An ADU, or accessory dwelling unit, is a self-contained second home built on the same lot as an existing single-family house. In real estate, an ADU has to include its own kitchen, bathroom, and sleeping area so someone can live there independently of the main residence. You may know these units by their informal names: granny flat, in-law suite, backyard cottage, casita. Whatever the label, the defining feature is the same — a complete secondary dwelling sharing one parcel with a primary home.
ADUs have become one of the fastest-growing categories of residential construction because they let homeowners add housing without changing the look of a single-family neighborhood. The rules that govern size, placement, ownership, and use vary widely by city and state, but a consistent set of federal financing programs and tax provisions applies no matter where you build.
The Four Types of ADU
The physical form of an ADU depends on your lot, your budget, and what your local zoning code allows. Nearly every unit falls into one of four categories.
- Detached. A standalone structure in the backyard or side yard, completely separate from the main house. This is the classic backyard cottage. It offers the most privacy for both parties but costs the most to build because it needs its own foundation, walls, roof, and utility hookups.
- Attached. Shares at least one wall with the primary residence, often as a wing added to the back of the house or a bump-out with its own entrance. Construction costs run lower than detached units because you’re extending existing systems rather than starting from scratch.
- Internal conversion. Carves a self-contained unit out of space already inside the home, usually a finished basement, attic, or attached garage. Conversions tend to be the least expensive option because the shell already exists.
- Junior ADU (JADU). A smaller unit contained entirely within the walls of the primary home, typically capped at 500 square feet. JADUs may share a bathroom with the main house and usually require only a basic efficiency kitchen rather than a full range.
The distinction between a JADU and a standard ADU matters for permitting. Because JADUs stay inside the home’s existing footprint, they face fewer structural requirements and lower costs. The shared-bathroom option and smaller size limit, though, make them less attractive as market-rate rentals.
Zoning and Building Code Rules
Every jurisdiction sets its own standards for ADU size, placement, and design. The specifics differ, but the same categories of regulation show up almost everywhere.
Size limits. Most codes cap ADU square footage. A common ceiling for detached units is around 1,200 square feet, while attached ADUs are often limited to 50 percent of the primary home’s living area. JADUs are generally capped at 500 square feet. Some codes also set a minimum of 200 or 250 square feet to ensure habitability.
Setbacks. Your ADU typically needs to sit a minimum distance from your side and rear property lines, often four feet, though older lots can trigger tighter constraints. Front setbacks usually match whatever applies to the main house.
Height. Detached ADUs are often limited to 16 feet on lots with single-family homes. Attached units may be allowed up to 25 feet or the height of the primary dwelling, whichever is lower. Two-story detached designs face stricter scrutiny in many places because of privacy concerns from neighbors.
Parking. Parking mandates have been loosening across the country. A growing number of states no longer require a new off-street space for an ADU, and even jurisdictions that still require parking often waive it near public transit or when the property already has a driveway or garage.
Safety features. Building codes require smoke detectors and carbon monoxide alarms in every ADU. Bedrooms need emergency egress windows meeting minimum size standards. Basement conversions usually require window wells or larger openings to meet these rules.
Impact fees. Larger ADUs may trigger fees that fund local infrastructure like schools, roads, and sewer capacity. Many jurisdictions waive these fees for units under 750 square feet, which is one reason smaller ADUs are so popular.
HOA Restrictions
If your property is governed by a homeowners association, read the covenants, conditions, and restrictions (CC&Rs) before spending money on plans. HOA rules have historically been one of the biggest barriers to ADU construction, and many CC&Rs contain language that effectively prohibits adding a second dwelling.
That landscape is shifting. A growing number of states have passed laws preventing HOAs from outright banning ADUs on lots zoned for single-family use. Under these laws, CC&R provisions that prohibit or unreasonably restrict ADU construction are void and unenforceable. HOAs in those states can still impose reasonable design standards covering materials, architectural style, or aesthetic consistency, but they cannot use those standards as a backdoor way to block construction.
Not every state has adopted these protections. Where no such law exists, your HOA may have the final say. Start with the CC&Rs and contact your board before engaging an architect; finding a prohibition after you’ve paid for design work is an expensive lesson.
Cost and Timeline
From initial planning through final inspection, an ADU project typically takes 8 to 12 months. The planning and permitting phase alone usually runs two to four months, depending on review speed and whether your design needs revisions. Construction itself generally adds another four to six months for a detached unit, less for a garage conversion or interior build-out.
