Building an ADU for House Hacking Income

You can add an accessory dwelling unit (ADU), a second, smaller home on your lot such as a backyard cottage, an addition, or a converted garage, and rent it out wherever your zoning allows one and no enforceable homeowners association rule or deed restriction blocks it. California Civil Code sections 714.3 and 4751 invalidate private restrictions that effectively prohibit or unreasonably restrict qualifying ADUs or junior ADUs on lots zoned for single-family use.

In California, Washington, and Arizona's larger cities, state law now requires local governments to allow ADUs on single-family lots and limits the conditions they can attach. Plan on a permit process of several steps, a build cost that a statewide California survey put at a median of $150,000, and a payback period you can estimate by dividing total cost by the yearly rent left after expenses. For how an ADU fits with other approaches, see what is house hacking.

Where state law requires cities to allow ADUs

Other states have their own ADU laws, and where no state law applies, your city or county zoning code decides whether an ADU is allowed, how large it can be, and whether you must live on the property. An owner-occupancy rule matters if you may later move out and rent both homes. Zoning and permits for house hacking explains how to find and read these rules.

Typical permit steps

1. Check zoning and the site. Confirm the ADU is allowed and note the setbacks (required distance from property lines), height, size, and parking limits. Ask whether your sewer or septic system and electrical service can carry a second unit.
2. Get plans drawn. Ask your building department for its submittal checklist, which usually starts with a site plan and floor plan.
3. Apply for the building permit. Some cities add a planning review. In California, if a home already exists on the lot, the city must approve or deny a complete application within 60 days; if it denies, it must list what is missing and how to fix it.
4. Pay fees and arrange utility connections. Ask for a written estimate of permit, school, and impact fees before you commit.
5. Pass inspections. Schedule each inspection the permit requires and get final sign-off before a tenant moves in.
6. Register the rental if required. Check whether your city requires a rental license and whether it limits short-term rentals.

Cost by ADU type

The most-cited cost data comes from a UC Berkeley survey of California ADU owners, released in April 2021: the median build cost was $150,000, or $250 per square foot. About 37% of ADUs cost less than $100,000 and 71% cost less than $200,000. Construction costs have changed since that survey, so use these figures as a starting range and get at least three written bids.

A basement unit is an interior conversion with its own issues (ceiling height, moisture, escape windows); see basement apartment conversion.

Financing options

• Home equity line of credit (HELOC). A HELOC lets you borrow against your equity as needed, usually at a variable rate, during a draw period that often lasts about 10 years. After that you repay, often over 10 to 20 years, and payments are often much higher. The house is the collateral, so falling behind can cost you your home. A HELOC leaves your first mortgage and its rate in place.
• Cash-out refinance. You replace your mortgage with a larger one and take the difference in cash. FHA caps a cash-out refinance at 80% of the home's value. Freddie Mac's average 30-year fixed rate was 7.32% on October 22, 2026, so if your current rate is lower, refinancing raises the rate on your entire balance, not just the new money.
• Renovation loans. FHA's standard 203(k) loan can fund building an attached ADU or renovating an existing attached or detached ADU, but not building a new detached ADU. Fannie Mae's HomeStyle Renovation loan can fund accessory units and other outbuildings when local zoning allows them.
• Counting the rent to qualify. When you buy, or refinance without taking cash out of, a home that already has an ADU, and you have no rental history on it, FHA lets the lender count 75% of the ADU's market rent (or the lease rent, if lower) as income, capped at 30% of your total qualifying income, and requires two months of payments in reserves after closing. FHA does not count ADU rent for a cash-out refinance, and a 203(k) loan counts only 50% if you have no rental history on the property. FHA borrowers on a home with an ADU also sign HUD form 92561, agreeing not to rent any part of it for less than 30 days.

Payback calculation

Payback in years = total cost / (yearly rent - vacancy - repairs - owner-paid utilities - added property tax - added insurance).

Example. A detached ADU with a $180,000 all-in bid (design, permits, fees, and construction) rents for $2,000 a month. Assume tenants pay all utilities directly. The tax and insurance figures are assumptions; get your county assessor's estimate and an insurance quote.

$180,000 / $19,020 = 9.5 years to recover the cost if you pay cash, about a 10.6% yearly return on the $180,000. If you borrow the full amount on a HELOC at an assumed 8.5% and pay interest only, interest is $15,300 a year, which leaves $3,720 a year until principal payments begin. Calculating rental income for house hacking shows how to set the rent figure from local listings.

On taxes, you report the ADU's rent on Schedule E and can depreciate its construction cost (not the land) over 27.5 years, about $6,545 a year on $180,000. When you sell, a unit separate from your own home does not get the home sale exclusion, so you allocate part of the gain to the ADU and pay tax on it. See house hacking tax benefits.