Rental income is the number one reason California homeowners build ADUs, and this estimator grounds that decision in real data: official HUD Fair Market Rents for your exact ZIP code, by unit size. Enter your ZIP and the size of the unit you're planning, and it projects monthly and annual rental income over 30 years — accounting for vacancy and rent growth along the way.
In this guide:
- How to Use This Calculator
- Understanding Your Results
- About HUD's Fair Market Rent Data
- Vacancy & Rent Growth Defaults
- Assumptions & Limitations
How to Use This Calculator
| Field | What to Enter |
|---|---|
| Property ZIP Code | The 5-digit ZIP where the ADU will be built. The estimator covers all 2,400+ California ZIP codes in HUD's dataset — if you see "Enter a valid California ZIP code," the ZIP isn't in HUD's California data (verify the digits, or try an adjacent ZIP). |
| ADU Size | The unit's bedroom count, from Studio to 4-Bedroom. This selects which of HUD's five rent tiers applies — and the bar chart shows all five side by side so you can weigh building bigger. |
| Vacancy Rate (annual) | The share of each year you expect the unit to sit empty (default 7.7% ≈ four weeks between tenants). |
| Rent Increase Rate (annual) | How much you expect rent to grow per year (default 3.5%, California's long-run average). |
Understanding Your Results
The Rental Income Analysis cards project three figures at the 5-, 10-, 20-, and 30-year marks: the Monthly Rent after compounding growth, the Gross Annual Rent (monthly × 12), and the Effective Annual Rent — what actually lands in your pocket after the vacancy allowance.
The bar chart compares today's HUD rent across all five unit sizes for your ZIP, with your selected size highlighted. This is often the most decision-relevant view on the page: in many California ZIPs, the step from a 1-bedroom to a 2-bedroom adds several hundred dollars a month, while the step from 3 to 4 bedrooms adds much less per square foot of extra construction.
The line chart tracks gross versus effective annual rent over 30 years — the widening dollar gap between them is your cumulative vacancy cost. The collapsible table itemizes every year (monthly rent, gross rent, vacancy loss, effective rent, and cumulative income) and can be downloaded as a CSV.
About HUD's Fair Market Rent Data
Rent figures come from the U.S. Department of Housing and Urban Development's Small Area Fair Market Rents (SAFMRs) — the ZIP-code-level version of the rent standards HUD publishes every fiscal year to administer housing programs like Section 8. We use the FY2026 dataset, which covers roughly 2,400 California ZIP codes with separate figures for studios through 4-bedroom units.
Three things are worth knowing about how HUD builds these numbers:
- They are gross rents — HUD's figures include the cost of essential tenant-paid utilities, not just the base rent.
- They target roughly the 40th percentile of rents paid for standard-quality units that recently turned over — by design, a modest, below-median benchmark. A brand-new ADU with modern finishes typically commands more than the SAFMR, so treat the estimate as conservative.
- They're updated annually from Census survey data and rent-inflation factors, so figures reflect recent market conditions with some lag.
Because the data is ZIP-level rather than city- or county-level, it captures real neighborhood differences — two ZIPs in the same city can differ by $1,000+ per month. A handful of border-area ZIPs (like Lake Tahoe-adjacent 894xx codes) appear in the data because HUD assigns them to California metro areas.
Vacancy & Rent Growth Defaults
The 7.7% vacancy default is roughly the long-run U.S. average rental vacancy rate — about four weeks per year. California's chronically undersupplied markets usually run tighter (often 3–5% in coastal metros), so the default leans conservative; well-priced ADUs in high-demand areas frequently re-rent within days.
The 3.5% rent growth default tracks California's long-run average annual rent increase. Two practical notes: first, AB 1482 caps increases for covered units at 5% plus local CPI (10% max) per year — many ADU arrangements are exempt, but check yours. Second, growth this smooth never happens in practice; rents plateau for years and then jump. Over a 30-year projection, the compounding average is what matters.
Assumptions & Limitations
The projection applies one constant growth rate and vacancy rate for 30 years and starts from HUD's deliberately modest benchmark — your actual achievable rent depends on the unit's condition, amenities, parking, privacy, and local demand. The estimate is gross income only: to net out operating costs, feed it into the Break-Even Calculator or the full Cash Flow & ROI Calculator.
