• Detached. Backyard cottage, tiny house, or standalone secondary structure.
• Attached. Above-garage unit, attached addition, or above-living-area unit.
• Internal conversion. Basement conversion or in-law suite carved out of the existing footprint.
• Legal vs unpermitted. Only a permitted unit counts. Unpermitted "casitas," garage conversions without a certificate of occupancy, and undocumented basement apartments do not count for financing or for using the rental income to qualify.

Beginning with AB 2299 and SB 1069 in 2017, California passed successive rounds of legislation in 2017, 2019, and 2021 making ADUs dramatically easier to permit and build. The state preempted many local restrictions:

• Cities cannot require owner occupancy of the primary residence.
• Setbacks and parking requirements were relaxed or eliminated.
• Permitting timelines were capped.
• By-right approval is required for many ADU configurations.

Oregon and Washington followed with similar laws. Other states are gradually adopting parts of the framework. The result is a national surge in ADU construction as a way to increase housing supply and let homeowners create rental income on existing property.

• To use the ADU's rental income on a qualifying basis, the ADU must be permitted (legal) under local zoning. Unpermitted units do not count.
• Some lenders require 12 months of established rental history before counting the income; others accept appraised market rents on a newly built ADU.
• Fannie Mae and Freddie Mac have both expanded their ADU rental income guidelines in recent years, making the income easier to use across more scenarios.
• We work with lenders going to the most permissive published rules where allowed. Send the scenario and we will tell you exactly how the rental income would be treated.

Highly market-dependent. Use these as a starting point, not a quote.

• Detached new construction. $200K to $500K+ depending on size, market labor cost, and finish level. Coastal California and high-cost metros are at the top of the range.
• Garage conversion. $100K to $200K. Faster timeline and lower cost than detached new build because the shell already exists.
• Internal conversion. $75K to $150K. Basement conversion or carving out an in-law suite from existing living area. Often the cheapest path.

Costs have risen significantly since 2020. Budget a contingency of 10-20% on top of your base construction estimate.

• Local zoning varies widely. Setback rules, height limits, parking requirements, lot-coverage rules, unit-size caps. Verify with the planning department before designing the unit.
• HOA restrictions can override state law. In some jurisdictions a homeowners association can still prohibit ADUs even where state law permits them. Check the CC&Rs before assuming you can build.
• Owner-occupancy requirements vary. California prohibits cities from requiring owner occupancy on ADU permits; other states still allow that requirement. Affects whether you can move out and rent both units.
• Short-term rental laws affect your rental strategy. Many cities ban or restrict STR in residential zones. Confirm whether your rental projection (long-term, mid-term, short-term) is even legal in your jurisdiction.

Illustrative only. Returns vary by market, rent, and operating cost.

• ADU construction cost: $300,000.
• Resulting ADU monthly rent: $2,500.
• Annual gross rent: $30,000.
• Roughly 10% cash-on-cash return before considering tax benefits or appreciation. After operating expenses, the net cash yield is lower; after factoring depreciation and tax shelter on the rental portion, the after-tax return is often higher.
• Often outperforms a separate property purchase because you skip the land cost, the down payment on a new property, and the operating cost of a second standalone home.

• Rental income from the ADU is taxable on Schedule E.
• Depreciation is available on the ADU portion of the property. Cost segregation can accelerate depreciation on components with shorter useful lives. See our cost segregation page.
• Mortgage interest has to be allocated between the personal portion (your primary residence) and the rental portion (the ADU). The personal portion may be deductible as home mortgage interest if you itemize; the rental portion is deductible on Schedule E.
• This is general information, not tax advice. Consult a CPA specializing in real estate before assuming any specific tax outcome.

• Underestimating cost and timeline. ADU projects routinely run over budget and over schedule. Budget contingency. Pad your timeline.
• Building without proper permits. Unpermitted ADUs cannot be used to qualify for rental income, cannot be legally rented, and can trigger code-enforcement penalties. Always permit.
• HOA blocking the ADU. Check the CC&Rs before designing.
• Short-term-rental restrictions. If your investment model depends on STR income, confirm STR is legal in your city or county before construction.
• Picking the wrong loan structure. Using a HELOC during a long build can expose you to rate movement; cash-out is rigid; construction loans have more underwriting. The right product depends on your equity, your liquid cash, and your timeline.

• Loan-to-cost ratio: typically 80-90%. Borrower funds the remaining 10-20% out of pocket.
• Borrower equity required. Cash in the deal, plus existing-property equity, plus contingency reserves.
• Interest-only during construction. You pay interest only on the balance drawn so far, not on the full loan commitment.
• Converts to permanent loan at completion. Once the ADU is built and certified for occupancy, the loan converts to a conventional permanent mortgage, either one-time-close (already arranged at the start) or two-time-close (refinance into the permanent loan when ready).
• For deeper construction-loan math (interest reserves, draw schedules, builder profit allocation), see our construction loan calculator.