ADU Rules by State: Accessory Dwelling Unit Guide
Accessory Dwelling Units (ADUs) — also called in-law suites, granny flats, backyard cottages, or secondary units — have seen major regulatory changes in the last five years. What was once heavily restricted or outright prohibited is now encouraged in most states as a response to housing shortages. If you're considering building an ADU, the rules in your specific city still matter — but the general trend is toward more permissive regulations.
Types of ADUs
There are three main types. A detached ADU is a separate structure in the backyard — the classic "backyard cottage." An attached ADU shares a wall with the primary home, like a converted garage or addition. A Junior ADU (JADU) is created entirely within the existing primary residence — a converted basement, attic, or large bedroom — and is typically limited to 500 square feet.
- Detached ADU: separate backyard structure, typically 400–1,200 sq ft
- Attached ADU: shares wall with primary home, often a garage conversion
- Junior ADU (JADU): entirely within primary residence, max 500 sq ft
California's ADU Reform: The National Model
California passed sweeping ADU reform starting in 2017 and continuing through multiple bills (SB 9, AB 2221, SB 897). Key provisions now apply statewide: cities cannot require more than a 4-foot setback for detached ADUs, cannot require owner-occupancy as a condition of approval, must approve or deny applications within 60 days, and cannot require additional parking for ADUs within half a mile of public transit. Impact fees are prohibited for ADUs under 750 square feet.
These reforms effectively override local restrictions in California, making it one of the most ADU-friendly states in the country.
Size Limits Across the Country
Outside California, ADU size limits vary from city to city. Common limits for detached ADUs range from 600 to 1,200 square feet, often capped at a percentage of the primary dwelling (typically 40–60%). Some cities also have minimum sizes (400 sq ft is common) to ensure units are actually livable.
Height limits for detached ADUs are typically 16–25 feet, with most cities at 20 feet or the height of the primary structure, whichever is less.
ADU Rules by State: Where Things Stand in 2025
The national trend is toward ADU legalization and preemption of overly restrictive local rules, but the landscape varies considerably by state.
California — The gold standard for ADU reform. Statewide law (AB 2221, SB 897) preempts local restrictions on setbacks (max 4 ft), owner-occupancy requirements, parking, and impact fees on ADUs under 750 sq ft. Every single-family lot in the state may have at least one ADU plus one JADU by right.
Oregon — A 2017 law (HB 2007) required all cities over 10,000 in population to allow ADUs by right on single-family lots. Oregon was the first state after California to mandate ADU access statewide. Setbacks and size limits remain locally defined within state guidelines.
Washington — HB 1337 (2023) requires cities to allow at least two ADUs on single-family lots, bans owner-occupancy requirements, and prohibits parking minimums for ADUs near transit. Cities had until the end of 2024 to update their codes.
Montana — SB 245 (2023) eliminated local prohibitions on ADUs statewide, making Montana one of the more sweeping single-session ADU reform states. Cities may still regulate design and size but cannot simply prohibit them.
Maine — LD 2003 (2022) requires municipalities that allow single-family housing to also permit at least one ADU per residential lot. A significant reform for a traditionally home-rule state.
Texas — No statewide ADU mandate. Rules vary enormously by city. Austin has progressively liberalized ADU rules and eliminated owner-occupancy requirements. Houston, which has no traditional zoning, allows ADUs with minimal restrictions in most areas. Dallas and San Antonio have more traditional local restrictions.
Florida — No statewide ADU mandate. Rules vary by city and county. Miami-Dade, Broward, and Palm Beach counties each have distinct ADU policies. Some Florida municipalities are permissive; others still prohibit detached ADUs entirely.
New York — New York City passed ADU reform legislation in 2023 legalizing basement and cellar conversions (long a gray area). Outside NYC, ADU rules are entirely local. Suburban communities in Westchester, Nassau, and Suffolk counties vary widely.
Arizona — Phoenix allows ADUs on most single-family lots. Arizona has considered statewide preemption but as of 2025 cities still set their own rules. Scottsdale and Tempe have updated their ordinances to be more permissive.
Colorado — Denver has been a leader in ADU reform, passing ordinances allowing ADUs in most residential zones. Colorado passed HB 1152 (2024) requiring cities over 1,000 population to allow ADUs on single-family lots — bringing it closer to the California/Oregon model.
- Most permissive (statewide preemption): California, Oregon, Washington, Montana, Maine, Colorado (2024)
- Permissive by major cities, no statewide mandate: Texas (Austin, Houston), New York (NYC)
- Locally variable, no statewide mandate: Florida, Arizona
- Check your city: even in reform states, setbacks, size limits, and design standards are local
Permit Costs and Timeline
ADU permits are among the more expensive residential permits — typically $500–$2,500 for the building permit itself, plus potential impact fees, school fees, and utility connection fees. California and some other states cap or waive these fees for small ADUs.
Timeline from application to approval typically runs 4–12 weeks for a straightforward ADU. Construction adds another 3–6 months depending on complexity and contractor availability.
Frequently Asked Questions
Can I build an ADU on any residential lot?
Not always. Many cities have minimum lot size requirements (often 5,000–7,500 sq ft) for detached ADUs. In California, these minimums cannot be applied to ADUs that would fit within 800 square feet. Check your city's specific rules.
Do I need to live on the property to build an ADU?
Owner-occupancy requirements vary significantly. California eliminated its owner-occupancy requirement in 2020. Many other cities still require the owner to live in either the primary dwelling or the ADU. This is one of the most variable rules in ADU regulations.
How many ADUs can I build on my property?
Most single-family residential lots are limited to one ADU plus one JADU. Some cities allow two ADUs on a single-family lot. Multifamily properties often have different rules.
Does an ADU require separate utilities?
Requirements vary. Some cities require separate utility meters; others allow shared utilities with the primary dwelling. Separate metering adds cost but gives you the ability to separately track usage or bill a tenant.
Can I rent out my ADU on Airbnb?
This depends on both your city's short-term rental rules and any owner-occupancy requirements. In cities that require owner-occupancy, you must live on the property. Short-term rental of ADUs may be additionally restricted. Check both your ADU regulations and your short-term rental ordinance.