What Is an ADU?
An ADU — Accessory Dwelling Unit — is a secondary, self-contained living unit on the same property as a primary residence. It has its own entrance, kitchen, bathroom, and living/sleeping area. ADUs go by many names depending on the region: in-law suite, granny flat, mother-in-law apartment, carriage house, backyard cottage, or casita.
ADUs can be created in three ways: converting existing space (a garage, basement, or attic), adding onto the existing house (an attached addition with its own entrance), or building a new detached structure (a backyard cottage or converted shed). Converting existing space is the most affordable route because the shell already exists.
Why ADUs Are Booming in 2026
New state laws: Over a dozen states have passed ADU-friendly legislation in the past three years, streamlining permitting, eliminating minimum lot size requirements, removing owner-occupancy mandates, and overriding restrictive local zoning. California, Oregon, Washington, Colorado, New York, Connecticut, Montana, and Vermont have all enacted significant ADU reforms. These laws make it legally easier and cheaper to build an ADU than at any time in modern history.
Housing shortage: The United States has a shortage of approximately 4 to 7 million housing units. ADUs add supply without new land development, making them popular with policymakers and community planners. Several cities now offer grants, fee waivers, and pre-approved plans to encourage ADU construction.
Rental income: With mortgage rates and housing costs elevated, homeowners are looking for ways to offset housing expenses. An ADU generating $1,000 to $2,500 per month in rent can cover a significant portion of a mortgage payment. This "house hacking" strategy has become one of the most popular personal finance moves of 2026.
Multigenerational living: Aging parents, adult children returning home, and extended family needs drive demand for separate-but-close living arrangements. An ADU provides independence and privacy while keeping family nearby.
How Much Does an ADU Cost?
Garage conversion ADU: $20,000 to $80,000. The most affordable option because the structure already exists. You add insulation, drywall, flooring, a bathroom, kitchenette, and separate entrance.
Basement conversion ADU: $40,000 to $120,000. More expensive than a garage because basements often need waterproofing, egress windows, and more extensive plumbing work.
Detached new construction ADU: $100,000 to $300,000. A brand-new backyard cottage built from the ground up. The most expensive option but offers the most design flexibility.
Prefab/modular ADU: $60,000 to $150,000 installed. Factory-built units delivered to your property and set on a prepared foundation. Faster than traditional construction but limited in customization.
How Much Rent Can an ADU Generate?
ADU rental rates depend entirely on your market:
High-cost markets (SF, LA, NYC, DC, Boston): $1,800 to $3,500/month
Mid-cost markets (Denver, Portland, Austin, Seattle): $1,200 to $2,200/month
Lower-cost markets (Midwest, Southeast, smaller cities): $800 to $1,500/month
At these rates, most ADU conversions pay for themselves in 3 to 7 years, after which the rental income is essentially profit minus maintenance and vacancy costs.
Do ADUs Increase Home Value?
Yes, significantly. A legal, permitted ADU increases property value by 20 to 35% in most markets. The increase comes from two factors: the additional square footage and the income-generating potential. Appraisers in ADU-friendly markets now have established methods for valuing properties with rental units.
The key word is "legal." An unpermitted ADU may add no appraised value and can actually decrease value by creating disclosure obligations and code violation risks.
Getting Started
The fastest and most affordable way to build an ADU is to convert existing space. Check out our garage to rental studio guide or basement to rental apartment guide for complete step-by-step instructions. Use our cost calculator to estimate your specific project.