Accessory Dwelling Units (ADUs): Cost, Rent, and ROI

What an ADU Adds to a Property

An accessory dwelling unit is a secondary, self-contained living space on the same lot as a primary home. It can be a converted garage, a basement conversion, an attached addition, or a detached backyard unit. For investors, an ADU turns one income stream into two without buying more land.

ADUs appeal because they add rental income and often property value on land you already own. But construction costs are significant, and local zoning, permitting, and rental rules vary widely, so the analysis has to be specific to your jurisdiction.

Estimate the Build Cost

• Detached new-build units are usually the most expensive per square foot.
• Garage and basement conversions can be cheaper because the structure exists.
• Budget for design, permits, utility connections, and impact fees, not just construction.
• Add a contingency of 10 to 20 percent for surprises in older homes.
• Confirm whether your lot allows an ADU and what size limits apply before spending on plans.

Calculate the Return

The core investment question is what the ADU costs to build versus the net rent it produces. If a unit costs 150,000 dollars all-in and rents for 1,600 dollars a month, gross annual rent is 19,200 dollars. After operating costs, net rent might be roughly 15,000 dollars, a cash-on-cash return around 10 percent on the build.

Also weigh the value added to the property. A well-built ADU can raise appraised value and future sale price, though the value added is rarely equal to the full build cost, so treat appreciation as a bonus rather than the core justification.

Watch the Rules and the Risks

Zoning determines whether an ADU is even allowed, how large it can be, whether it can be rented separately, and whether short-term rental is permitted. Some areas have streamlined ADU approval; others make it difficult. Verify the rules before committing capital.

The main risks are cost overruns, long permitting timelines, and financing. Because ADUs are add-on projects, they often require cash, a construction loan, or a HELOC rather than a standard purchase mortgage.

When an ADU Makes Sense

An ADU tends to work best where rents are high relative to build cost, zoning is permissive, and you plan to hold the property long enough to recover the investment through rent. It is a value-add play that rewards patience and careful local research.

Frequently Asked Questions