421 Blackstone Dr
San Rafael 94903 · Marinwood · Unincorporated Marin · Coordinates: 38.03823, -122.54642
Executive Verdict · Public-Records Synthesis
Zoned R1 in Unincorporated Marin with likely ADU & SB 9 infill eligibility and mapped CAL FIRE high fire hazard severity zone (local responsibility area) due diligence; benchmark value $1,333,000 ($1,173,000–$1,493,000).
Upsides & Opportunities
- ✓ ADU & SB 9 infill pathways screen as likely eligible
Zoned R1 supports ministerial ADU/JADU review plus SB 9 both two-unit development and ministerial urban lot split on a 10,149 sq ft lot. Local precedent is active with 1 ADU permit within ¼ mile over the last 36 months.
Source: Unincorporated Marin zoning · CA Gov. Code 66314 & 65852.21
- ✓ Documented permitted upgrades (+$24,640 contributory est.)
Public building records show issued construction/renovation work on this parcel (1 total permit, latest Apr 21, 2021).
Source: County of Marin Community Development Agency, Building Permits (unincorporated Marin) (parcel APN)
Watchouts & Due Diligence
- ! Mapped flood awareness zone (Zone X: minimal flood hazard)
all of the lot of the parcel intersects Zone X: minimal flood hazard. Outside mandatory federal SFHA insurance rules, but check local drainage and finished-floor requirements.
Source: FEMA National Flood Hazard Layer (parcel polygon)
- ! CAL FIRE High Fire Hazard Severity Zone (Local Responsibility Area) (all of the lot of lot)
Located in a mapped Fire Hazard Severity Zone. Expect Chapter 7A ignition-resistant building standards, defensible-space inspections, and stricter property insurance underwriting.
Source: CAL FIRE FHSZ (map Mar 24, 2025)
- ! 6 slow or stalled nearby projects within ½ mile
Closest lead: 125 new homes (0.27 mi E, entitled). Inspect timeline history before assuming near-term completion.
Source: County of Marin Community Development Agency, Building Permits (unincorporated Marin) · HCD APR (½-mile radius)
- ! Year 1 Prop 13 tax reset (~$15,780/yr at $1,333,000)
Sale triggers reassessment to 1.00% base + ~0.15% voter debt (~$1,315/mo). Confirm exact Mello-Roos / CFD special assessments on the county tax bill.
Source: CA Const. Art. XIIIA (Prop 13) · County Tax Collector
Parcel & Assessor
- Lot size:
- 10,149 sq ft (0.23 ac)
- Assessor use:
- Single-Resid. - Improved
- Addresses on lot:
- 421 BLACKSTONE DR
Zoning & Land Use
R1 — Residential Single Family (Unincorporated Marin)
State Housing Pathways Screen (ADU & SB 9)
- ADU or junior ADULikely eligible
Zoned R1 (single-family residential): state law allows ADUs where homes are allowed.
- SB 9: two homes on the lotLikely eligible
Zoned R1, a single-family residential zone.
- SB 9: split into two lotsLikely eligible
Zoned R1, a single-family residential zone. The lot is 10,149 sq ft by the assessor, room for two lots of at least 1,200 sq ft, each at least 40% of the original.
Mapped Hazard Zones on This Lot
- CAL FIRE Fire Hazard Severity Zone
- High Fire Hazard Severity Zone (Local Responsibility Area) (all of the lot, map Mar 24, 2025)
- FEMA National Flood Hazard Layer
- Zone X: minimal flood hazard (all of the lot) · FIRM panel 06041C0291D, effective May 4, 2009
- CGS Alquist-Priolo Earthquake Fault Zone
- Not in an Alquist-Priolo Earthquake Fault Zone
Surrounding Rent & Value Benchmarks (Public Records)
Range: $1,173,000 – $1,493,000 · Status: Off market
HUD Small Area FMR (ZIP 94903) · FY 2026
Studio $2,880 · 1BR $3,450 · 2BR $4,180 · 3BR $5,340 · 4BR $5,530
Developments Deserving Closer Investigation & Active Area Builders
- 125 new homes (Entitled)0.27 mi E
In "Entitled" for 23 mo (since Dec 6, 2024), exceeding the 292d median and 313d 90th percentile for new multifamily / condos / mixed-use permits in Unincorporated Marin (n=21). High-impact scale (125 housing units, 124 deed-restricted affordable across 0 permits).
- New commercial building at 200 Marinwood Ave (Applied)0.43 mi SE
Active in "Applied" for 3.9 yr (first record Dec 14, 2022) with no new permit activity recorded in 3.9 yr. High-impact scale ($35,000,000 declared valuation across 2 permits).
- Accessory dwelling unit at 2558 Heatherstone Dr (Applied)0.26 mi SE
In "Applied" for 5.5 yr (since Apr 29, 2021), exceeding the 253d median and 686d 90th percentile for new single-family home permits in Unincorporated Marin (n=121).
- Accessory dwelling unit at 287 Blackstone Dr (Issued)0.29 mi E
Active in "Issued" for 19 mo (first record Apr 23, 2025) with no new permit activity recorded in 14 mo.
Entity-resolved builders & contractors in ½ mile: Roland Franz Construction (1 on parcel, 2 in ½ mi · Addition (adds square footage)); Owner Builder (46 in ½ mi · Remodel / interior alteration); Owner (29 in ½ mi · Remodel / interior alteration); Kl Construction (3 in ½ mi · Addition (adds square footage)); Gelardi Construction (3 in ½ mi · Remodel / interior alteration).
Permit & Project History on This Parcel (1 permits, 0 grouped projects)
| Record # | Category | Status | Applied / Issued | Valuation | Description |
|---|---|---|---|---|---|
| 167873 | Remodel / interior alteration | Issued | Iss. Apr 21, 2021 | $56,000 | Kitchen & Bath Remodel |