On most single-family lots in jurisdictions with modern ADU rules, you can have one standard accessory dwelling unit plus one junior ADU, for a total of two units in addition to the main house. Some places cap you at a single ADU with no junior option, a smaller number allow two full-sized ADUs, and multifamily properties follow a different formula entirely. The number of ADUs you can have on your property depends on your state law, your city’s zoning code, and what physically fits inside your lot’s setback and coverage rules. As of mid-2025, 18 states have passed laws broadly allowing homeowners to build and rent ADUs.1Mercatus Center. A Taxonomy of State Accessory Dwelling Unit Laws 2025
The Typical Allowance on a Single-Family Lot
The most common configuration in ADU-friendly jurisdictions is one standard ADU plus one junior ADU on a single-family lot. A standard ADU is a self-contained residence with its own kitchen, bathroom, sleeping area, and entrance, and it can be either attached to the main house (including a garage conversion) or detached as a freestanding backyard structure. A junior ADU, or JADU, is a smaller unit carved out of the existing footprint of the primary home, typically capped at 500 square feet, often with an efficiency kitchen rather than a full one and sometimes sharing a bathroom with the main house.
The JADU category matters because jurisdictions that recognize it usually treat it as a separate allowance from a standard ADU. That’s why one of each is the common ceiling. Some areas allow only a single ADU and no JADU. A smaller number permit two full-sized ADUs, usually with tighter size or lot coverage restrictions.
Even when state law allows two units on paper, your specific lot might only support one. A 5,000-square-foot lot with a large house and a detached garage may have room for a JADU inside the house but no space for a detached ADU in the yard. The headline number is the legal maximum; what you can actually build is whatever survives the physical and regulatory filters below.
Allowances on Multifamily Properties
Duplexes, triplexes, and apartment buildings follow different rules than single-family lots, and the total allowance is often higher. Two paths are typical.
Many jurisdictions allow conversion of existing non-livable spaces inside a multifamily building, such as storage rooms, laundry areas, or basements, into ADUs. The cap on these conversion ADUs is often 25% of the existing unit count, so a 12-unit building might qualify for up to three. Separately, some jurisdictions allow up to two detached ADUs on a multifamily property, independent of any interior conversions. That means a multifamily parcel can sometimes carry more ADUs than a single-family lot of the same size.
What Cuts the Allowance Below the Headline Number
Five factors do most of the work in deciding what your property can actually accommodate.
Zoning District
Your property’s zoning classification is the starting point. Single-family residential zones are the primary target for ADU legislation, but not every residential zone qualifies, and mixed-use or some commercial zones may also permit ADUs. Your city’s online zoning map, usually on the planning department’s website, tells you which district you’re in.
Size and Setbacks
Maximum ADU size most commonly falls between 750 and 1,200 square feet, with some jurisdictions using a percentage of the main home’s living area instead of a fixed cap. States with strong ADU laws often guarantee a minimum allowable size, ensuring a homeowner can build an ADU of at least 800 square feet as long as it fits within required setbacks.1Mercatus Center. A Taxonomy of State Accessory Dwelling Unit Laws 2025 Setbacks for ADUs are typically four feet from side and rear property lines, though local rules can differ.
Lot Coverage
Most zoning codes set a maximum percentage of the lot that can be covered by structures. If your home and garage already consume most of that allowance, there may not be room for a detached ADU, no matter what the unit count rules say. Some older ordinances also impose minimum lot sizes for ADU construction, though the trend among states passing ADU laws is to eliminate minimum lot size requirements and let setbacks and coverage control what fits.
Parking
Parking requirements used to kill many ADU projects, especially when the natural location was a converted garage. States with strong ADU laws now prohibit local governments from requiring more parking for a lot with an ADU than for one without.1Mercatus Center. A Taxonomy of State Accessory Dwelling Unit Laws 2025 If your state hasn’t addressed parking, check whether your city still requires it, because this is often the factor that quietly makes a second unit infeasible.
HOA Rules
If your property is in a homeowners association, the CC&Rs may prohibit ADUs or impose stricter rules than local zoning. Some states have passed laws preventing HOAs from outright banning ADUs where the local government allows them, but the protection isn’t universal. Pull your governing documents before spending money on design or permits.
State Law Sets the Floor, Local Zoning Sets the Rest
State-level ADU legislation has moved quickly. As of July 2025, 18 states have adopted laws broadly allowing ADU construction and rental, with 10 classified as having strong laws and 8 having weaker ones.1Mercatus Center. A Taxonomy of State Accessory Dwelling Unit Laws 2025 Strong-law states prevent local governments from imposing three specific obstacles: owner-occupancy requirements, additional parking mandates, and discretionary review processes that can delay or block permits.
The practical point for counting units: state law is a floor your city can’t drop below. A city might try to impose rules that state law has already prohibited, and knowing the state standard gives you leverage. If your state hasn’t passed an ADU law, your local zoning code has the final word, and allowances in those states vary widely from city to city.
Owner-occupancy rules are worth checking separately. These require the owner to live on-site as a condition of having the ADU at all. Strong-law states prohibit them for standard ADUs, meaning you can rent out both the main house and the ADU. JADUs are an exception in some states and still require the owner to live in either the JADU or the primary residence. Short-term rental restrictions are a different question: many jurisdictions prohibit ADU stays shorter than 30 days, which effectively blocks vacation rental platforms.
A Boundary Worth Knowing: Financing Only Recognizes One ADU
Even where zoning allows two ADUs, conventional mortgage financing through Fannie Mae currently recognizes only one ADU per parcel.2Fannie Mae. Special Property Eligibility Considerations If you build a second unit, it may not contribute to your appraised value when you sell or refinance under conventional lending guidelines. FHA expanded its ADU financing options in late 2023 and now allows lenders to count ADU rental income toward a borrower’s qualifying income on eligible transactions.3U.S. Department of Housing and Urban Development. FHA INFO 2023-89 The gap between what zoning allows and what lenders recognize is worth factoring in before committing to a second unit.
How to Confirm the Number for Your Property
Your local planning department gives the definitive answer. Before calling, look up your property’s zoning designation on your city or county’s online zoning map, then find the ADU section of your municipality’s zoning code. It will specify how many units are allowed in your zone, maximum sizes, setback requirements, and any special conditions. Many cities now publish dedicated ADU pages that consolidate these rules in plain language.
If your state has an ADU law, read it alongside your city’s code. The state law sets the floor, and comparing the two tells you whether your city is complying or trying to impose restrictions state law has already preempted. For properties with unusual characteristics such as historic district designation, steep slopes, or environmental overlays, talk directly to a planner. These factors can reduce your allowance below what the base zoning code suggests, and they rarely show up cleanly in the written rules.