Methodology

How we estimate ADU cost

Cost figures on every city page and on the costs hub are produced by a regression model with two layers:

Layer 1 — per-square-foot baseline by ADU type (LA-anchored)

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:

• GreatBuildz — LA-focused builder-aggregator blog
• Abodu — California prefab vendor
• Angi — national aggregator (US generic)
• CALI ADU — Los Angeles design-build (premium positioning)
• Andalusia Drafting — California drafting services
• Maison Remodeling — San Jose remodeller
• GatherADU — Los Angeles HQ ADU guide
• VerifiedADU — California editorial

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

Layer 2 — regional multiplier vs LA baseline

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×	—

What the cost model does not capture

The regression captures all-in design-build cost (architecture, engineering, permits, materials, labour, soft costs). It does not capture lot-specific variables that materially shift real quotes:

• Site work — grading, retaining walls, soil-bearing fixes on hillside lots can add $10,000–$60,000.
• Utility upgrades — panel upgrades from 100A to 200A, sewer-line extensions, water-meter upgrades typically add $5,000–$25,000.
• High-end finishes — quartz/stone counters, custom cabinetry, wide-plank flooring, designer fixtures can push 20–40% above the “typical” range.
• Permit fees — vary by city; baseline assumes ~$5,000–$15,000. Cities with affordable-ADU bonus programs (e.g. Sacramento) may waive fees entirely; cities without can charge $20,000+.

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.

How we verify city-specific ordinance data

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:

• The exact source URL
• The fetch method (live page, archived snapshot, manual curation from a Municipal Code section)
• The fetch date
• Any caveats specific to that city's data

For cities without verified ordinance data, the city page shows the California state-law floor (post-AB-2221, AB-976, AB-1033 effective 2024) with no city-specific overlay. State-law minimums apply universally and are not city-specific data.

What we do not do

• We do not publish AI-generated values. Where automated scrapes returned values that contradict state law (an LA scrape returned a 5-foot rear setback, the state floor is 4 feet), we did not save those values. The scrape was rejected and the city manually entered from the official Municipal Code section.
• We do not infer values. If a city's planning page does not state, for example, the maximum ADU size, the city page shows “state default” and links to the city planning department. We do not guess.
• We do not republish stale data without re-verification. Cities update ordinances on their own schedule. Each entry in city_ordinances.json carries the verification date; we re-verify quarterly.

Cities deferred for future verification

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.

How we source the licensed-builder directory

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:

1. Fetch the CSLB master file (244,112 contractors as of the most recent fetch).
2. Filter to active California B-class general contractor licences (102,004 records).
3. Filter to records with ADU-specialist branding in the registered business name — substrings “ADU”, “granny”, “casita”, “backyard”, “in-law”, etc. (144 records remain after this filter; documented in data/adu_builders_ca.csv).
4. Group by city using the registered business address.

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).

What this filter misses

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.

How we report ADU permit history

Annual ADU permit counts on every city page are sourced from the Big West Coast ADU Dataset published by BuildinganADU.com, which aggregates and normalises permit records from California city building departments. The dataset attributes 88,885 ADU permits to California jurisdictions across the 2013–2022 window.

For our deeper data analysis of this permit history, see California ADU Permits 2013–2022: A Data Analysis — original analysis of geographic concentration, ordinance-grade-vs-volume correlation, and the long tail of cities still permitting almost no ADUs.

Known gap

The Big West Coast Dataset stops at calendar year 2022. The California Department of Housing and Community Development (HCD) now collects ADU permit reporting at the state level via the Annual Progress Report (APR) framework. We expect a publishable 2023–2025 update from BuildinganADU.com or directly from HCD in 2026; we will republish the analysis when that data drops.

Update cadence and known gaps

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	Big West Coast Dataset	Annual (when published)	2024 (covers 2013–2022)
State law summary	California legislative tracker	After each ADU bill enacted	April 2026 (post AB-1332)

State legislative activity drives off-cycle updates. AB-2221 (effective 2023, setback reductions and ministerial-review tightening), AB-976 (effective 2023, owner-occupancy preemption extended to 2030), AB-1033 (effective 2024, condominium conversion of ADUs), and AB-1332 (effective 2025, pre-approved-plan framework) each triggered a content review on every city page in the relevant scope.

Corrections process

If a value on this site is wrong, please email hello@backyardadu.net. 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.

About the editor

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: May 2026.