In brief
- The law is on your side. On a lot zoned for homes, with a house on it or planned, the city must review an ADU by objective rules, and where a home already exists, decide within 60 days of a complete application.
- Your city still sets most of the details. Size limits above what the state guarantees, design, front setbacks and many site rules are local, and they differ from city to city.
- The lot usually decides the outcome. Easements, utilities, fire access, hazard zones and the true property lines settle most real projects.
- Public records answer part of it. Zoning, lot size, hazard zones and permit history are public. Easements, utility capacity and survey lines generally are not.
- A good report shows its sources. Whatever you pay for, each answer should name the record it came from, the date it was checked and what it could not determine.
Three layers decide it
Think of feasibility as three filters, applied in order. Each one can only narrow what the one before it allowed.
A feasibility check that stops at the first layer tells you an ADU is legal. Only the third layer tells you whether one fits, and at what cost.
What state law settles
California's ADU law is in Government Code §§ 66310–66342. These are the points it settles for every city and county. For the full picture, including junior ADUs, multifamily lots and the changes that took effect January 1, 2026, read ADUs 101: A Plain-Language Guide to Accessory Dwelling Units in California.
| Question | What state law says | Section |
|---|---|---|
| Where are ADUs allowed? | On lots zoned to allow single-family or multifamily homes that have a home, or will have one | § 66314 |
| How is it reviewed? | Ministerially, by objective standards: no hearing and no discretionary review | § 66317 |
| How long can the city take? | 15 business days to say whether the application is complete. On a lot with an existing home, 60 days to approve or deny a complete application, after which it is deemed approved | § 66317 |
| How many units on a single-family lot? | One ADU and one junior ADU inside the house or an existing accessory structure (which may grow by up to 150 sq ft for an entrance), plus one new detached ADU with 4-ft side and rear setbacks. From January 1, 2027: two new detached ADUs | § 66323(a) |
| What can't local rules block? | An 800 sq ft ADU with 4-ft side and rear setbacks. Lot coverage, floor-area, open-space, front setback and minimum lot size rules can't prevent it | § 66321(b)(3) |
| Is there a minimum lot size? | A city cannot set one for ADUs | § 66314(b)(1) |
| Must the owner live on site? | Not for an ADU. For a junior ADU, only if it shares a bathroom with the main house | §§ 66315, 66333 |
| Impact fees? | None for an ADU of 750 sq ft or less, or a junior ADU of 500 sq ft or less. Above that, proportional to the main home's size. From January 1, 2027, charged only on the area above 750 sq ft, for up to two ADUs on a lot | § 66311.5 |
| Can an HOA prohibit it? | Not on a lot zoned for single-family homes: HOA and deed rules that prohibit or unreasonably restrict an ADU are void. Reasonable restrictions are allowed | Civ. Code §§ 4751, 714.3 |
| Fire sprinklers? | Can't be required in an ADU if the main home isn't required to have them | § 66323(d) |
None of these guarantees that a particular backyard can hold a building. State law also lets a city designate where ADUs may be permitted, based on the adequacy of water and sewer service, traffic flow and public safety (§ 66314(a)).
Coming January 1, 2027. Two bills signed on September 29, 2026 change the rules above from the new year: AB 956 requires cities to approve two new detached ADUs on a single-family lot instead of one (and lets them decline a junior ADU on a lot that has both), and SB 1117 limits impact fees on an ADU over 750 sq ft to the area above 750 sq ft, for up to two ADUs on a lot. Neither has an urgency clause, so both take effect on January 1, 2027. A city's own rules and forms may lag behind.
What your city decides
Within the state floor, each city or county writes its own ADU standards. They usually cover:
- Maximum size and height above what the state guarantees, and how much of the lot or rear yard structures may cover.
- Front setbacks, siting and separation from the main house.
- Design rules such as materials, windows facing neighbors, and standards for historic properties.
- Site rules from other codes: fire department access, protected trees, grading, flood and hazard-zone requirements.
- Process: submittal checklists, fees, and whether the city offers preapproved ADU plans.
Start with your city's or county's ADU web page and its zoning code. If the city's ordinance conflicts with state law, or was never submitted to the state's housing department (HCD), state standards apply instead. In San José, for example, the City lets owners choose between its own standards and the State standards, but not a mix of both; our San José page walks through that city's checklist.
What the lot decides
These are the questions that settle most real projects, where each answer comes from, and whether free public records can settle it.
