If four taxpayers aren't paying the amount of taxes required to fund the infrastructure needed by four taxpayers then your city is already bankrupt and zoning and planning is irrelevant.

> If it was 600 new units rather than 4

If it was literally only 4 units then you shouldn't need any new infrastructure anyway. Nothing should be operating with margins that thin to begin with.

If it's 4 units here, 4 units there then it adds up to hundreds which is the thing you admit isn't a problem.

At least in the San Jose area, the sewage treatment was largely built 50 years ago, when it was doing a lot of agriculture, and flows are way lower with people than they were with food processing and other industrial processing. The new plant that's in progress needs and has way less capacity, even though the standards are much much much tighter.

At the one house per acre level though, you can likely manage with septic, which is incremental sewage treatment capacity, and not a huge municipal undertaking (there is record keeping and regulation and what not, so it's not nothing)

Precisely.

> Sewage treatment plants have finite capacity, can't be built on a 'serves 20 units' incremental scale

Nor would they need to be. If your sewage treatment plant has a capacity for 100,000 units and you have 99,981 existing units, you already need a new sewage treatment plant regardless of where you put the new units, unless your plan is to never approve any new construction anywhere in the city, which is manifestly unreasonable.

> and oftentimes new plants have much stricter requirements than grandfathered in one.

And whose doing is that?