What Is an ADU?
An accessory dwelling unit (ADU) is a secondary housing unit on a single-family residential lot. ADUs go by many names depending on region and local terminology — granny flats, in-law suites, backyard cottages, secondary suites, carriage houses, and casitas. They can be detached structures built in a backyard, attached additions to an existing home, conversions of existing spaces like garages or basements, or even small internal units carved out of a primary residence. A subtype called a Junior ADU (JADU), recognized in California law, allows a unit of up to 500 square feet created within the walls of an existing structure. ADUs have become one of the most effective solutions to the housing shortage across the United States. They add housing supply without requiring new infrastructure, help homeowners offset mortgage costs through rental income, and allow multi-generational living on a single lot. As of 2026, at least 18 states have passed laws specifically governing ADU development, and thousands of cities have adopted their own ADU-friendly regulations. Understanding your state and local rules before starting any ADU project is essential — the rules for setbacks, size limits, permits, and fees vary significantly even between cities in the same state.
States With the Most Permissive ADU Laws
California leads the nation in ADU-friendly legislation. Since 2017, the state has passed a series of bills that systematically preempt local restrictions. AB 68 (2019) eliminated owner-occupancy requirements and reduced setbacks to 4 feet. AB 881 (2019) prohibited cities from imposing setbacks greater than 4 feet and waived impact fees for ADUs under 750 square feet. SB 9 (2022) authorized urban lot splits on single-family parcels, allowing up to 4 units where only 1 was previously allowed. AB 2533 (2024) further streamlined permitting standards. California's preemption is full: cities cannot require owner-occupancy, charge excessive fees, impose minimum lot sizes, or require parking within half a mile of transit. Oregon followed with House Bill 2001 in 2019, which required all cities with more than 10,000 residents to allow ADUs by right in residential zones. Portland was already ADU-permissive before HB 2001, with parking requirements for ADUs eliminated as early as 2013. Washington passed HB 1337 in 2023, expanding ADU rights statewide and preempting local owner-occupancy and parking mandates. Colorado enacted HB 24-1175 in 2024, requiring municipalities over 1,000 residents to allow ADUs by right in single-family zones, eliminating owner-occupancy requirements and capping permit fees. These four states represent the leading edge of ADU reform and offer the most predictable regulatory environment for homeowners planning an ADU project.
States With Moderate ADU Regulations
A second tier of states has passed ADU or housing reform legislation that expands ADU rights without fully preempting local zoning authority. These states allow ADUs more broadly than before but still leave cities with some flexibility to set design standards or size limits. Connecticut passed PA 21-29 in 2021, allowing ADUs by right in all residential zones across the state while permitting municipalities to set reasonable design standards. Maine enacted LD 2003 in 2022, requiring cities and towns to allow ADUs on single-family lots without special permit requirements. New Hampshire codified ADU rights in RSA 674:71-73, requiring municipalities to permit at least one ADU per single-family lot by right. Vermont's Act 47 (2023) and Act 47 (2023) both advanced statewide ADU allowances. Virginia Section 15.2-2291 requires localities to allow ADUs on single-family lots. Utah SB 174 (2021) requires municipalities to allow ADUs in areas with existing single-family or two-family zoning, with statewide preemption taking effect for Salt Lake City and other municipalities. North Carolina HB 488 (2023) establishes a right to build ADUs on single-family lots, which has created statewide preemption for Charlotte, Raleigh, and other NC cities. In these states, local jurisdictions retain authority over design compatibility, setback specifics (within reason), and architectural standards — but they cannot ban ADUs outright. Homeowners in moderate-regulation states should still verify local rules, but they can expect a path to approval.
