ADU units, or accessory dwelling units, are smaller, self-contained second homes built on the same lot as a single-family house, each with its own kitchen, bathroom, sleeping area, and entrance. You’ll hear them called in-law suites, backyard cottages, carriage houses, or granny flats, but the legal concept is the same: a secondary residence that shares a property, a deed, and a parcel number with a primary one. Rules vary by city and state, but the structure of what you can build, how you can own it, and how you can finance it follows a consistent pattern across most of the country.
The Four Types of ADUs
The type you build drives almost everything else: cost, permitting difficulty, utility work, and how the unit lives on the lot.
Detached ADUs
A detached ADU is a standalone structure built separately from the main house. These are the backyard cottages and carriage houses most people picture. They need their own foundation and typically separate utility connections for water, sewer, and electricity. Because they’re built from scratch, detached units cost the most, running roughly $300 to $400 per square foot nationally, but they offer the most privacy for both the homeowner and any tenant.
Attached ADUs
An attached ADU shares at least one wall with the primary residence. It might extend from the rear or side of the house, or sit above a garage. Costs run a bit lower, roughly $250 to $350 per square foot, because the structure ties into existing framing and foundation. The tradeoff is less privacy and tighter design constraints, since the addition has to work with the existing roofline, siding, and utility systems.
Interior Conversions
Converting a basement, attic, or attached garage reuses space inside the home’s existing footprint. These are the most affordable path to an ADU, often $200 to $250 per square foot, because you’re working inside an existing structure rather than pouring a new foundation. The construction challenges usually involve meeting fire safety requirements, adding proper egress windows, and installing separate utility meters where the jurisdiction requires them.
Junior ADUs
A junior accessory dwelling unit (JADU) is a smaller subcategory built entirely within the walls of an existing or proposed single-family home. Size is tightly capped, with 500 square feet as the standard maximum in jurisdictions that recognize them. The key distinction is that a JADU can share a bathroom with the main house as long as there’s interior access between the two, which eliminates one of the biggest costs of a full ADU. JADUs still need a separate exterior entrance and their own cooking facilities, though the kitchen can be a basic efficiency setup with a sink, small appliances, and counter space rather than a full range and oven.
What Makes a Structure Legally an ADU
An ADU is legally subordinate to the primary residence on the lot. It cannot exist without the main house, and zoning codes prevent it from being treated as the primary structure. Local ordinances typically require the main dwelling to be built or at least under construction before an ADU can be permitted.
What separates an ADU from a spare room or a finished shed is independent habitability. Fannie Mae’s definition is a good benchmark: the unit must include space for living, sleeping, cooking, and bathing independent of the primary residence, and it must be accessible without going through the main house.1Fannie Mae. Accessory Dwelling Units (ADUs) That independent-entrance requirement shows up in most local codes, though a few jurisdictions make exceptions for interior conversions.
The International Residential Code, which most U.S. jurisdictions have adopted with local amendments, sets the baseline. Every ADU needs a dedicated sleeping area, a bathroom with a toilet and bathtub or shower, and a functional kitchen with a sink, cooking appliance, refrigeration, and adequate counter space. These standards exist so the unit is genuinely habitable, not just a finished room with a microwave plugged in.
Fire safety is where the code gets more involved. When you create a separate dwelling unit within or attached to an existing structure, the code typically requires a one-hour fire-resistance-rated separation between the two units: fire-rated drywall, insulation, and sealed penetrations. That rating can be cut to a half-hour if a residential sprinkler system is installed. Detached ADUs don’t need this interior separation but must comply with fire setback distances from property lines and other structures. Fire safety requirements often drive a significant share of renovation costs for attached and conversion ADUs.
Size, Setbacks, and Parking
Most jurisdictions cap ADU size, though specific numbers vary. Common ceilings for detached ADUs range from 800 to 1,200 square feet. Some localities tie the maximum to the primary home, allowing an attached ADU up to 50% of the main house’s living area with an absolute cap of 1,200 square feet. JADUs are almost universally capped at 500 square feet. A few jurisdictions set minimum sizes as well, typically 150 to 200 square feet, to prevent units too small to be livable.
Setback rules dictate how close an ADU can sit to property lines. A four-foot setback from side and rear lot lines is one of the more common standards for new detached and attached ADUs. Conversions of existing structures, such as turning a legal garage into an ADU, are often exempt from setback requirements beyond what fire safety demands. Corner lots sometimes carry additional restrictions on exterior side yards. Check your local zoning code before drawing up plans, because setback violations can halt construction and force expensive redesigns.
Parking requirements have loosened dramatically over the past decade. Where a space is still required, the maximum a jurisdiction can typically impose is one per ADU or per bedroom, whichever is less. Many localities waive parking entirely when the ADU is within a half-mile walking distance of public transit. When a garage or carport is demolished or converted to create the ADU, most jurisdictions no longer require the lost parking spaces to be replaced elsewhere on the lot. Guest parking cannot be required under most current frameworks.
