SB 1211 for Duplex in San Mateo
Own a duplex in San Mateo? SB 1211 lets you stack 3 new units on top of your existing 2: 2 detached + 1 interior. At San Mateo's median rent of $3,000/mo, that's ~$108K in additional gross annual rent.
The San Mateo scenario
Picture the typical case: a 2-unit duplex on a San Mateo block built between 1955 and 1975, with surface parking that hasn't been fully utilized in years. SB 1211 turns that parking into 2 detached units plus 1 interior conversion — 3 new doors that didn't exist before 2024.
Local tip for San Mateo: 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 2-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 | 2 |
| Detached ADUs (SB 1211) | 2 |
| Interior conversions | 1 |
| Total new units | 3 |
| Median 1BR rent in San Mateo | $3,000/mo |
| Est. additional gross rent | $108K/yr |
| Estimated total project cost | ~$806K |
| Year-1 NOI estimate | ~$81K |
| 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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