ADU homes, short for accessory dwelling units, are smaller self-contained residences built on the same lot as a primary single-family house. Each one has its own kitchen, bathroom, sleeping area, and entrance, but it stays legally tied to the main property and cannot be sold off to a separate buyer. Homeowners build them to house aging parents, create rental income, or get more use out of a lot they already own. Roughly 18 states now limit how strictly local governments can ban them, and the number continues to grow as housing shortages spread.
What Legally Counts as an ADU
HUD’s Single Family Lender’s Handbook defines an ADU as a habitable living unit with its own entrance that is “subordinate in size” to the primary dwelling. The main home and the ADU together form one real estate interest, which is why you can’t carve off the ADU and sell it like a condo. It travels with the deed.
The feature that separates an ADU from a guest suite or finished basement is the kitchen. A real ADU needs permanent cooking facilities along with a bathroom and a dedicated sleeping space. A bedroom with a mini-fridge doesn’t qualify. A pool house without a stove doesn’t qualify. Building departments use the cooking setup as the bright line between an independent dwelling and an extra room.
Types of ADUs
Which form makes sense depends on your lot, your budget, and how much construction disruption you can live with.
- Detached. A standalone building in the yard, fully separate from the house. Most privacy, highest cost, because it needs its own foundation, walls, roof, and utility runs.
- Attached. Shares at least one wall with the main home and has its own entrance. Cheaper than detached because you’re extending systems that already exist.
- Interior conversion. Turns existing space, usually a basement or attic, into a private apartment. Often the least expensive option since the shell is already there.
- Garage conversion. Repurposes an existing garage. Common because the structure exists, but you give up parking and storage.
- Junior ADU (JADU). A smaller unit, typically capped at 500 square feet, carved out of the main home’s existing footprint. A JADU can share a bathroom with the main house but must have its own cooking area, even if it only fits small plug-in appliances.
Prefab vs. Stick-Built
Factory-built ADUs are now a serious alternative to conventional construction. A prefab is manufactured off-site and delivered in sections or as a complete unit, usually under a single contract that bundles design, permits, finishes, and site work. A stick-built project means separate contracts for architecture, engineering, framing, plumbing, and finishes, each with its own potential for overruns and delays.
Timing differs too. A prefab ADU can go from contract to move-in in roughly 11 months. Stick-built timelines are harder to predict because weather, labor, and material backorders all weigh on them. If budget certainty and speed matter, price out prefab before committing to conventional construction.
Common Zoning and Size Rules
Every city and county writes its own ADU rules, so there’s no single national standard. The same patterns show up almost everywhere, though, and California’s law has shaped a lot of them.
- Maximum size. Most jurisdictions cap detached ADUs around 800 to 1,200 square feet. Some tie the limit to a percentage of the main home’s floor area, often 50%, whichever is smaller.
- Setbacks. A four-foot minimum from side and rear property lines is the most common standard for detached ADUs. The reason is emergency access and drainage, not just appearance.
- Height. Detached ADUs are frequently limited to 16 to 18 feet to limit shadowing of neighboring lots. Attached ADUs usually follow the main home’s height rules.
- Lot coverage. Many codes limit the total share of the lot that structures can cover, which can effectively rule out a larger ADU on a small property.
Owner-Occupancy Requirements
Some jurisdictions require the owner to live in either the main house or the ADU. The intent is to keep ADUs as neighborhood housing rather than investor rentals, but the rule discourages construction because it eliminates the option of renting both units if you later move. States passing new ADU laws are trending toward banning these local mandates. Cities that have repealed them, including Los Angeles and Seattle, saw noticeable jumps in ADU permit applications afterward.
Short-Term Rental Restrictions
If your plan is Airbnb, read the local rules before you break ground. Many cities either prohibit short-term rentals in ADUs outright or allow them only on owner-occupied properties. Some let you use either the main house or the ADU as a short-term rental, but not both at once. Fines run $500 per day or more in some places, and the booking platforms increasingly share data with enforcement agencies. Even where the city allows short-term use, your HOA or deed restrictions can prohibit it independently.
HOAs and Deed Restrictions Can Still Block You
This is where ADU plans quietly die. A city zoning code can allow ADUs while your homeowners association forbids them, and in most states the local zoning law does not override private covenants. Only one state has explicitly barred HOAs from banning ADUs on single-family lots. Everywhere else, a CC&R that limits outbuildings or caps the number of dwelling units on a lot can stop the project. Read your covenants before paying an architect. If the HOA bans ADUs, your only route is usually a covenant amendment through a membership vote, which typically needs a supermajority.
Permits and Inspections
Every jurisdiction requires a permit, and skipping the process creates problems that are expensive to fix later. You’ll submit a site plan, architectural floor plans, structural plans, and utility plans showing how the ADU will connect to water, sewer, and electrical service. Many departments require a licensed engineer’s seal on structural work.
Plan review typically takes 30 to 60 days after submission. Some jurisdictions offer expedited review for ADUs, especially in states with pro-ADU laws that require faster turnaround. Permit fees range widely: a simple garage conversion may cost a few thousand dollars, while a detached new build in a high-cost area can run $10,000 or more in permits before impact fees for water, sewer, and schools. Impact fees add another $1,000 to $7,000 in many places.
