California

Also, if you don't have adequate parking space in an area, it results in cars spilling out into the nearest street parking.

That being said, if you want to increase housing density and you want available parking, you either gotta convert some low-rise buildings into parking and some into higher-density housing, or you gotta build parking garages, which cost substantially more than ground parking.

kagis

https://dcplm.com/blog/cost-of-building-a-parking-garage/

The cost per space varies based on the location and design of the parking garage.

1. A surface lot is $1,500-$10,000 per space (economical).
2. An above ground garage is $25,000-$35,000 per space (balanced).
3. An underground garage is $35,000-$50,000 per space (expensive from excavation).
4. Automated parking garages can vary greatly, but typically fall within the range of above-ground garages.

Any parking mandate adds to the cost of the housing. That being said, I generally think that that's worthwhile (and frankly, my experience has been that parking mandates generally aren't high enough for existing apartments). If you have a one-parking-slot-per-apartment mandate, you also have a number of people in those apartments who have multiple cars.

considers

I do kind of wonder how much it would cost to do low-rise parking garages. Usually when I see them, they're in high-value areas, downtowns and stuff, and very tall.

https://f.hubspotusercontent20.net/hubfs/9476621/2021%20Parking%20Structure%20Cost%20Outlook.pdf

This has multistory parking garage construction costs for different locations. It looks like parking garage construction cost in San Francisco is about 50% higher than a lot of the country. I wouldn't expect materials cost to vary much from state to state. Some of that is maybe earthquake code compliance. Some probably labor cost.

EDIT: One other point I'd add is that if you have one parking spot, it's also possible to fit multiple motorcycles/mopeds in it. I know one person who was in an apartment who kept her motorcycle and another one in her parking space. Some places in (arid parts of) California are pretty amenable to motorcycle use.

EDIT2: If you figure, back of the napkin, that housing has an expected 10% ROI and the price difference between a surface parking slot and a multistory parking garage parking slot is maybe $25k, then it's gonna increase annual rent for an apartment by maybe $2,500 or $208/mo, which I kinda suspect is gonna be rather outweighed by potential decrease in rent from more supply of housing being available; the price difference between "high demand area" housing and "low demand area" housing is a lot more than that.

https://constructioncoverage.com/research/cities-with-the-most-expensive-rents

The top four most-expensive US cities median rental price for a 1-bedroom apartment rental here are all in California: San Jose ($3,223), San Francisco ($2,705), San Diego ($2,534), and Los Angeles ($2,358).

The four least expensive cities are St. Louis, MO ($1,059), Cleveland, OH ($1,046), Oklahoma City, OK ($1,010), and Tulsa, OK ($994).

That's a lot of difference there to work with.