| Question | Where the answer comes from | Public records alone? |
|---|---|---|
| Which city or county has jurisdiction, and how is the lot zoned? | County assessor record; city or county zoning map | Yes |
| How large is the lot and what shape? | County parcel map | Roughly. Parcel maps are not surveys |
| Where exactly are the property lines and existing buildings? | A boundary survey | No |
| Was the main house built with permits? Any open code case? | City permit history and code enforcement records | Partly. Older permits may not appear in online systems |
| Is there an easement through the buildable area? | Preliminary title report and recorded documents | Rarely from free maps. The title report is the reliable source |
| Is the lot in a flood zone? | FEMA flood maps | The zone, yes. Elevation requirements depend on the design |
| Is it in a fire hazard severity zone? | State Fire Marshal hazard maps | Yes |
| Is it in an earthquake fault, liquefaction or landslide zone? | California Geological Survey maps | The zone, yes. Ground conditions need a geotechnical report |
| Is it historic, or in the Coastal Zone? | City or county planning department | Usually, from local records |
| Can fire crews reach it, and is there enough water at the hydrant? | Fire department rules; a water-flow letter from the water provider | No |
| Can sewer, water and electric service handle another unit? | Utility providers; a site inspection | No |
| Protected trees, slope, drainage? | City tree rules; a site visit | Partly |
| Do HOA rules apply? | The HOA's governing documents | No (they are private), though state law limits what they can restrict |
How to check your own lot
You can answer the public-record questions yourself, for free, in about this order:
- Find the parcel. Look up the property on your county assessor's website to get its parcel number (APN), lot size and whether it is inside a city.
- Find the zoning. Use your city's or county's zoning map. ADU rules follow the zoning district.
- Read the local ADU rules. Your city's ADU page usually links its ordinance, a submittal checklist and any preapproved plans.
- Pull the permit history. Search the city's permit portal for the address, or request records. Check that the main house and any garage or shed you plan to convert were permitted.
- Check the hazard maps. Search the address in the FEMA Flood Map Service Center, the State Fire Marshal's Fire Hazard Severity Zone maps and the California Geological Survey's EQ Zapp.
- Read your title report. The one from your purchase lists recorded easements. If you don't have it, a title company can prepare a preliminary report.
- Ask the utilities and the city. Ask your water provider about fire flow at the nearest hydrant, and ask the city whether it offers a pre-application meeting.
If steps 1 to 5 raise no problem, the remaining questions are physical: lines, easements, utilities and access. That is the point where paying for professional help usually starts to make sense.
What a feasibility report should include
Whoever prepares it, a useful ADU feasibility report covers four groups of questions, and shows its work for each:
- Eligibility: jurisdiction, zoning, the main house's permit status and any open code case.
- Standards: which size, height and setback limits apply, under which ordinance or code section, and which ADU types the lot allows.
- Site constraints: hazard zones, easements, fire access, trees and slope, with the map or document each finding came from.
- Services and cost drivers: utility connections, fees and anything that needs a letter, a survey or an inspection.
For every line, look for three things: the figure or finding, the source document or dataset, and the date it was checked. Be cautious of reports that give a single yes-or-no score without sources, that never say what they could not determine, or that mainly lead to one builder's product.
What a feasibility study costs
Published prices vary with how much of the work is records research and how much is people on site. On the websites of nine California providers we checked on October 22, 2026, we found:
| Type | Published price | What it usually covers |
|---|---|---|
| Online screening | Free | An address-based check of zoning and state rules |
| Paid online report | About $75 to $200 | A written report from public data; some include setbacks, buildable area and a cost estimate |
| Study by a builder | About $500 to $1,000 | Records research, often with a preliminary site plan; some credit the fee toward construction |
| Consultant or architect study | About $2,000 to $3,000 | An on-site inspection, utility review and design work |
Prices change, and the cheaper options rely on the same public records you can check yourself. What the higher prices buy is the lot layer: someone measuring, checking utilities and drawing what fits.
Where LedgeX fits
LedgeX is building the records part of this work as a Property File: one page per parcel, where every fact shows the source it came from and the date it was retrieved, and anything public records can't answer is marked "Can't determine" instead of guessed. LedgeX is in private testing, with an initial focus on San José single-family properties. It does not cover other California cities yet.
Property File
- Zoning districtKnown limitR-1-8 Single-Family ResidenceCity of San José zoning layer · retrieved 2026-10-02
- Unit sizeNeeds your inputThe allowed size depends on the unit you plan to build.
- Private easementsCan't determinePublic records don't show private easements. A title report will.
Illustrative example. Figures are for a sample lot, not a real parcel.
What this page does not establish
- Whether an ADU fits on any specific property. This page explains how to check; it does not check a parcel.
- Your city's rules. Local standards vary. Only your city's or county's current code and planning staff can confirm them.
- Every part of state law. The table summarizes the points that most often affect feasibility. Junior ADUs, multifamily lots, the Coastal Zone and legalization of existing units have more detail.
- Current prices. The price ranges are what providers published on October 22, 2026, and are not quotes or endorsements.
- Law after the verification date. Last verified October 24, 2026. California amends ADU law most years.
Sources
- California Government Code, Title 7, Division 1, Chapter 13, Accessory Dwelling Units (§§ 66310–66342)
- California Civil Code § 4751, ADUs in common interest developments
- California Civil Code § 714.3, deed restrictions on ADUs
- AB 956 (Chapter 791, Statutes of 2026) and SB 1117 (Chapter 809, Statutes of 2026), chaptered text
- California Department of Housing and Community Development, ADU Handbook (updated March 2026)
- FEMA Flood Map Service Center
- Office of the State Fire Marshal, Fire Hazard Severity Zones
- California Geological Survey, EQ Zapp: California Earthquake Hazards Zone Application
- Prices published on the websites of California ADU feasibility providers, checked October 22, 2026
This page is general information, not legal, architectural or financial advice. Confirm requirements for a specific property with the local planning department and a qualified professional before relying on them.