> build properties to rent out

That (generally) increases your tax assessed value. Whenever you renovate or build out - they will factor that in and tax you more. Your taxes don't stay flat with building more. You're incentivized to hold and not build - if anything. Zoning isn't in your favor either - you can't just add 10 units to your SFH. Remember - people bought when they couldn't afford to renovate - so they couldn't build out and then hold at peak development. They bought at lowest development and held. It's only after the property got really expensive that they could because they moved, rented the place out, and started raking it in.

So, what people do here is keep the properties as close to the same as possible because it's too late to build out more. (And zoning) If they're able to "renovate" in such a way where they can put up an in-law unit that adds like $100-200k in assessed value, they'll do that because then they can charge $2500+/month for that unit. Ideally, in their case, they'll just convert the garage and never get taxed more. But it adds housing but at a worse quality for both the main home owner and, likely, the person in the in-law than if they just built out a lot of nice apartments somewhere. If you have a "historical" house in San Jose - you can renovate the interior all you want and get no increase in tax assessed value as long as you don't modify the exterior. In one home I was in, they gutted the entire interior of a 900sqft 2-bedroom house into a 5-bedroom 2-bath so that they could rake in $5000+/month in rent out of 5 students going to SJSU. This resulted in 0 increase in tax assessed value - it's a common move and we can probably agree shoving 5 adults into 900sqft isn't ideal. A lot of people buy homes and will renovate them to add as many bedrooms as possible - each new bedroom is like $1000+ in rent you can charge. But - remember, the interior space doesn't increase. You just cram more people in the same square footage - different than how apartments would build up and give people the same amount of living space.

To give you another idea of why you wouldn't probably build out - my neighbor bought his house for $1.2m (This was like 2013). They spent who knows what demolishing the old home and putting up a new one. The house isn't like 5x bigger or anything - it's modestly larger than what was there. We'll say maybe 2x the interior square footage. Mostly because they added a second story on the new design. The tax assessed value is now $2.3m for that property (nearly double). If you're a landlord - would you spend $500k+ on renovating/building a property to be taxed on it too? Nah - better to just hold and rent. Besides, zoning gets complicated. There are only so many SFHs that you can make bigger and bigger to get more rent.

There's a reason why almost all the housing in the bay area is outrageously expensive but has nearly original interiors from when it was first made from 1930-1975 - it's cause people don't wanna improve the property because it will be rented anyway. The cost to improve isn't worth it - it doesn't outweigh the tax increase and the minor gains in rent. The cost to change the interiors to add more bedrooms in the same square footage is the only thing that is a sometimes worthwhile investment.