for them to make affordable housing projects profitable. They can only push down their fixed costs so much before they’re erecting mud huts in far-flung exurbs; and they can only raise their prices so much before their affordable apartments are no longer affordable.

The reality is that funding for affordable housing has to come from the government. In hot markets like San Francisco or New York City, where the mayor says his affordable housing plan is actually ahead of schedule, there are enough renters at the high end that requiring builders to include affordable apartments in their developments can be balanced out to a certain extent. This can also be used as a tool for the economic integration of cities. Which is nice. But even here, it’s not enough. Ultimately, if we want to create large (meaning “anywhere close to sufficient to meet demand”) amounts of affordable housing, it will have to be paid for with tax dollars, either through tax breaks and incentives for developers or through a huge boost in funding for public housing. Our city’s public housing authority already has $17 billion in unfunded capital needs. The cost of adding tens or hundreds of thousands of units and maintaining them would be high. It would require a large permanent commitment of funds, and a lot of tax money out of our pockets.

But like other human needs, it’s always better to pay up front. If you don’t, you end up paying more on the back end. Just like paying for preventative health care is cheaper than waiting until you’re sick, paying for public housing is better than being afflicted with widespread homelessness, as all of these unaffordable cities are. Housing is the biggest cost most people have. It is intimately tied to the existence of poverty. To tackle poverty, we need affordable housing. To have affordable housing, we need to spend a lot of money.

It’s a good investment.