How every numeric claim on BackyardADU is sourced and verified. The cost model, ordinance data, builder directory, and permit history each have a documented protocol below. Citations link to the original published source where available.
Cost figures on every city page and on the costs hub are produced by a regression model with two layers:
Per-square-foot ranges are calibrated to the median of published 2026
builder pricing aggregated from eight California-relevant sources. Raw
data (per-source quoted ranges) lives in
data/cost_quotes.csv in the build pipeline:
For each ADU type we take the median of medians across sources and set the low/high anchors at 0.6× / 1.6× the median to reflect observed real-quote spread. Current per-square-foot baseline (Los Angeles, all-in design-build):
| ADU type | Low | Typical | High | Default size |
|---|---|---|---|---|
| Detached new-build | $180 | $300 | $480 | 750 sqft |
| Attached | $150 | $240 | $380 | 700 sqft |
| Garage conversion | $ 90 | $150 | $240 | 400 sqft |
| Junior ADU (JADU) | $ 90 | $150 | $240 | 400 sqft |
| Prefab | $135 | $225 | $360 | 600 sqft |
| Basement | $105 | $175 | $280 | 600 sqft |
City-specific cost ranges apply a regional multiplier to the LA-anchored baseline. The seven regions and their multipliers reflect observed labour-rate and permit-fee differentials documented in our source set:
| Region | Multiplier | Examples |
|---|---|---|
| Bay Area | 1.30× | SF, Oakland, San Jose, Berkeley, Palo Alto |
| Central Coast | 1.05× | Santa Barbara, SLO, Monterey, Santa Cruz |
| LA / Orange County | 1.00× | LA, Long Beach, OC core |
| San Diego | 0.95× | SD city + county coastal |
| Sacramento | 0.85× | Sacramento, Roseville, Folsom, Davis |
| Inland Empire | 0.85× | Riverside, San Bernardino, Ontario, Palm Springs |
| Central Valley | 0.75× | Fresno, Bakersfield, Stockton, Modesto |
| Default (any other CA city) | 0.95× | — |
The published per-square-foot ranges in our source set are typical design-build pricing for a buildable lot with standard utilities and mid-grade finishes. That covers architecture, engineering, building permits at a typical fee schedule, materials, labour, and the general-contractor margin. It does not consistently capture lot-specific or scope-specific variables that materially shift real quotes:
We currently treat each of the eight sources as one observation and take the median of medians. This gives equal weight to a national aggregator (e.g. Angi), a vertical specialist (Abodu, prefab-only), and an LA-focused design-build (CALI ADU). A statistically tighter approach would weight by sample size, geography, and scope coverage — data which most published guides do not disclose. We treat the median-of-medians range as a scoping range, not a forecast, and verify against city-specific real quotes when those become available. The equal-weighting assumption is documented here so readers can judge for themselves.
For these reasons, the cost grid on each city page should be treated as a scoping range, not a quote. The standard guidance is to get three written bids from CSLB-licensed B-class general contractors before committing.
Out of California's roughly 540 incorporated cities, we currently have city-specific ordinance data verified for 19 cities: Anaheim, Berkeley, Burbank, Chula Vista, Encinitas, Fremont, Garden Grove, Glendale, Long Beach, Los Angeles, Pasadena, Sacramento, San Diego, San Francisco, San Jose, Santa Ana, Santa Barbara, Santa Monica, and Santa Rosa. Together they cover roughly 76% of California ADU permit volume since 2017.
For these 19 cities, every value displayed on the city page (max ADU size,
max height, side and rear setbacks, parking requirements, notable local
provisions) is sourced from the city's own planning department web page
and verified via direct fetch. Each entry in
data/city_ordinances.json includes a
_data_provenance field documenting:
For cities without verified ordinance data, the city page shows the California state-law floor (Government Code sections 66310 to 66342, as they read in October 2026) with no city-specific overlay. State-law minimums apply universally and are not city-specific data.
city_ordinances.json carries the verification
date; we re-verify quarterly.Seven cities planned for the original verification batch (Oakland, Santa Maria, Santa Cruz, Palm Springs, Westminster, Rosemead, Palo Alto) hit anti-bot blocks on their respective city CMSes that prevented automated extraction. Rather than publish unverified values, those city pages currently show the state-law floor. Re-attempt is queued for the next verification cycle using browser-emulating fetch (Firecrawl stealth) or manual curation from each city's Municipal Code section.