Ownership and Selling
The foundational rule across nearly all jurisdictions: an ADU cannot be sold separately from the primary home. Both structures sit on a single deed and transfer together. You cannot subdivide the lot to carve out a new parcel for the ADU. That restriction is what keeps the unit legally “accessory.”
Owner-occupancy rules are more of a mixed bag. Some jurisdictions require the property owner to live in either the main house or the ADU, which prevents the entire property from becoming an absentee-owned rental. Others have moved away from the rule entirely. As of mid-2025, at least 11 of the 18 states with broad ADU legislation had banned local governments from imposing owner-occupancy requirements. Where it still applies, violating it can trigger fines and jeopardize the ADU’s legal status.
Buyers considering a property with an existing ADU should review recorded covenants and local compliance certificates. An unpermitted ADU creates title complications that can derail a sale. Lenders flag unpermitted structures during appraisal, and insurance companies may refuse to cover them. Getting the unit retroactively permitted before listing is almost always the smarter play.
Short-Term Rental Restrictions
This is the regulation that catches the most ADU owners off guard. A growing number of cities prohibit using an ADU as a short-term vacation rental, defined as stays under 30 consecutive days, or allow it only if the owner lives on-site. Some jurisdictions that offered construction incentives or fee waivers require the owner to forgo short-term rentals for a set number of years in exchange. If your plan is to list the unit on a vacation rental platform, check your local ordinance first. Penalties can include permit revocation, not just fines.
Financing an ADU
The biggest misconception about ADU financing is that you need a special loan product. Fannie Mae treats an ADU the same as any other home feature or improvement, which means it can be financed with a standard purchase loan, a refinance, or an affordable lending product. For homeowners adding an ADU to an existing property, the HomeStyle Renovation mortgage lets you roll construction costs into a single loan.1Fannie Mae. Accessory Dwelling Units (ADUs) FHA borrowers have options too: the FHA now allows up to 75% of estimated ADU rental income to help qualify for a mortgage on a property with an existing ADU, and 50% of projected rental income when financing a new conversion through the 203(k) rehabilitation program.
One useful wrinkle for conventional loans: Fannie Mae allows rental income from an existing ADU on a one-unit primary residence to count as qualifying income, capped at 30% of total qualifying income.2Fannie Mae. Rental Income Lenders apply a 25% haircut to gross rent to account for vacancy and maintenance, so if the ADU rents for $2,000 per month, only $1,500 counts toward your income.
Taxes and Insurance
If you rent out your ADU, the income goes on Schedule E of your federal return.3Internal Revenue Service. Topic No. 414, Rental Income and Expenses You can deduct mortgage interest allocable to the ADU, property taxes, insurance premiums, repairs and maintenance, utilities you pay for the unit, advertising costs, and property management fees. You can also depreciate the ADU itself, since residential rental property uses a 27.5-year straight-line schedule under the general depreciation system.4Internal Revenue Service. Publication 527 (2025), Residential Rental Property Depreciation begins when the unit is placed in service, meaning the day it’s ready and available for rent, not necessarily the day a tenant moves in.
Building an ADU will increase your property taxes, but generally only on the value of the new construction. Most jurisdictions assess the ADU improvement separately without triggering a full reassessment of the existing home. The assessor typically values the ADU based on construction costs or a market comparison as of the date of completion. For conversions, the scope of the renovation matters: light modifications may add modest assessed value, while a gut renovation taken down to studs can be assessed as entirely new construction.
On insurance, don’t assume your existing policy covers it. A standard homeowners policy may cover an ADU as an “other structure” on the property, but coverage limits for secondary structures are often set at just 10% of your dwelling coverage, which is nowhere near enough to rebuild a detached ADU. Contact your insurer before or during construction to add adequate coverage. If you’re renting the unit out, you’ll likely need a landlord or lessor policy endorsement, which covers lost rental income if the ADU becomes uninhabitable due to a covered event. An umbrella policy is worth considering since it extends liability protection beyond your base limits.
The Permitting Process
ADU permits are handled at the local level, and the process ranges from straightforward to genuinely complicated depending on where you live. Most jurisdictions now require ministerial (non-discretionary) approval for ADUs that meet all zoning and building code requirements, meaning the planning department can’t deny the permit based on neighborhood objections or subjective design concerns. Several states mandate that local agencies process ADU applications within 60 days.
The typical sequence: verify that your property is eligible under local zoning, prepare architectural plans that meet building code, submit a permit application with the required forms and fees, wait for plan review, respond to any correction requests from the building department, and receive your building permit. Construction then proceeds under periodic inspections until the unit passes a final inspection and receives a certificate of occupancy.
Where things slow down is pre-application review. Depending on your jurisdiction and lot, you may need a geotechnical report, fire flow study, environmental review, or historic resource review before you can even submit. Impact fees and utility connection charges vary enormously, from under $1,000 to over $20,000, and some jurisdictions have waived or reduced these fees to encourage ADU production. Ask your local planning department for a complete fee schedule before finalizing your budget, because permit and impact fees can add 10% to 15% on top of hard construction costs.