Municipal permit and plan-check fees range widely, from a few hundred dollars in smaller towns to $8,000 or more in high-cost metro areas. Impact fees, where they apply, add substantially to that figure. Budget for these costs early because they’re due before construction begins.
Construction costs depend heavily on where you live, the type of ADU, and the level of finish. National ranges run roughly $150 to $400 per square foot for most projects, with high-cost coastal cities pushing above that and rural areas falling below. A 600-square-foot detached unit in a mid-cost market might land between $120,000 and $240,000 all-in, including permits, site preparation, and utility connections. Internal conversions and garage conversions come in significantly cheaper because the structure already exists.
Financing an ADU
Three major federally backed loan programs now explicitly support ADU construction, which has broadened financing options well beyond the traditional home equity line of credit.
FHA Loans
The Federal Housing Administration updated its policies in late 2023 to accommodate ADUs. Under Mortgagee Letter 2023-17, borrowers using FHA financing can count projected rental income from an ADU toward mortgage qualification. The lender uses 75 percent of either the fair market rent identified by the appraiser or the rent in an existing lease, whichever is lower. The ADU income cannot exceed 30 percent of the borrower’s total effective income used for qualification.1U.S. Department of Housing and Urban Development. FHA Single Family Housing Policy Handbook Borrowers can use the FHA’s Standard 203(k) Rehabilitation program to finance converting an existing structure, such as a garage or basement, into an ADU.2U.S. Department of Housing and Urban Development. FHA INFO 2023-89
Fannie Mae
Fannie Mae treats an ADU the same way it treats any other home improvement. You can finance one with any standard Selling Guide product, including a conventional purchase or refinance loan. Borrowers who qualify for a HomeReady loan can include rental income from an existing ADU to help meet income requirements.3Fannie Mae. Accessory Dwelling Units (ADUs) The HomeStyle Renovation loan specifically covers building a new ADU as part of a renovation, with total loan amounts up to 75 percent of the lower of purchase price plus renovation costs or the as-completed appraised value.4Fannie Mae. HomeStyle Renovation
Freddie Mac
Freddie Mac’s CHOICERenovation mortgage covers both building a new ADU and renovating an existing one. Borrowers using ADU rental income to qualify can count up to 75 percent of the lease amount, capped at 30 percent of total qualifying income, the same thresholds as FHA. Freddie Mac requires at least one borrower to complete landlord education for purchase transactions unless they already have a year of property management experience.5Freddie Mac Single-Family. Accessory Dwelling Units Freddie Mac also allows a no-cash-out refinance to pay off short-term financing used to build the ADU, letting you fund construction with a construction loan and roll it into a permanent mortgage.
Can You Sell an ADU Separately
In most jurisdictions, no. An ADU and the main house sit on a single parcel with a single legal description, and they transfer together as one real estate transaction. This is fundamentally different from a condominium, where each unit has its own deed and can be bought or sold independently.
The rule is starting to change in a few places. California, Oregon, and parts of Washington and Texas now allow or are moving toward allowing ADUs to be sold as separate condominium units. Under those frameworks, a local government opts in by passing an ordinance that permits the ADU to be split into its own condominium interest, creating two separately conveyable units on one lot. This remains the exception. If you’re building an ADU with the expectation of selling it independently, verify that your jurisdiction has actually adopted a separate-sale ordinance before committing.
Some jurisdictions also require the property owner to live in either the main house or the ADU as a primary residence. These owner-occupancy requirements are recorded as covenants that bind future owners, not just the person who built the unit. The trend nationally has been to relax or eliminate these requirements because they discourage ADU construction and are difficult to enforce.
Property Tax and Rental Income
Property Tax Reassessment
Adding an ADU will raise your property tax bill. In most jurisdictions, the assessor keeps the existing tax base for the original home intact and adds the assessed value of the new construction on top of it. You’re not getting a full reassessment of the entire property, just a supplemental assessment reflecting the value the ADU adds. Report the completed construction to your county assessor promptly; unreported improvements can trigger back taxes and penalties when eventually discovered.
