California ADU 101 (for Homeowners) — 2025 Edition

Expert guide for California homeowners considering an ADU. Written in clear language, with public, verifiable sources. Citations use numbered references and a source list at the end.

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TL;DR

• ADUs are legal statewide and approved ministerially (no hearings) if you meet objective rules. Cities must act within 60 days on a complete ADU/JADU application. [2][3]
• A minimum 800 sq ft ADU with 4-ft side/rear setbacks must be allowed; many sites qualify for 16–18 ft heights (context dependent). [2][3]
• Parking is often not required (e.g., within ½ mile of transit) and no replacement parking is required for garage/carport conversions. [2][3]
• Impact fees are $0 for ADUs under 750 sq ft; above that, fees must be proportional to the primary home. [3]
• Owner-occupancy is prohibited for ADUs (you may rent both units); JADUs still require owner-occupancy. [4][5]
• Pre-approved (standard) ADU plans are being rolled out statewide and can shorten plan check; the law provides faster 30-day decisions for local pre-approved plan submittals. [6]
• Financing has improved: FHA, Freddie Mac, and Fannie Mae now allow ADU rental income (with conditions) to help borrowers qualify. [12][13][14]

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What counts as an ADU (and a JADU)?

An Accessory Dwelling Unit (ADU) is a self-contained home on the same lot as a primary residence. It has its own kitchen, bath, and entrance, and can be detached, attached, or a conversion of existing space (e.g., garage). [2][3]

A Junior ADU (JADU) is ≤ 500 sq ft and carved entirely within an existing single-family home (or attached garage). It requires an efficiency kitchen and an exterior entrance; owner-occupancy is required (owner lives in the primary or the JADU). [2][4]

Why this matters: Qualifying ADUs are approved ministerially using objective standards—no subjective hearings. [2][3]

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Core statewide rules you can rely on

Ministerial approval & timeline (60 days). Your city/county must issue a decision within 60 days of a complete application (when there's an existing home on the lot). Reviews apply objective standards only. [2][3]

Baseline buildable envelope that must be allowed. Every jurisdiction must allow at least one 800 sq ft ADU with 4-ft side/rear setbacks regardless of lot coverage/FAR/min lot size. Height floors include 16 ft (detached) and 18 ft in certain transit/multifamily contexts; attached ADUs may follow the primary dwelling height subject to statute. [2][3]

Parking you may not need. Cities cannot require ADU parking in several cases (e.g., within ½ mile of public transit, conversions, historic districts), and cannot require replacement when a garage/carport is converted or demolished to build an ADU. [2][3]

Impact & utility fees. ADUs < 750 sq ft are exempt from impact fees. For ≥ 750 sq ft, fees must be proportional to the primary dwelling. Water/sewer connection/capacity charges must be proportionate to actual burden; interior conversions often avoid new connections. [2][3]

Fire sprinklers. ADUs do not require sprinklers if the primary dwelling doesn't have them; creating an ADU does not trigger sprinklers for the main home. Always confirm local fire access/safety requirements. [2][16]

Owner-occupancy & rentals. No owner-occupancy may be imposed for ADUs (made permanent by AB 976). JADUs still require owner-occupancy. Many jurisdictions require ≥ 30-day rentals for ADUs—check your city's short-term rental rules. [2][4][5]

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The approval path (and how to go faster)

1. Feasibility first. Check setbacks, height, utilities, easements/trees, and fit the 800 sq ft / 4-ft "safe harbor." If near transit or on certain multifamily lots, you may qualify for 18 ft height. [2][3]

2. Pick a plan path.

   • Local pre-approved (standard) plan: cities/counties maintain catalogs or vendor lists; state law supports 30-day decisions on those applications. [6]
   • Custom design: more flexibility, typically longer plan check.

3. Ministerial plan check. Respond to objective comments only. Cities can't apply standards that would block the 800/4-ft allowance. [2][3]

4. Permits & build. Coordinate utilities early; confirm no replacement parking for garage conversions; verify any fire access needs and whether sprinklers apply based on the main home. [2][16]

Fast-track reality: If available, use pre-approved plans. LA City, LA County, San José, and Sacramento all run programs that shorten review. [7][8][9][10]

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Costs, rents, and financing (2025 snapshot)

Typical costs (planning bands). Rigorous statewide research (pre-pandemic) found average ADU costs around the $160k–$180k range, with regional variation (Bay Area higher; LA somewhat lower on a $/sf basis). 2024–2025 labor/material inflation pushes turnkey costs higher, but ≤ 750 sq ft units often save five figures in fees. [19][2][3]

Rental demand. Long-term rental demand for ADUs is strong; many cities require ≥ 30-day leases to preserve housing supply. [2]

Financing has improved.

• FHA allows 75% of estimated ADU rent in certain cases; 203(k) can finance ADU work. [12]
• Freddie Mac permits counting ADU rental income for qualifying (guide conditions apply). [13]
• Fannie Mae supports underwriting with ADU income under specified caps and documentation. [14]

ROI tip: If your program works at ≤ 749 sq ft, you'll avoid impact fees by statute—often the cleanest "yield per dollar" for rentals or multigenerational use. [3]

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Local snapshots (what to expect by market)

Los Angeles (City)

• Standard Plan Program: catalog of pre-reviewed ADU designs—significantly faster plan check. [7]
• Coastal Zone parcels may require extra coastal review; hillside/fire zones can add safety standards.
• City policy aligns with state baselines (e.g., 800/4-ft, parking exemptions); ADUs typically restricted to ≥ 30-day rentals. [2]

Orange County (cities + unincorporated)

• Follows state baselines on setbacks/heights/fees/parking; county guidance highlights fee rules (exemption < 750 sq ft; proportional ≥ 750 sq ft). [20]
• Many cities restrict ADUs to ≥ 30-day rentals; coastal cities add Coastal Zone considerations. [2]

