The 20% affordable component of Builder's Remedy means you need to propose at least 5 homes so one of them can be affordable.
That said, a "missing middle" program is a really helpful thing to help cities get their Housing Element into compliance so if you think there is a place where this density would be helpful, send a comment to your city council and cc HousingElements@hcd.ca.gov.
Perhaps it's less lucrative, but it would still allow for increasing the housing stock without putting a 50 unit building in the midst of an existing SFH neighborhood. Developers will also need to weigh the desirability of what they build into the equation. There's quite possibly a sweet spot of desirability and affordability at 3 dwellings per lot.
As an example, I live on a multi-dwelling lot (effectively town-homes without shared walls) in a predominately SFH neighborhood. It's very high density by California standards, but it's quite desirable and not apartment-block style living.
Unfortunately, many California cities are so incredibly far behind what's actually needed for equitable housing costs that the only practical way for them to catch up is a lot of 50-unit buildings built yesterday.
That said, a "missing middle" program is a really helpful thing to help cities get their Housing Element into compliance so if you think there is a place where this density would be helpful, send a comment to your city council and cc HousingElements@hcd.ca.gov.