Utility metering is worth sorting out early. Many jurisdictions now require separate electrical meters for new dwelling units. Water service can often stay shared with the existing meter, but adding a second unit can shift your wastewater billing from a single-family rate to a multi-family rate. A separate water meter costs several thousand dollars to install, so decide during design rather than after construction.
Once the permit is issued, inspectors sign off at key stages: foundation, framing, plumbing rough-in, electrical rough-in, insulation, and final walkthrough. Each has to pass before work moves to the next. The final inspection clears the way for a certificate of occupancy, the legal document that authorizes anyone to live in the unit.
Construction Costs
Costs vary heavily by region, type, and how much of the structure already stands. Nationally, expect roughly $150 to $300 per square foot, though projects in high-cost markets can top $600 per square foot. Typical total ranges by type:
- Basement or garage conversion. $60,000 to $150,000.
- Attached new construction. $100,000 to $216,000.
- Above-garage build. $128,000 to $225,000.
- Detached new construction. $110,000 to $285,000.
Conversions cost less because the shell already stands. Detached builds cost the most because everything, including foundation and utility runs, has to be built from scratch. After geography, the next biggest cost swing is finishes: rental-grade materials versus higher-end fixtures and appliances can shift the per-square-foot cost by 40% or more.
Financing an ADU
Few homeowners pay cash. Several loan programs now specifically accommodate ADU projects.
FHA 203(k) Rehabilitation Loans
The FHA Section 203(k) program lets borrowers finance a home purchase and the cost of building an eligible ADU in a single mortgage, and single-family homes with ADUs are explicitly listed as eligible property.1HUD.gov. 203(k) Rehabilitation Mortgage Insurance Program The Standard 203(k) covers major structural work with no maximum renovation cost but a $5,000 minimum; the Limited 203(k) handles non-structural work up to $75,000.2HUD.gov. Buying a House That Needs Rehabilitation or Renovating Your Home Most new ADU builds involve structural work, so the Standard version is the usual path. The Standard loan also requires an FHA-approved 203(k) consultant.
Fannie Mae and Freddie Mac
Both agencies let lenders count ADU rental income toward mortgage qualification. The property must be a one-unit principal residence, only income from one ADU counts, and the qualifying ADU rental income cannot exceed 30% of your total qualifying income. When projecting that income from a lease or market rent appraisal, the lender multiplies gross monthly rent by 75% and treats the remaining 25% as lost to vacancy and maintenance.3Fannie Mae. Rental Income
Freddie Mac’s CHOICERenovation program goes further, letting borrowers finance new ADU construction or renovation of an existing unit as part of a purchase or refinance. That’s useful if you’re buying a property with an older garage or outbuilding you plan to convert. Freddie Mac allows one ADU on one-, two-, and three-unit properties, and the ADU must comply with local zoning unless it falls under specific exceptions for non-conforming units.4Freddie Mac Single-Family. Accessory Dwelling Units
Taxes When You Rent It Out
Rental income from an ADU is taxable. Every dollar a tenant pays in rent, including advance rent and lease cancellation payments, must be reported in the year received. If a tenant pays one of your expenses, like a utility bill, that counts as rental income too. Security deposits aren’t income as long as you intend to return them, but any portion you keep becomes taxable that year.5Internal Revenue Service. Publication 527, Residential Rental Property
Deductible expenses offset a lot of the income. You can deduct the mortgage interest allocated to the ADU, property taxes, insurance, maintenance, utilities you pay for the unit, and depreciation of the structure itself.6Internal Revenue Service. Topic No. 415, Renting Residential and Vacation Property Depreciation is the big one: you spread the ADU’s construction cost (not land value) over 27.5 years, creating a paper deduction that lowers taxable rental income without additional cash outlay. Report rental income and expenses on Schedule E (Form 1040).5Internal Revenue Service. Publication 527, Residential Rental Property
One narrow exception: if you rent the ADU for fewer than 15 days in a tax year, you don’t have to report the rental income at all, though you also can’t deduct the related expenses.5Internal Revenue Service. Publication 527, Residential Rental Property
Insurance and Property Tax Impact
Standard homeowner’s insurance usually covers an attached or interior ADU as part of the main dwelling, and a detached ADU may fall under the policy’s “other structures” coverage. That coverage is often capped at 10% of your dwelling coverage, which may not be enough to rebuild. More importantly, renting the unit introduces liability that standard homeowner’s policies aren’t built for. If a tenant or their guest is hurt in the ADU, your homeowner’s policy may deny the claim. Before renting, ask your insurer about a landlord endorsement or a separate landlord policy. Umbrella coverage is worth considering for additional liability protection.
Property taxes will rise. Your existing home typically isn’t reassessed just because you added an ADU; the assessor adds the new construction’s value to your current assessed value. If the ADU adds $150,000 in value and your rate is 1%, that’s roughly $1,500 per year in additional tax. Rental income usually covers it, but build the figure into your numbers from the start.
What Happens If You Build Without a Permit
Operating an unpermitted ADU is a gamble that rarely pays off. Enforcement varies, but penalties commonly include daily fines until the violation is cured, orders to remove the structure, and loss of rental income. Some jurisdictions fine $500 per day for renting an unpermitted unit. The bigger problem comes at sale: title searches and buyer inspections flag the unit, and lenders won’t finance a property with an unpermitted dwelling. You’ll end up either legalizing the unit at your expense, which often costs more than permitting it correctly the first time, or demolishing it before closing. The permit process is slow, but it protects the investment in ways shortcuts never will.