They will need to triple or quadruple the capacity of the water, electrical and sewer grids that were sized for the current density.
And convince people to part with their cars because street parking will become impossible.
The denser the place, the more efficient the infrastructure use. 50 families in a single apartment building use massively fewer pipes & power lines than 50 families in single family homes. But the tax revenue for the city can still be the same. In a denser area, those families are less likely to own or need cars, and if they do, they drive less.
On the other hand, low-density suburban neighborhoods are often so inefficient that the maintenance cost of their infrastructure is more than the tax revenue the residents bring in. (Here's a good overview of that: https://www.youtube.com/watch?v=7IsMeKl-Sv0)
It's more true to say that "we don't have the infrastructure for single family homes" than it is to say that we don't have the infrastructure for apartment buildings.
Do you own a home? Would you willingly let it be replaced alongside surrounding properties with an apartment complex? After all, it's more efficient.
You shouldn't own a washing machine either, that's silly. A laundromat helps reduce the need to own the appliances and create a more social environment.
And we haven't even begun talking about that car you own. Gross. Public transportation is so much more efficient.
Oh you actually wanted to travel somewhere and not have to spend 4 hours of your day to go there, take care of your business, then come home? Weird.
IOW, there's plenty of slack in existing cities, especially the older coastal cities, which were originally planned and built for far greater density than permitted now, and at a time when per capita usage was far greater. If anything, continuing to incentivize urban sprawl is the surest way to overburden water infrastructure, especially supply.
Most water consumption these days comes from the state's agriculture, which bringing more residents in would not really impact since denser housing would not really go in areas that are rural.
“We’ve reduced water, power, meat, and space consumption by XX%” is not good news. That just means we’re getting poorer. Good news would be “we’ve halved the cost of water, power, meat, and space, so everyone have a good time”.
https://ktla.com/news/california/please-conserve-water-touri...
In the Monterey Peninsula area residential building permits have been restricted for years due to a persistent water shortage. That's been going on since well before the current drought.
Also, infrastructure like water treatment plants are expensive and scale well, so you can spread the cost over more people.
But yes, there are problems that come with density and they have to be thought through.
Convince nothing, price them out. The state is not obligated to pay for permanent storage for your private property on public land. If you want a car, you need to pay for its permanent storage.
Consider the street closures during the pandemic for outdoors eating. The parking crowd has screeched endlessly about the traffic and parking issues caused by this, but the majority of the populace love these areas now. A bunch of them are actually becoming permanent.
Similarly, the cute, street car suburbs with high walkability and insufficient parking are absurdly expensive everywhere I’ve ever lived. Turns out people like living places that are nice, and that sufficient parking is a relatively minor deciding factor.
Building a new walkable neighborhood is fine. Trying to drastically alter an existing neighborhood is nearly impossible.
Of course, the counter argument is “but Paris has a good metro system”, to which I say maybe we should fix that.
Also, cars are literally the least space efficient means of transit ever devised. Depending on cars for transit means everyone has to live further apart in order to make roads and parking big enough to accommodate all the cars[0], making commutes worse.
Cars are also time inefficient over longer distances, trains (if we bothered to make them) and airplanes trivially beat cars.
0 - Underground parking solves the storage issue at a ludicrous cost. In many modern buildings the cost of the underground parking structure is often more than the cost of the apartment building above it.
That said, as others have mentioned, urban living arrangements can be more efficient. There is a big question though if that is compatible with the historical CA dream ideal. Sure, that ideal is becoming more and more incompatible with reality, but people want what they want. CA could offer me a free condo in a high-rise urban environment and I'd still leave because that's not something I would ever want to live in.