[arcticbull]

That's the risk, right. You have to account for that or you're doing a really bad job of building a real-estate business. This is often cited as a reason multi-family homes (or larger) are better for landlords than individual units: if you have an apartment you rent out and you have a vacancy, that's a 100% vacancy rate. If you buy a 10-plex and you have a vacancy thats a 10% vacancy rate.

Luckily, or not depending on how you look at it, SF properties and rents tend to be fairly predictable as compared to other markets in large part due to the artificial supply constraints imposed by city council, and the huge demand of the tech industry which tends to weather downturns pretty well. If you can afford $4500/month in rent for a 1 bedroom, chances are you're not going to be fired first.