Accessory Dwelling Units (ADUs): Zoning, Permits, and Taxes

The laws and regulations governing accessory dwelling units sit in four layers: state statutes that often override local bans, local zoning codes that control where and how big the unit can be, building codes that set safety standards, and federal tax and fair housing rules that kick in once you rent it out. As of 2025, at least 18 states have passed laws broadly allowing homeowners to build and rent these units, with Arizona, California, and Washington leading in removing barriers to construction.¹ What follows walks through each layer, because clearing one doesn’t mean you’ve cleared the others.

State Preemption and HOA Rules

The first question is whether you are even allowed to build. State legislatures have increasingly answered yes by preempting local rules that would otherwise block construction. A growing number of states now prevent local governments from imposing minimum lot sizes that would disqualify smaller properties, banning ADUs outright, or requiring discretionary hearings that add months to the approval process.¹

Some states have gone further and voided HOA rules that prohibit ADUs. Where this preemption exists, a homeowners association cannot enforce CC&R provisions that ban secondary units on single-family lots, even if those restrictions were in the original deed. This is not universal. In states without HOA preemption, your association’s covenants may still block construction regardless of what your local zoning allows. Check both your state law and your HOA governing documents before you spend money on plans.

Zoning, Setbacks, and Size Limits

Local zoning controls where the unit can sit on your property. Setback requirements dictate how far any structure must be from property lines, and a four-foot rear and side yard setback is common in jurisdictions that have adopted ADU-enabling legislation. Front-yard setbacks for the primary home typically still apply, which pushes most ADUs behind or beside the main house.

Lot coverage limits cap the total percentage of your parcel that buildings can occupy. If your existing home already uses most of the allowed footprint, your options may be limited to converting interior space or building upward.

Size is capped by the building code. The International Residential Code, which most jurisdictions use as a starting point for local building regulations, caps ADUs at the lesser of 1,200 square feet or 50 percent of the primary home’s floor area, with a minimum of 190 square feet.² Your local jurisdiction may adopt different thresholds, but those figures are the most widely referenced baseline, and the intent is to keep the ADU clearly secondary to the main house.

Building Code Requirements

An ADU must meet the same fundamental safety standards as any other residential construction. The IRC provides the framework that most local building departments adopt, sometimes with amendments, covering structural integrity, fire safety, plumbing, electrical systems, and energy efficiency.

Height limits for detached ADUs commonly fall in the 16- to 25-foot range, depending on the jurisdiction and whether the unit includes a second story. Ceiling heights inside the unit must be at least seven feet in habitable rooms, with exceptions for sloped ceilings where at least half the required floor area still meets that threshold.

Fire safety requirements depend on several factors, including the total number of dwelling units on the property and the unit’s distance from fire department access. A standalone detached ADU on a property with one existing home generally does not trigger a sprinkler requirement, because codes typically require sprinklers starting at three or more units in a single building. An ADU added to an existing two-unit building creates a third unit and usually does require sprinklers. Fire department access rules can independently require sprinklers if the ADU sits too far from the street for firefighters to reach quickly.

Energy efficiency standards apply to ADUs just as they do to primary homes. Depending on your state, you may need to meet specific insulation values, window efficiency ratings, and climate-control system requirements.

Permits, Review, and Inspections

Permit applications require a professional site plan showing every existing and proposed structure on the lot, architectural elevations depicting the exterior from each side, and detailed floor plans with room dimensions. Garage conversions and attached additions follow different application paths than standalone detached builds, so confirm the application type with your planning department before paying for drawings.

The trend in states with ADU-enabling legislation is toward ministerial review, meaning the planning department evaluates your application against a checklist of objective standards rather than exercising subjective design judgment. If the project meets every standard, the permit gets approved. This is a deliberate departure from the model where a planning commission or neighborhood hearing could reject a project because neighbors objected. Several states now mandate this streamlined approach, with approval deadlines that commonly fall around 60 days.

After permit issuance, inspections occur at key construction milestones: foundation, framing and rough mechanical systems, and final completion. Passing the final inspection results in a certificate of occupancy, which is the legal authorization to use the space as a dwelling. Without it, renting or occupying the unit violates local housing codes regardless of how well-built it is.

Occupancy and Rental Restrictions

Most jurisdictions attach use restrictions to ADUs that go beyond standard rental regulations. Three show up in some form almost everywhere.

Owner-occupancy requirements. Many local ordinances require the property owner to live in either the main house or the ADU. The trend in recent state ADU legislation is to relax or eliminate these requirements, but they remain common, particularly in jurisdictions that adopted ADU rules before the recent wave of state preemption laws.

Short-term rental prohibitions. Renting an ADU for fewer than 30 days on vacation platforms is frequently banned. Jurisdictions that allow ADU construction often do so to increase long-term housing supply, and short-term tourist rentals undermine that goal. Violations can carry daily fines that accumulate quickly.

No separate sale. An ADU is legally tied to the primary residence and almost universally cannot be subdivided and sold as a separate parcel or condominium. A handful of jurisdictions have begun exploring exceptions, but for practical purposes, you should assume the ADU can only be transferred with the entire property.

