SB 1211 for Fourplex / 4-unit in San Mateo
A 4-unit fourplex / 4-unit in San Mateo qualifies for up to 5 new homes under SB 1211 — 4 detached ADUs and 1 interior conversions. At San Mateo's median rent of $3,000/mo, that's ~$180K in additional gross annual rent.
The San Mateo scenario
The scenario most San Mateo fourplex / 4-unit owners face: an existing building, mature trees, and a back lot that's currently doing nothing. SB 1211's 4-ft setback and 18-ft height envelope fits 4 detached units behind that primary structure with room for fire-access in front.
San Mateo-specific note: Peninsula tech rents; very strong upside.
Small multifamily (2–4 unit) design playbook
Small multifamily lots are usually constrained on width before they're constrained on count. With a 4-unit existing footprint, the binding constraint becomes side-yard setbacks (4 ft min) and the 18-ft height limit. Two-story stacked-flat ADUs over surface parking are the highest-yield typology.
Pro-forma snapshot
| Existing units | 4 |
| Detached ADUs (SB 1211) | 4 |
| Interior conversions | 1 |
| Total new units | 5 |
| Median 1BR rent in San Mateo | $3,000/mo |
| Est. additional gross rent | $180K/yr |
| Estimated total project cost | ~$1,344K |
| Year-1 NOI estimate | ~$135K |
| Stabilized cap rate at cost | 10.0% |
| Replacement parking required | No (§66313) |
| Approval pathway | Ministerial · §66314 |
| Typical permit timeline in San Mateo | 75-130 days |
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