San José / South Bay

• Preapproved ADUs (vendor plan library) enable much faster permits for eligible sites. [9]
• Santa Clara County runs a pre-approved program for unincorporated areas. [11]
• Costs trend higher in the Bay Area—tight feasibility/budget control recommended. [19]

Sacramento (City & County)

• Shelf-Ready ADU plans (free) from the City; County also publishes a shelf-ready program. [10][11]
• ADU-friendly permitting; all-electric plan sets available; leverage ≤ 750 sq ft to minimize fees. [2][3]

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Beyond California: quick comparatives for WA & OR

• Washington (EHB 1337, 2023–2025): Cities planning under GMA must allow two ADUs on single-family lots in urban areas and remove several local barriers (owner-occ, some parking/lot-size constraints). Implementation guidance via WA Dept. of Commerce. [17]
• Oregon (ORS 197.312): State law prohibits owner-occupancy and off-street parking requirements for ADUs inside urban growth boundaries; state guidance and case law limit extra siting constraints. [18]

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Common homeowner questions (California)

Q: Can my city deny my project for subjective reasons (neighborhood "character")? A: No. Qualifying ADUs must be approved ministerially using objective standards. [2][3]

Q: Do I need to replace my garage parking if I convert it? A: No—state law forbids requiring replacement parking when converting/demolishing a garage/carport for an ADU. [2][3]

Q: Do I need fire sprinklers? A: Generally only if the main house has them. Building an ADU does not trigger sprinklers in a non-sprinklered primary dwelling. [2][16]

Q: Can I short-term rent (Airbnb) my ADU? A: Often no. Many cities (and state-exempt ADUs) require ≥ 30-day leases. Check local STR ordinances. [2]

Q: What if my city's ordinance seems stricter than state law? A: HCD actively reviews and enforces compliance. Non-compliant ordinances are void to the extent they conflict with state law. [1][2]

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How ADU Pilot can help (feasibility → permit → build)

At ADU Pilot, we specialize in feasibility research for California homeowners:

1. Zoning & site check against state baselines and your city's rules (setbacks, height, lot coverage, easements, fire access).
2. Pre-approved plan fit vs custom trade-offs for rent and build-cost ROI.
3. Fee & timeline model (flag ≤ 750 sq ft impact-fee advantages; parking exemptions; sprinkler implications).
4. Permit pack tuned to objective requirements to reduce comments and hit the 60-day statutory window. [2][3]

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Sources

[1] California HCD — Accessory Dwelling Units (official portal): https://www.hcd.ca.gov/policy-and-research/accessory-dwelling-units [2] California HCD — Accessory Dwelling Unit Handbook (latest): https://www.hcd.ca.gov/policy-and-research/accessory-dwelling-units (see "Handbook" PDF) [3] California Government Code § 65852.2 (ADUs): https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=GOV&sectionNum=65852.2 [4] California Government Code § 65852.22 (JADUs): https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=GOV&sectionNum=65852.22 [5] AB 976 (2023) — Owner-occupancy prohibition for ADUs extended/made permanent: https://leginfo.legislature.ca.gov/faces/billNavClient.xhtml?bill_id=202320240AB976 [6] AB 1332 (2023) — Local pre-approved ADU plans & 30-day decisions for those applications: https://leginfo.legislature.ca.gov/faces/billNavClient.xhtml?bill_id=202320240AB1332 [7] Los Angeles (LADBS) — ADU Standard Plan Program: https://www.ladbs.org/services/plan-check-permit/standard-plan-program/adu-standard-plan [8] Los Angeles County — Pre-Approved ADUs: https://pw.lacounty.gov/building-and-safety/adu/pre-approved-adus [9] City of San José — Preapproved ADUs: https://www.sanjoseca.gov/your-government/departments/planning-building-code-enforcement/building-division/homeowners/adu/preapproved-adus [10] City of Sacramento — Shelf-Ready ADU Plans (free): https://adu.cityofsacramento.org/shelf-ready-plans [11] Santa Clara County — Pre-Approved ADU Program: https://plandev.santaclaracounty.gov/adu/pre-approved-adu-program [12] HUD / FHA (Oct 16, 2023) — Press release on ADU rental income in underwriting: https://www.hud.gov/press/press_releases_media_advisories/hud_no_23_237 [13] Freddie Mac — ADU resource (income & appraisal guidance): https://sf.freddiemac.com/working-with-us/accessory-dwelling-units [14] Fannie Mae — ADUs (underwriting/eligibility updates): https://singlefamily.fanniemae.com/originating-underwriting/underwriting/other-sources-income/accessory-dwelling-units [15] (Optional backup) HCD Technical Help / FAQs (rental duration norms, objective standards): https://www.hcd.ca.gov/policy-and-research/accessory-dwelling-units [16] City of San Diego DSD — Residential sprinklers (technical bulletin; ADU context): https://www.sandiego.gov/development-services/forms-publications/technical-bulletins [17] Washington Dept. of Commerce — ADU Implementation (EHB 1337): https://www.commerce.wa.gov/growth-management/housing/planning-for-housing/accessory-dwelling-units/ [18] Oregon DLCD — ADU guidance (ORS 197.312; owner-occ/parking preemption): https://www.oregon.gov/lcd/UP/Pages/ADU.aspx [19] Terner Center (UC Berkeley) — Reaching California's ADU Potential (costs, production trends): https://ternercenter.berkeley.edu/research-publications/reaching-californias-adus-potential [20] Orange County Public Works — ADU Flyer (fees/standards summary): https://ocpw.oc.prod.acquia-sites.com/sites/ocpwocip/files/2023-09/ADU_Flyer_070821.pdf