$1,789,000
421 W 39th Ave, San Mateo 94403
Size check
The listing's square footage against the county assessor's record and the permits on file.
The listing and the county record agree within 10%.
What can explain a difference
- Space built without a permit.
- Permitted work the assessor has not recorded yet.
- An ADU, a finished basement or a converted garage counted in one figure and not the other.
- Different measuring. Listing agents and the assessor often measure the same house differently.
AddressIntel AI Briefing
The optimal play is stealth density: primary + ADU + JADU, leveraging the generous backyard space of this clean parcel to add high-value accessory units. Free of geologic hazards, the site offers an easy path through ministerial approval pipelines. The neighborhood features high-income households that value additional multigenerational living space.
Property Permit History
Recent Neighborhood Remodeling (San Mateo)
Parcel & Zoning Info
ADU Potential
Flag / irregular · Corner lot · ~96 ft frontage × 115 ft deep, a tucked-away rear unit is very private; access is typically via the pole driveway. A second street frontage lets an ADU take its own entrance and address.
Screening heuristic based on lot size, existing coverage, lot orientation, and hazard overlays. CA ADU law is ministerial statewide; verify max size, setbacks, and JADU owner-occupancy with the local planning dept.
Redevelopment Potential
Meaningful unused floor-area headroom under current zoning, room to add square footage. Zoning allows roughly 2,892 sqft of gross floor area on this lot.
Screening estimate comparing existing living area to the city's single-family floor-area limit. FAR figure is a discretionary-review trigger, not a hard cap. Treat as approximate. Source: San Mateo Muni Code 27.18.060 (R1-B/C default; district unresolved) (as of 2026-08-24). Verify FAR, coverage, setbacks, and height with the local planning dept.
Hazard Snapshot
Flood Zone
Fire Severity
Liquefaction
Fault Zone
Not a substitute for a Natural Hazard Disclosure (NHD) report. Zones reflect available GIS data and may not cover all localities. Liquefaction and fault flags: modified from California Geological Survey, Web Service of Official Maps of Alquist-Priolo Earthquake Fault Zones and Web Service of Official Maps of Seismic Hazard Zones (2025).
Nearby Public Schools
Laurel Elementary
San Mateo-Foster City
Abbott Middle
San Mateo-Foster City
Hillsdale High
San Mateo Union High
School boundaries are subject to change. Nearest school distance does not guarantee enrollment eligibility. Check with the local district. Data provided by CDE.
Proprietary Intelligence
Market Heat Index
83/100
Based on recent sales velocity and local demand in 94403.
Flippability Score
70/100
Measures neighborhood renovation density vs. recent permits on this property.
Bidding War Prob.
58%
Likelihood of multiple offers based on local list-to-sale ratios and DOM.
Proforma ROI Calculator
Back-of-the-napkin deal analysis across development strategies.
Zoning allows roughly 2,892 sqft on this lot, and the default builds to that envelope. The FAR figure is a discretionary-review trigger, not a hard cap.
Median of 12 recent single-family sales in San Mateo (94403): $1,140/sqft, plus 5% because new houses sold at a median 1.07x their comps across 239 real rebuilds. Sized within 25% of the 2,900 sqft build, because smaller houses sell for more per foot and pricing a rebuild off them overstates resale.
Commission, transfer tax and closing. Deducted from resale before profit, so the ROI below is net of the exit rather than gross.
Interest on the purchase price for the whole project plus half the build budget, since construction draws ramp up. The default is 7% over 33 months, the median purchase-to-sale time of 239 real Peninsula and South Bay rebuilds. Set the rate to 0 for a cash purchase, or use your own lender's terms.
Projected Profit
Warning: Project is mathematically underwater.
Recent Comparables (San Mateo 94403)
Sized to the modelled 2,900 sqft build: sales within 25% of it. Smaller houses sell for more per foot, so pricing a rebuild off them overstates resale.