Every contractor listed on a city page or in our full directory is sourced from the California State License Board's Public Data Portal (the CSLB master file). The pipeline:
data/adu_builders_ca.csv).For each builder we display the registered business name, mailing address, license number, classification, and date issued. Visitors can verify the licence status independently at the CSLB Instant License Check (linked from every builder card).
The CSLB-name-based filter undercounts. Many real ADU builders have generic business names (“Smith Construction Inc”) and would not match the substring filter even if 40% of their projects are ADUs. The 144-builder count is a high-precision, low-recall lower bound on the actual ADU specialist universe in California. Phase 2 of the directory will augment with Yelp Fusion API and Google Business Profile scrapes to widen recall.
We do not accept payment for placement. Builders cannot pay to appear in the directory or to rank higher within it. Builders cannot remove their entries while their CSLB licence is active. Future paid listings (planned but not yet active) will be marked as “Featured” and will not displace organic ranking.
Annual ADU permit counts on every city page come from two sources. For 2013–2017 we use the Big West Coast ADU Dataset published by BuildinganADU.com, which aggregates and normalises permit records from California city building departments (HCD's Annual Progress Report framework has no ADU line item before 2018). For 2018–2025 we use California HCD's Housing Element Annual Progress Report Table A2 (October 2026 release, published by HCD on 2026-10-02), filtered to ADU rows with an issued building-permit date. The full merged file is downloadable at /data/.
For our deeper data analysis of this permit history, see California ADU Permits 2013–2025: A Data Analysis — original analysis of geographic concentration, ordinance-grade-vs-volume correlation, and the long tail of cities still permitting almost no ADUs.
HCD's data is revised continuously as jurisdictions amend their filings, and the first release of any reporting year understates it (2025 went from 25,289 permits in the April 2026 release to 41,844 by the October release, and the 2024 total has been revised both up and down). We refresh the permit layer whenever HCD re-publishes the file, so figures can move slightly between refreshes.
Build times on city pages and in the build times report are the number of days from a building permit's issue date to the certificate of occupancy (or other form of readiness, such as final inspection) in the same HCD file. Because the state file holds one row per project per reporting year, we link a project's rows by parcel within a jurisdiction. A city's figure is shown only when at least 40 completed ADU projects can be measured and at least 40% of the ADUs it permitted in 2018–2023 have a completion on record. The Los Angeles figures are checked against the City's own permit system. The full method, the audit and the limits are on the report page. No individual property is shown anywhere on this site.
| Data layer | Source | Refresh cadence | Last refresh |
|---|---|---|---|
| Cost regression | Builder pricing pages | Quarterly (April / Oct) | April 2026 |
| City ordinances | City planning pages | Quarterly | April 2026 |
| CSLB builder file | CSLB Public Data Portal | Quarterly | April 2026 |
| Permit history | HCD APR Table A2 (+ Big West Coast for 2013–2017) | Quarterly | October 2026 (covers 2013–2025) |
| Build times | HCD APR Table A2, permit and completion dates linked by parcel | With each permit refresh | October 2026 |
| State law summary | California Legislative Information (bill text and Government Code) | After each ADU bill enacted | October 2026 (through the 2026 session) |
State legislative activity drives off-cycle updates. AB-2221 (effective 2023, front-setback limits and written reasons for a denial), AB-976 (effective 2024, owner-occupancy ban made permanent), AB-1033 (effective 2024, separate sale as a condominium where a city opts in), and AB-1332 (programs due by January 1, 2025, pre-approved ADU plans) each triggered a content review on every city page in the relevant scope.
If a value on this site is wrong, please email hello@decisionlab.com.au. State the page URL, the specific value you believe is incorrect, the value you believe is correct, and your source. Page-specific corrections are typically actioned within 24 hours; broader methodology questions within 2 business days.
Corrections are logged with the date and the source change in the
site's CHANGELOG (visible at CHANGELOG.md in
the build repository). We do not silently update content — each
revision is dated and documented.
BackyardADU is published by Jarrod, the named editor. For more on editorial standards, conflicts of interest, and how the site is funded, see about the editor.
Last updated: October 2026 (permit layer refreshed to HCD's October 2026 release and build times added; cost, ordinance and CSLB layers last verified April 2026).