Rental Income Reporting
Rent collected from an ADU tenant is taxable income. You report it on Schedule E (Form 1040) along with deductible expenses like maintenance, insurance, property management fees, and the portion of property taxes and mortgage interest allocable to the rental unit.6Internal Revenue Service. Publication 527 (2025), Residential Rental Property Advance rent counts as income in the year you receive it, regardless of the period it covers. Security deposits don’t count as income unless you keep some or all of the deposit because the tenant broke the lease.
Depreciation
You can depreciate the ADU’s construction cost, excluding land value, over 27.5 years using the straight-line method under MACRS.7Office of the Law Revision Counsel. 26 U.S. Code 168 – Accelerated Cost Recovery System Depreciation begins when the unit is ready and available for rent, not when you actually find a tenant. If you converted personal-use space, such as a garage you’d been using for storage, the depreciable basis is the lower of the space’s fair market value or your adjusted basis on the date you converted it to rental use.6Internal Revenue Service. Publication 527 (2025), Residential Rental Property Depreciation is a powerful deduction, but the IRS recaptures it when you sell: you’ll owe tax on the accumulated depreciation at a rate of up to 25 percent.
The 14-Day Rule
If you rent the ADU for fewer than 15 days in a year, the rental income is completely tax-free, but you also cannot deduct any rental expenses for those days. Once you cross the 14-day threshold, all rental income becomes reportable and you can begin deducting expenses proportionally.8Office of the Law Revision Counsel. 26 U.S. Code 280A – Disallowance of Certain Expenses in Connection With Business Use of Home, Rental of Vacation Homes, Etc. This matters most for homeowners who occasionally rent their ADU on short-term platforms rather than keeping a year-round tenant.
Rental Restrictions and Fair Housing
Many jurisdictions limit how you can rent your ADU. Short-term rental bans are increasingly common, with some cities requiring minimum lease terms of 30 days, six months, or a full year for ADUs. The rationale is that ADUs were permitted to expand long-term housing supply, not to create more vacation rentals. Before listing your unit on a short-term platform, check your local ordinance. Violations can result in fines and, in some jurisdictions, revocation of your ADU permit.
The federal Fair Housing Act applies to ADU rentals. You cannot discriminate against tenants based on race, color, religion, national origin, sex, familial status, or disability. There is a limited exemption for owner-occupied buildings with no more than four units, a category that typically includes a homeowner renting a single ADU, but the exemption covers only certain provisions of the Act.9U.S. Department of Housing and Urban Development. Fair Housing Equal Opportunity for All Discriminatory advertising is never exempt, regardless of how many units you own or whether you live on the property.
Insurance Considerations
A standard homeowners policy covers structures on your property, but coverage may not be adequate once you add a rental unit. Most policies provide some protection for “other structures” on the lot, typically up to 10 percent of the dwelling coverage limit. A 400-square-foot detached ADU that cost $160,000 to build will quickly exceed that cap.
Contact your insurer before construction begins. You’ll likely need either an endorsement extending coverage to the ADU or a separate landlord policy if you plan to rent it. Landlord policies cover property damage to the rental structure and liability for tenant injuries. If you plan to use a short-term rental platform, confirm that your policy covers transient guests; most standard homeowners and landlord policies exclude short-term rental activity, and you may need a specialized rider. An umbrella policy adds a broader layer of liability protection and is worth considering once you become a landlord, even on a small scale.
Effect on Resale Value
A permitted ADU generally increases your property’s market value, though by how much depends on location, construction quality, and local demand for rental housing. Industry estimates suggest a 20 to 35 percent bump in overall property value for homes with a permitted ADU, with detached units at the higher end and JADUs at the lower end. The return looks better still when you factor in rental income collected during the years you own the property.
Appraisers handle homes with ADUs differently from standard single-family properties. The appraiser may use the income approach, estimating value based on the rent the unit can generate, or the sales-comparison approach using properties that also have ADUs. Fannie Mae requires at least one comparable sale with an ADU when ADU rental income is being used to qualify the borrower.5Freddie Mac Single-Family. Accessory Dwelling Units In markets where ADUs are still rare, fewer comparable sales can make the appraisal process more difficult and potentially suppress the unit’s contributory value.
One factor matters more than most homeowners realize: permit status. An unpermitted ADU is a liability, not an asset. Lenders and appraisers either ignore it or treat it as a risk. If you inherited or purchased a property with an existing unpermitted unit, legalizing it through your local planning department is usually worth the cost, both for your own protection and for the property’s eventual resale value.