509 Northern Ave
Mill Valley 94941 · Tamalpais · Unincorporated Marin · Coordinates: 37.88046, -122.53917
Executive Verdict · Public-Records Synthesis
Zoned RA-B2 in Unincorporated Marin with likely ministerial ADU eligibility and mapped CAL FIRE moderate fire hazard severity zone (local responsibility area) due diligence; benchmark value $2,662,000 ($2,343,000–$2,981,000).
Upsides & Opportunities
- ✓ Ministerial ADU / JADU pathway likely eligible
Zone RA-B2 allows residential use qualifying for ministerial ADU review under state law (5 ADU permits permitted within ¼ mile in 36 months).
Source: Unincorporated Marin zoning · CA Gov. Code 66314
- ✓ Documented permitted upgrades (+$518,410 contributory est.)
Public building records show issued construction/renovation work on this parcel (4 total permits, latest Jun 9, 2025).
Source: County of Marin Community Development Agency, Building Permits (unincorporated Marin) (parcel APN)
- ✓ Above-average reinvestment in Tamalpais
50 substantial renovation & housing permits over 24 months (20.5 per 1,000 parcels vs. 16.4 citywide baseline) (#3 of 22 neighborhoods).
Source: County of Marin Community Development Agency, Building Permits (unincorporated Marin) (Tamalpais)
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 Moderate 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)
- ! Delayed or open permit activity on this parcel
Accessory dwelling unit at 509 Northern Ave (aka 511): Active in "Issued" for 4.3 yr (first record Jul 26, 2022) with no new permit activity recorded in 3.6 yr.
Source: County of Marin Community Development Agency, Building Permits (unincorporated Marin) (review-time benchmark)
- ! 4 slow or stalled nearby projects within ½ mile
Closest lead: Accessory dwelling unit at 875 Marin Dr (0.5 mi SW, applied). Inspect timeline history before assuming near-term completion.
Source: County of Marin Community Development Agency, Building Permits (unincorporated Marin) · HCD APR (½-mile radius)
Parcel & Assessor
- Lot size:
- 9,453 sq ft (0.22 ac)
- Assessor use:
- Single-Resid. - Improved
- Addresses on lot:
- 509 NORTHERN AVE; 511 NORTHERN AVE
Zoning & Land Use
RA-B2 — Residential Agriculture (Unincorporated Marin)
State Housing Pathways Screen (ADU & SB 9)
- ADU or junior ADULikely eligible
Zoned RA-B2 (single-family residential): state law allows ADUs where homes are allowed.
- SB 9: two homes on the lotCan't tell
Zoned RA-B2 (single-family residential): we can't tell whether it counts as a single-family zone for SB 9. Residential agriculture, estate and restricted: large-lot residential; confirm SB 9 with the county.
- SB 9: split into two lotsCan't tell
Zoned RA-B2 (single-family residential): we can't tell whether it counts as a single-family zone for SB 9. Residential agriculture, estate and restricted: large-lot residential; confirm SB 9 with the county.
Mapped Hazard Zones on This Lot
- CAL FIRE Fire Hazard Severity Zone
- Moderate 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 06041C0468F, effective March 16, 2016
- CGS Alquist-Priolo Earthquake Fault Zone
- Not in an Alquist-Priolo Earthquake Fault Zone
Surrounding Rent & Value Benchmarks (Public Records)
Range: $2,343,000 – $2,981,000 · Status: Off market
HUD Small Area FMR (ZIP 94941) · FY 2026
Studio $2,730 · 1BR $3,270 · 2BR $3,960 · 3BR $5,060 · 4BR $5,240
Developments Deserving Closer Investigation & Active Area Builders
- Accessory dwelling unit at 509 Northern Ave (aka 511) (Issued)On this parcel
Active in "Issued" for 4.3 yr (first record Jul 26, 2022) with no new permit activity recorded in 3.6 yr.
- Accessory dwelling unit at 511 Northern Ave (Issued)On this parcel
Active in "Issued" for 3.7 yr (first record Mar 15, 2023) with no new permit activity recorded in 3.7 yr.
- Accessory dwelling unit at 875 Marin Dr (Applied)0.5 mi SW
In "Applied" for 2.6 yr (since Apr 11, 2024), exceeding the 253d median and 686d 90th percentile for new single-family home permits in Unincorporated Marin (n=121). Record changed to "updated" in snapshot Nov 4, 2026.
- New home at 560 Alta Way (Applied)0.36 mi N
In "Applied" for 3.3 yr (since Aug 10, 2023), exceeding the 253d median and 686d 90th percentile for new single-family home permits in Unincorporated Marin (n=121). High-impact scale (1 housing unit, $975,000 declared valuation across 1 permit).
Entity-resolved builders & contractors in ½ mile: Owner Builder (1 on parcel, 90 in ½ mi · Remodel / interior alteration); Abodu (1 on parcel, 2 in ½ mi · Accessory dwelling unit (new or conversion)); Distribugen (1 on parcel, 1 in ½ mi · Solar / battery storage); United Solar (1 on parcel, 1 in ½ mi · Accessory dwelling unit (new or conversion)); Owner (45 in ½ mi · Remodel / interior alteration).
Permit & Project History on This Parcel (4 permits, 2 grouped projects)
| Record # | Category | Status | Applied / Issued | Valuation | Description |
|---|---|---|---|---|---|
| B42641 | Solar / battery storage | Issued | Iss. Jun 9, 2025 | $15,000 | 3.8kw roof mount solar |
| 173768 | Accessory dwelling unit (new or conversion) | Issued | Iss. Apr 11, 2023 | $40,000 | Factory-Built 2 Bedroom A D U |
| B32989 | Accessory dwelling unit (new or conversion) | Issued | Iss. Mar 15, 2023 | $12,600 | INSTALL A NEW 2.2KWDC PG&E GRID TIED PHOTOVOLTAIC SYSTEM, ROOF MOUNTED ON ADU IN BACKYARD |
| 172096 |