[shereadsthenews]

That can’t really be the reason because SF has approved numerous such mergers and expansion of gigantic single-family structures and they steadfastly forbid subdivision of buildings no matter how large into duplexes in the vast majority of the city. The city’s actions indicate that detached single-family structures are the strongly preferred type.

[IkmoIkmo]

Not sure, in my city (capital in western Europe) we have similar laws, both for division and mergers. They're built to protect the housing stock. Family homes can't be split up because it incentivises landlords to create many small units for young single tenants, students etc, while reducing homes available for families who need multiple rooms, who get pushed out the city. But similarly, you also can't just merge two homes, because it does the opposite, push out young/student/poor people and attract only families or those with the money to buy a larger unit.

Of course mergers/divisions can still happen, but it has to be approved by the municipality.

The popularity / trend of either mergers or divisions purely based on the market may change every five years or it may swing one way for decades. The point of these laws is for the municipality to exert control over this, because it's an elected body with normative ideas about what the city should look like and who should be able to live there.

A protective law for small units doesn't negate that at the same time there could be good reason to have a protective law for large units.

Second, my city doesn't just do it to control the overal housing stock. It also does it to steer individual neighbourhoods a certain way. It may be that the aggregate distribution of small and large units is completely fine in the city, but that one neighbourhood has only family units, and another only single-person units. It tends to be that, while some concentration/clustering of characteristics is helpful (e.g. a family vs a student neighbourhood), it's also healthy for a neighbourhood to maintain a bit of a mix. Healthy municipalities of highly livable cities often try to steer this mix, such that you don't end up with de facto segregation. Rich and poor, young and old, educated and uneducated, various ethnicities and trades all living together instead of segregated. This also means municipalities tend to want to create laws to protect a certain size of housing stock, could be small or large units, by requiring approval for both mergers and divisions, to protect certain citizen's groups. The fact laws for both mergers and divisions exist certainly isn't necessarily strange.

Anyway I'm relatively unfamiliar with SF, never lived there, so honestly I've got no clue what's actually happening here.