Reporting Rental Income

If you rent your ADU, every dollar of rent you collect is taxable income that you report on Schedule E of your federal return. This includes not just monthly rent checks but also advance rent payments, lease cancellation fees, and any expenses your tenant pays on your behalf, such as utility bills in your name.³

You can deduct ordinary and necessary expenses for managing and maintaining the rental portion of your property. Common deductions include mortgage interest allocated to the ADU, property taxes, insurance, maintenance, repairs, advertising for tenants, and utilities you pay for the unit.³ Improvements, meaning things that add value or extend the property’s life, cannot be deducted immediately but are recovered through depreciation.

Depreciation

The IRS treats a residential rental ADU as depreciable property with a 27.5-year recovery period under the general depreciation system.⁴ You can deduct a portion of the ADU’s construction cost (not the land value) each year over 27.5 years. On a $200,000 build, that works out to roughly $7,270 per year. It is one of the most valuable tax benefits of owning rental property, with a catch at sale time.

Passive Activity Loss Rules

Rental real estate is classified as a passive activity, which means if your ADU expenses exceed your rental income in a given year, you generally cannot use that loss to offset wages or other active income. There is an important exception. If you actively participate in managing the rental, which includes making decisions about tenants, approving repairs, and setting lease terms, you can deduct up to $25,000 in rental losses against your other income. That $25,000 allowance begins to phase out when your adjusted gross income exceeds $100,000 and disappears entirely at $150,000.⁵ Losses you cannot use in the current year carry forward and can be deducted in future years or fully recouped when you sell.

Capital Gains When You Sell

When you sell your home, you can exclude up to $250,000 in capital gains from your income ($500,000 for married couples filing jointly) as long as you owned and used the property as your principal residence for at least two of the five years before the sale.⁶ An ADU complicates this in two ways.

First, the portion of your gain attributable to depreciation deductions you claimed on the ADU is not eligible for the exclusion. If you claimed $50,000 in depreciation over the years, that $50,000 is taxed when you sell, regardless of the overall exclusion.⁶ Second, if part of the property was used for rental and not as your principal residence, gain allocated to those periods of nonqualified use may also fall outside the exclusion. Run the numbers before you list the property.

Fair Housing Obligations

Renting an ADU makes you a landlord, which brings federal Fair Housing Act obligations. You cannot discriminate against prospective tenants based on race, color, religion, sex, disability, familial status, or national origin. The Fair Housing Act does include an exemption for owner-occupied buildings with no more than four units, which covers most ADU situations.⁷

That exemption is narrower than many homeowners realize. Even when it applies, you cannot publish discriminatory advertising. If your rental listing expresses a preference or limitation based on any protected class, you have violated federal law regardless of the small-landlord exemption.⁷ State and local fair housing laws often provide additional protections beyond the federal floor, so the exemption that exists under federal law may not apply under your state’s rules.

Insurance

Your existing homeowners policy almost certainly does not automatically cover a new ADU, particularly a detached structure. Contact your insurer before construction begins, because building a new structure changes the property’s risk profile in ways that could leave you uninsured for construction-phase damage or liability.

For a detached ADU, you will likely need an endorsement that increases your policy limits or provides separate coverage for the new structure. If you rent the unit, standard homeowners insurance may not cover landlord-related risks, and you may need a separate landlord policy or endorsement covering property damage, liability from tenant injuries, and loss of rental income if the unit becomes uninhabitable. Premiums will increase to reflect the added replacement cost, liability exposure, and rental activity.

Requiring your tenant to carry renter’s insurance is not legally mandated in most places, but it is standard practice and gives both parties a layer of protection.

One Note on Financing

Federal mortgage policy has caught up with the shift. Both Freddie Mac and Fannie Mae now allow ADU rental income to help borrowers qualify for a loan. Under Freddie Mac’s guidelines, projected ADU rental income can count on a primary residence for purchases and no-cash-out refinances, with the rental income limited to 75 percent of the documented lease amount, and it cannot exceed 30 percent of the total income used to qualify. The ADU must be legally permitted or legally nonconforming, and buyers without prior landlord experience must complete landlord education before closing.⁸ The FHA expanded its ADU policies in late 2023 along similar lines, allowing lenders to count ADU rental income when underwriting FHA-insured mortgages.⁹ The “legally permitted” requirement is the connection back to everything above: a unit built without permits or occupancy approval does not qualify, no matter how livable it is.

1
Mercatus Center at George Mason University. A Taxonomy of State Accessory Dwelling Unit Laws 2025
2
International Code Council. IRC 2024 Appendix BC – Accessory Dwelling Units
3
Internal Revenue Service. Tips on Rental Real Estate Income, Deductions and Recordkeeping
4
Internal Revenue Service. Publication 527, Residential Rental Property
5
Office of the Law Revision Counsel. 26 USC 469 – Passive Activity Losses and Credits Limited
6
Office of the Law Revision Counsel. 26 USC 121 – Exclusion of Gain from Sale of Principal Residence
7
U.S. Department of Housing and Urban Development. Fair Housing – Equal Opportunity for All
8
Freddie Mac. Accessory Dwelling Units (ADU) Fact Sheet
9
U.S. Department of Housing and Urban Development. FHA INFO 2023-89 – Accessory Dwelling Units