States Without ADU-Specific Legislation
Many states leave ADU policy entirely to local jurisdictions. No statewide ADU law does not mean ADUs are banned — it means rules vary city by city and county by county. Texas has no statewide ADU law. However, Austin's Development Code Title 25, Section 25-2-774 creates a robust local ADU framework allowing secondary apartments up to 1,100 square feet. The 2023 HOME Ordinance further allows up to 3 total units on any Austin SF-zoned lot. Dallas, Houston, and San Antonio each have their own local ADU allowances. Houston's lack of traditional zoning means ADU restrictions are effectively minimal compared to most cities. Florida enacted HB 1339 in 2024, making it one of the more recent states to establish ADU rights, requiring local governments to allow ADUs in residential zones. Before that reform, Florida cities like Miami, Tampa, and Orlando operated entirely under local rules. In states without statewide ADU legislation — including Illinois, Georgia, Tennessee, Michigan, and Pennsylvania — homeowners must research city-specific codes. PropertyZoned's city pages compile the current local rules for all 100 major US cities, giving you a starting point before you call your local planning department.
How Much Does an ADU Cost?
ADU costs vary significantly by city, project type, and local construction labor rates. All permit fee figures below are sourced directly from the city data compiled on PropertyZoned. In Los Angeles, total permit fees typically run $2,000 to $8,000 depending on ADU size and project valuation. Impact fees are waived for ADUs under 750 square feet under California state law (AB 881). In San Francisco, total permit fees range from $3,000 to $10,000, reflecting the city's higher valuation-based fee schedule. In Portland, permit fees range from $3,000 to $8,000 for plan check and building permit, plus Portland's System Development Charges (SDCs) for transportation, parks, and water/sewer — which can add $10,000 to $25,000 for a new detached ADU. In Austin, total fees including significant Water/Wastewater and Transportation impact fees typically run $3,000 to $15,000. In Seattle, permit fees run $4,000 to $12,000 with school impact fees additional. Construction costs on top of permit fees depend heavily on ADU type: a garage conversion in LA might cost $50,000 to $150,000 all-in, while a new detached ADU in San Francisco can reach $300,000 to $500,000. Attached ADUs (room additions) typically fall between those ranges. Prefabricated ADU units have grown popular in California and Oregon as a way to reduce construction costs and timelines — some manufacturers offer complete units installed for $100,000 to $200,000. Always budget for architectural/design fees (5-15% of construction cost) and unexpected site conditions.
ADU Permit Process: Step by Step
The ADU permit process follows a similar general sequence in most jurisdictions, though timelines and specific requirements vary significantly. Step 1: Confirm your zoning. Look up your property on your city's zoning map to verify that your zone allows ADUs and whether any overlay districts (historic, hillside, coastal) apply. Step 2: Review city-specific requirements. Check your city's ADU rules for maximum size, setbacks, height limits, parking requirements, and utility connection rules. PropertyZoned's city pages summarize these for all 100 major US cities. Step 3: Hire an architect or designer. Most cities require stamped architectural plans for ADU permits. Some cities (Portland, San Diego) offer pre-approved standard plan sets that can reduce design costs and review time. Step 4: Submit plans to your local building department. In California, ministerial approval is required within 60 days under state law. Portland's Bureau of Development Services processes standard ADUs in 4-8 weeks. Austin's DSD typically takes 2-4 months. Step 5: Address plan check corrections. First-round corrections are common — budget time for one or two revision cycles. Step 6: Pull the permit and begin construction. In most jurisdictions, the permit must be physically posted on-site during construction. Step 7: Schedule and pass inspections. Required inspections vary but typically include foundation, framing, rough electrical, rough plumbing, insulation, and final. Step 8: Receive your certificate of occupancy (CO). The CO legally authorizes the unit for occupancy and is required for legal rental. The entire process from initial research to CO typically takes 6 to 18 months.
Key Differences Between Cities
Even within the same state, ADU rules can vary significantly between cities. In California, Los Angeles permit fees typically total $2,000 to $8,000 while San Francisco runs $3,000 to $10,000 for similar projects — a meaningful cost difference on top of higher SF construction costs. LA allows detached ADUs up to 1,200 square feet; Portland caps detached ADUs at 800 square feet. Portland's no-parking requirement dates to 2013 while LA's parking waivers depend on transit proximity. Seattle permit fees ($4,000 to $12,000) are significantly higher than Portland ($3,000 to $8,000 before SDCs), and Seattle adds school impact fees while Portland's SDCs cover different categories. Houston's lack of traditional zoning creates a uniquely permissive environment where ADUs face fewer barriers than in almost any other major city. Austin's HOME Ordinance (2023) allows up to three total units on any SF-zoned lot, effectively making Austin one of the most permissive Texas cities for additional dwelling units — a direct result of local policy rather than any statewide law. These differences matter significantly for project feasibility. A site in Portland that would require $10,000 in SDCs might cost $3,000 in LA permit fees for the same project. Understanding city-by-city variation is essential before committing to a project budget.
