Accessory Dwelling Units — regulations, costs, builders by city

Accessory Dwelling Unit regulation is not one thing. It is a state-framework layered with a city overlay, and the gap between those two layers is wider than most homeowners realize. California pre-empts most local ADU rules through AB 68, AB 881, AB 2221, and SB 897, which together override city-level setback, parking, and owner-occupancy restrictions¹. Cities then layer bonus-density programs, condo-sale pathways, and design review on top. Texas has no state-level ADU framework at all, which means Austin owns its entire rulebook and Dallas owns a different one. Canada is province-led: Ontario passed Bill 23 More Homes Built Faster Act, and British Columbia's SSMUH (Small-Scale Multi-Unit Housing) legislation forces municipalities to permit up to four units on most lots. Seattle did its own heavy lift with the 2019 ADU-DADU reform and the 2023 follow-up. Denver passed Group Living in 2020. Phoenix has minimal state pre-emption, so Maricopa County and the city's own R1-6 zoning code do most of the work.

This page synthesizes across all eight cities AskBaily currently routes ADU projects into. It is designed for homeowners who are researching ADUs across multiple markets because they own property in more than one place, are relocating, or are evaluating which market actually lets them build the unit they want.

The four regulatory regimes — CA state pre-emption vs TX no-state vs Canadian province-led vs city-by-city

California sits at one extreme. The state has been systematically stripping cities of their ability to block ADUs since 2017. AB 68 eliminated owner-occupancy requirements through 2025. AB 881 removed minimum lot-size mandates. AB 2221 capped permit review times at 60 days and restricted design review to objective standards. SB 897 loosened height limits. The cumulative effect is that any California homeowner on a residential lot can expect to build at least one ADU and one JADU under state default standards, regardless of what the local zoning code says¹. City governments in California have responded by offering bonus programs (extra unit count, waived fees, expedited review) in exchange for affordability covenants or other public benefits. Los Angeles went one step further with AB 1033, letting homeowners sell ADUs as separate condo units.

Texas sits at the opposite extreme. There is no statewide ADU pre-emption law. Austin's HOME Initiative (Phase 1 in 2023, Phase 2 in 2024) legalized up to three units on most single-family lots and reduced minimum lot size. Dallas has a separate framework. San Antonio has a third. A homeowner in Houston plays by entirely different rules than one in Austin. This means the regulatory risk in Texas is all in city council composition, not state capital composition, and that flips the research burden back onto the homeowner.

Canada is province-led. Ontario's Bill 23 requires municipalities to permit up to three units on residential lots as of right. British Columbia's SSMUH legislation (late 2023) requires municipalities to allow up to four units on most lots and up to six on lots near transit. But municipalities still control design standards, parking minimums within provincial caps, and building-code enforcement. Toronto's laneway suite policy and secondary suite legalization predate Bill 23 and remain the operative framework for most homeowners. Vancouver's laneway house framework predates SSMUH and still defines the path for most detached ADU projects there.

Seattle and Denver are city-by-city laggards in the sense that neither Washington nor Colorado has passed California-style statewide pre-emption. Seattle's 2019 ADU-DADU ordinance and 2023 amendments eliminated owner-occupancy, raised height limits, and allowed two ADUs per lot². Denver's 2020 Group Living text amendment expanded household definitions and ADU allowances. Phoenix has done the least zoning reform of any city in this list and still relies on R1-6 base standards plus master-planned community architectural review for most ADU applications.

The 8 cities AskBaily currently covers — a comparison matrix

Each city deserves its own read, and each has its own dedicated pillar page linked at the bottom. Here is the comparison at one glance.

• Los Angeles operates under full California state pre-emption, plus AB 1033 condo-sale (adopted by LA City Council in 2024), plus SB 9 lot-split stacking on single-family lots. Two pillars: the AB 1033 condo-sale pathway and the SB 9 lot-split + ADU combination.
• San Diego operates under California state pre-emption plus the Complete Communities Housing Solutions program (CCHS), which grants bonus unit density in transit priority areas in exchange for affordability covenants³. One pillar covers the combined ADU + JADU + CCHS framework.
• Phoenix operates under Arizona's minimal state framework plus R1-6 base zoning plus master-planned-community architectural review for most suburban lots. One pillar covers AZ ROC licensing plus MPC ARC submission.
• Austin operates under the HOME Initiative, which legalized up to three units on most single-family lots and lifted lot-size minimums⁴. One pillar covers the HOME amendments and their interaction with Austin's preservation-district overlays.
• Seattle operates under the 2019 ADU-DADU ordinance and the 2023 follow-up amendments, allowing two ADUs per lot, raised height for DADUs, and eliminated owner-occupancy². One pillar.
• Denver operates under Group Living 2020 plus municipal general contractor registration plus DORA-licensed trades plus Wildland Urban Interface overlay for foothill-adjacent lots. One pillar.
• Toronto operates under Ontario Bill 23 plus the city's laneway suite policy plus the secondary suite legalization framework. Two pillars: laneway suites and secondary suite basement apartment legalization.
• Vancouver operates under BC SSMUH plus Vancouver's own laneway house framework plus seismic baseline requirements that exceed most North American standards. One pillar.

