Methodology
How we collect, verify, and publish ADU data.
Purpose
This research hub aggregates publicly available data on ADU regulations, permits, costs, and programs from government and institutional sources. Our goal is to make it easy for journalists, policymakers, and researchers to find and cite ADU data without digging through scattered government PDFs and raw spreadsheets.
Data Sources
Every headline statistic comes from one of these source categories. We do not use aggregator citations or unattributed data.
| Source | What We Use It For | Update Frequency |
|---|---|---|
| U.S. Census Bureau (ACS 5-Year) | Population, income, rent, housing units, multigenerational households | Annual (December) |
| HUD Fair Market Rents | Rental income benchmarks by metro area | Annual (October) |
| California HCD Annual Progress Reports | ADU permit counts and trends by jurisdiction | Annual |
| CareScout/Genworth Cost of Care Survey | Assisted living costs by state | Annual |
| Mercatus Center | State ADU law classifications and regulatory analysis | Annual (August) |
| FHFA UAD Aggregate Statistics | ADU property value data from appraisals | Periodic |
| RSMeans / Census C30 | Construction cost indices | Annual |
Original Calculations
We perform four types of original calculations. All use simple arithmetic on trusted source data. None require defending a complex methodology.
Care Gap Breakeven
Formula: Midpoint ADU construction cost ÷ Annual assisted living cost
Sources: Regional construction cost data and CareScout/Genworth Cost of Care Survey
California Permit Trends
Source: California HCD Annual Progress Reports
Method: Direct extraction and aggregation. No modeling or estimation. Year-over-year growth rates are basic percentage change calculations.
ADU Support Stack
Method: Manual verification of city government websites for the presence of 8 official support features. Binary yes/no scoring.
Each entry includes the source URL and date verified. The “Support Score” is a simple count of yes answers (0–8), not a weighted composite.
State-by-State Comparison Counts
Method: Arithmetic aggregation from Mercatus Center state ADU law classifications. Counts of states meeting specific regulatory criteria.
Limitations
- ADU construction cost estimates are regional approximations. Actual costs vary significantly by location, ADU size, site conditions, and labor market.
- State ADU law classifications are based on Mercatus Center taxonomy and may not reflect very recent legislative changes.
- California permit data depends on timely and accurate HCD Annual Progress Report submissions by individual jurisdictions.
- The ADU Support Stack reflects city government website content at the time of verification. Features may have been added or removed since the verification date.
- Care Gap breakeven calculations are simplified comparisons. They do not account for opportunity cost, maintenance, insurance, property tax increases, or time value of money.
Update & Correction Policy
Data is reviewed and updated quarterly. Each publication displays a “Last updated” date. Corrections are logged in a public changelog.
If you find an error, please email research@dwellingindex.com with the specific data point, the page URL, and the correct source if available.
Independence Statement
Dwelling Index publishes independent ADU data and analysis using public government sources. This research is not sponsored by any ADU builder, lender, or service provider. Research findings are published regardless of outcome. No sponsor review or approval is sought or given.