How to Get Started
Getting started on an ADU project begins with research, not contractors. Before you spend money on designs or permits, you need to know whether an ADU is feasible on your specific lot. Use PropertyZoned's ADU Feasibility Checker to evaluate your lot based on size, zone, and local requirements. The tool uses city data to give you a preliminary feasibility assessment in minutes. Next, look up your city's full ADU rules on PropertyZoned's city pages. Each city page covers allowed ADU types, maximum size and height, setback requirements, parking rules, permit fees, and permit timeline. We cite official municipal code sources and last-verified dates for every data point. If you are in a state with ADU-friendly legislation (California, Oregon, Washington, Colorado), you have additional protections against unreasonable local restrictions. Look up your state's ADU law summary on PropertyZoned's state pages. Once you have confirmed feasibility, your next steps are: hire an architect familiar with your local ADU code, obtain preliminary design plans, and consult with your planning department about any project-specific issues before submitting for permit. Many cities offer pre-application meetings at no cost where a planner will review your proposed project and flag issues before you spend money on full construction documents. Use these resources — they save time and money. PropertyZoned's comparison pages can also help you understand how your city's rules compare to similar markets, which is useful context for evaluating ADU builder bids and understanding whether local fees are typical or unusually high.
Frequently Asked Questions
Can I build an ADU in any state?
Not all states have ADU-specific legislation, but most cities allow some form of secondary dwelling. California, Oregon, Washington, and Colorado have the strongest statewide ADU protections. In states without ADU laws (like Texas), rules vary by city — Austin is permissive while other Texas cities may be more restrictive. Always check your local zoning code and state laws before planning a project.
Do I need a permit to build an ADU?
Yes, virtually all jurisdictions require a building permit for ADU construction. Most also require architectural plans reviewed and stamped by a licensed professional. In California, cities must process ADU permit applications ministerially within 60 days. Some cities also require a planning permit or design review if the ADU is in a historic district or sensitive overlay zone. Operating an unpermitted ADU can result in fines, orders to vacate tenants, and required demolition.
How long does it take to build an ADU?
The total timeline from initial research to move-in typically ranges from 6 to 18 months. Plan check alone takes 2-6 weeks in most California cities, 4-8 weeks in Portland, and 2-4 months in Austin. After permit approval, construction of a simple garage conversion takes 2-4 months; a new detached ADU takes 4-12 months. More complex projects in jurisdictions with design review or historic overlay requirements can take longer.
Can I rent out my ADU?
In most jurisdictions, yes — that is one of the primary reasons homeowners build ADUs. Long-term rentals (30+ days) are allowed in virtually all cities that permit ADUs. Short-term rentals (under 30 days via Airbnb/VRBO) are regulated separately and vary significantly: Los Angeles requires Home Sharing registration, San Francisco requires the host to be present on-site, and many cities ban whole-unit short-term rentals altogether. Check your city's short-term rental rules on PropertyZoned before assuming vacation rental income is an option.
What is the difference between an ADU and a JADU?
A Junior ADU (JADU) is a California-specific ADU subtype defined under California Government Code Section 65852.22. A JADU is limited to 500 square feet and must be created within the walls of an existing single-family residence or accessory structure — it cannot be a new detached structure. JADUs must share utility connections with the primary dwelling and do not require a separate bathroom (a bathroom can be shared). The main advantage of a JADU is lower construction cost since you are converting existing space, and California law grants the same by-right approval protections as full ADUs. JADUs are not recognized outside California.