Cost spectrum across all 8 markets

ADU costs are driven by three things: labor rates, land value (which drives finish-level expectations), and permitting duration. The spread across these eight markets is wide, and homeowners who assume coastal California pricing in Phoenix or Austin will overestimate by 2x.

Los Angeles and San Diego anchor the high end. A ground-up detached ADU in either city runs $200,000 to $600,000 depending on size, finish, and site conditions. The labor premium is real (union-adjacent trades, high workers' comp), and the permitting tail adds carrying cost.

Toronto and Vancouver sit in the mid-high range in Canadian dollars, roughly $200,000 to $400,000 CAD for a laneway or secondary suite. The weakness of the Canadian dollar against the US dollar makes this feel cheaper to cross-border owners, but material costs have risen substantially post-2022.

Phoenix and Austin sit at the low end. A detached ADU in Phoenix often comes in at $80,000 to $250,000 because lot sizes are generous, trades are less unionized, and permitting is faster. Austin runs slightly higher at $120,000 to $400,000 because of recent labor tightening.

Seattle and Denver sit mid-high at $180,000 to $500,000. Seattle's cost pressure is labor plus the Puget Sound seismic overlay plus long permit review times. Denver's cost pressure is labor plus the WUI overlay for foothill-adjacent lots plus the municipal GC registration step that eliminates handyman-grade bids.

These are ranges, not promises. A custom architect-designed ADU with premium finishes will clear the top of any of these ranges. A prefab or modular unit on a flat lot with existing utility stubs will come in below.

The four unlock patterns that repeat across cities

Four patterns show up everywhere once you read enough ADU codes.

Bonus density tied to transit or lot thresholds. San Diego's CCHS grants extra unit count near transit lines. Austin's HOME tiers unit count by lot size. Toronto's laneway policy requires lane access, which implicitly biases toward denser streetcar-era neighborhoods. The pattern is the same: cities trade density for location.

State-pre-empted setbacks. California caps ADU setbacks at 4 feet regardless of underlying zoning. BC SSMUH does something similar. Most cities without state pre-emption retain 5-to-10-foot setbacks. This one rule changes what fits on a small lot more than any other.

Condo-sale unlock. Los Angeles under AB 1033 is the first US city to let homeowners sell ADUs as separate condo units⁵. Other California cities are evaluating whether to adopt. No other jurisdiction in this list has a comparable mechanism yet, though Toronto's multiplex condominium structure approximates it.

Fire sprinkler parity. The rule most homeowners miss: if your primary residence has sprinklers, your ADU needs them; if it doesn't, it usually doesn't. This one rule can add $8,000 to $20,000. It shows up identically in California, Washington, Colorado, and Ontario building codes.

How to find an ADU-qualified contractor in each city

The generic pattern for contractor verification is the same everywhere: state or provincial credential, local permit-office registration, current insurance, and prior ADU project history. The specific credentials differ.

California requires a CSLB Class B General Building Contractor license. A Class C (specialty) license is not sufficient for ADU ground-up work. The CSLB public lookup confirms license status and workers' comp.

Texas has no state GC license at all. Austin verifies at the city permit office through contractor registration, certificate of insurance, and the city's contractor registration system. A contractor's ADU track record matters more here than credentialing, because credentialing is so thin.

Seattle requires Washington L&I contractor registration plus Seattle-specific licensing for certain scopes. Denver requires DORA-licensed trades plus a municipal GC registration number that shows up on every permit. Phoenix requires AZ ROC licensing at the appropriate dollar threshold.

Toronto contractors in Ontario should be HCRA-registered for new-home construction and Tarion-registered where applicable. Vancouver contractors should be registered with the BC Homeowner Protection Office for new residential construction warranty coverage.

The universal pattern: never hire off credential alone. Always pair credential + insurance + at least two completed ADU projects of similar scope in the same city.

How Baily routes your ADU project

The default industry pattern is that homeowners submit their scope to a lead aggregator, which resells it to 10 to 12 contractors who each pay a lead fee and then compete on price. That model compresses contractor margin, which compresses build quality, which lands on the homeowner as change-order surprises and abandoned projects.

Baily works differently. Homeowners scope their project with Baily once. Baily confirms license, insurance, and ADU-specific project history for the contractors in the relevant city. Then Baily matches to one pre-vetted ADU specialist per project. Contractors do not pay per-lead; the model is a percentage take on completed work. The incentive structure rewards completing projects well, not converting the most leads fastest.

Below are the ten city-specific ADU pillars covering each of the eight markets above in detail.