431 Wellesley Ave
Mill Valley 94941 · Tamalpais · Unincorporated Marin · Coordinates: 37.88318, -122.53623
Executive Verdict · Public-Records Synthesis
Zoned RMP-1.5 in Unincorporated Marin with likely ministerial ADU eligibility and zero mapped flood, fire, or fault hazard overlaps; benchmark value $2,309,000 ($2,032,000–$2,586,000).
Upsides & Opportunities
- ✓ Ministerial ADU / JADU pathway likely eligible
Zone RMP-1.5 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 (+$17,983 contributory est.)
Public building records show issued construction/renovation work on this parcel (4 total permits, latest Feb 4, 2021).
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. 16.3 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)
- ! 6 slow or stalled nearby projects within ½ mile
Closest lead: New home at 560 Alta Way (0.29 mi NW, 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)
- ! Year 1 Prop 13 tax reset (~$27,004/yr at $2,309,000)
Sale triggers reassessment to 1.00% base + ~0.15% voter debt (~$2,250/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:
- 23,087 sq ft (0.53 ac)
- Assessor use:
- Single-Resid. - Improved
- Addresses on lot:
- 431 WELLESLEY AVE
Zoning & Land Use
RMP-1.5 — Residential Multiple Planned (Unincorporated Marin)
State Housing Pathways Screen (ADU & SB 9)
- ADU or junior ADULikely eligible
Zoned RMP-1.5 (multifamily residential): state law allows ADUs where homes are allowed. On a multifamily lot the state allowance is different: ADUs in non-living space of the building, plus up to eight detached ADUs.
- SB 9: two homes on the lotUnlikely
Zoned RMP-1.5 (multifamily residential), not a single-family residential zone.
- SB 9: split into two lotsUnlikely
Zoned RMP-1.5 (multifamily residential), not a single-family residential zone.
Mapped Hazard Zones on This Lot
- CAL FIRE Fire Hazard Severity Zone
- Not in a State-mapped fire hazard severity zone
- 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: $2,032,000 – $2,586,000 · 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
- New home at 560 Alta Way (Applied)0.29 mi NW
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).
- Accessory dwelling unit at 403 Marin Ave (adu 403a) (Issued)0.36 mi S
Active in "Issued" for 2.9 yr (first record Dec 21, 2023) with no new permit activity recorded in 19 mo. High-impact scale (1 housing unit, $1,000,000 declared valuation across 1 permit).
- New home at 731 Bay Rd (Issued)0.39 mi SW
Active in "Issued" for 18 mo (first record May 12, 2025) with no new permit activity recorded in 15 mo. High-impact scale (1 housing unit, $955,000 declared valuation across 1 permit).
- New commercial building at 100 Harvard Ave (Issued)0.41 mi NE
Active in "Issued" for 23 mo (first record Dec 26, 2024) with no new permit activity recorded in 18 mo. High-impact scale ($6,500,000 declared valuation across 1 permit).
Entity-resolved builders & contractors in ½ mile: Wright Rhode Constr (1 on parcel, 1 in ½ mi · Addition (adds square footage)); Swell Services (1 on parcel, 1 in ½ mi · Solar / battery storage); American Solar (1 on parcel, 1 in ½ mi · Solar / battery storage); Lockwood Electric (1 on parcel, 1 in ½ mi · Electrical / panel); Owner Builder (107 in ½ mi · Remodel / interior alteration).
Permit & Project History on This Parcel (4 permits, 0 grouped projects)
| Record # | Category | Status | Applied / Issued | Valuation | Description |
|---|---|---|---|---|---|
| 167075 | Solar / battery storage | Issued | Iss. Feb 4, 2021 | $29,153 | Energy Storage System |
| 155057 | Solar / battery storage | Issued | Iss. Mar 30, 2017 | $26,000 | 7.6kw Roof Mounted Photovoltaic - Express |
| 145790 | Addition (adds square footage) | Issued | Iss. Feb 10, 2015 | $60,000 | Addition - Change Of Contr 7/30/15 T.D. 12/31/14 |
| 142875 | Electrical / panel | Issued | Iss. Jun 9, 2014 | $6,440 | Upgrade Electric Service - 100 To 200amps |