ADU Glossary
ADU Meaning: What Is an Accessory Dwelling Unit?
The complete guide to understanding ADUs — types, costs, benefits, regulations, and why millions of homeowners are building them.
Quick Answer
An ADU (Accessory Dwelling Unit) is a secondary housing unit on a single-family residential lot — also called a granny flat, in-law suite, or backyard cottage.
Updated April 2026
What Does ADU Mean?
ADU stands for Accessory Dwelling Unit — a self-contained living space located on the same property as an existing single-family home. An ADU has its own entrance, kitchen, bathroom, and sleeping area, making it a fully independent housing unit.
ADUs go by many names depending on the region and configuration: granny flats, in-law suites, backyard cottages, casitas, secondary suites, laneway houses, and ohana units. Regardless of the name, the legal definition is the same — a secondary residential unit on a lot zoned for single-family housing.
ADUs have become one of the most popular housing solutions in the United States, driven by state legislation that has dramatically simplified the approval process. California alone added over 20,000 ADU permits per year starting in 2020, and states like Oregon, Washington, and Colorado have followed with their own by-right ADU laws.
Types of ADUs
Not all ADUs are the same. The type you build depends on your lot, budget, and goals. Here are the five main categories:
🏠 Detached ADU
A standalone structure built separately from the main home, typically in the backyard. Offers maximum privacy and highest rental income potential. Cost: $150K–$350K.
🏘️ Attached ADU
An addition built onto the existing home, sharing at least one wall. Lower cost than detached ($100K–$250K) but less privacy for both occupants.
🏡 Junior ADU (JADU)
A small unit (500 sq ft max) carved out of existing space within the primary home. The most affordable option at $50K–$100K. Must share at least one wall with the main home.
🔧 Garage Conversion
Converting an existing garage into a living space. Takes advantage of existing structure to reduce costs ($80K–$180K). Zero setback requirements since the structure already exists.
⬆️ Above-Garage ADU
A second-story unit built on top of an existing or new garage. Preserves parking while adding living space. Cost: $150K–$300K.
ADU Size Limits by Type
| ADU Type | Max Size (CA) | Typical Cost | Timeline |
|---|---|---|---|
| Detached ADU | 1,200 sq ft | $150K–$350K | 8–14 months |
| Attached ADU | 1,200 sq ft* | $100K–$250K | 6–10 months |
| JADU | 500 sq ft | $50K–$100K | 3–6 months |
| Garage Conversion | Existing footprint | $80K–$180K | 3–6 months |
| Above-Garage | 1,200 sq ft | $150K–$300K | 8–12 months |
*Attached ADUs limited to 50% of primary home or 1,200 sq ft, whichever is less.
A Brief History of ADUs
Accessory dwelling units are not a new concept. Secondary homes on residential properties have existed for centuries — from carriage houses in the 1800s to post-WWII garage apartments that housed returning veterans. However, single-family zoning restrictions adopted in the mid-20th century made most ADUs illegal or impractical to permit.
The modern ADU movement began in earnest in 2016–2017 when California passed a series of landmark bills (SB 1069, AB 2299, AB 2406) that removed many local barriers to ADU construction. These laws prohibited cities from requiring discretionary approval, reduced parking requirements, and established reasonable size standards.
In 2020, additional California legislation (AB 68, SB 13) further streamlined the process by mandating 60-day ministerial approval, reducing setback requirements to 4 feet, eliminating impact fees for ADUs under 750 sq ft, and preventing homeowner association (HOA) bans on ADUs.
Other states quickly followed California's lead. Oregon became the first state to effectively legalize ADUs statewide in 2019 (HB 2001). Washington, Colorado, Montana, and Vermont have since passed similar legislation, creating a nationwide trend toward ADU legalization.
Who Builds ADUs and Why?
ADUs serve a wide range of homeowners with different goals. The most common motivations include:
Rental Income
The most common reason. ADUs generate $1,200–$3,500/month in rental income depending on the market. In high-cost areas like the Bay Area or Los Angeles, a well-built ADU can generate enough income to cover a significant portion of the homeowner's mortgage.
Aging-in-Place / Multigenerational Living
Families build ADUs for aging parents who want to live independently but nearby. This arrangement provides support while maintaining privacy — far more affordable than assisted living ($4,000–$8,000/month).
Property Value
ADUs add 20–35% to property value. In a market where the median home costs $750,000, that's $150,000–$260,000 in additional equity — often exceeding construction costs.
Housing Solutions
ADUs help address the housing crisis by adding gentle density to existing neighborhoods without changing their character. They're the most politically feasible form of new housing in single-family zones.
ADU Benefits at a Glance
Generate $1,200–$3,500/month rental income
Increase property value by 20–35%
Provide independent housing for aging parents
House adult children returning home
Create a home office or studio space
Add gentle density without changing neighborhood character
Take advantage of by-right approval — no public hearings
Qualify for tax benefits as rental property
ADU Regulations: State-by-State Overview
ADU regulations vary significantly by state. Here's a snapshot of current laws in the most ADU-friendly states:
| State | By-Right? | Max Size | Key Law |
|---|---|---|---|
| California | Yes | 1,200 sq ft | AB 68, SB 13, AB 1033 |
| Oregon | Yes | 900 sq ft | HB 2001 (2019) |
| Washington | Yes (cities >25K) | 1,000 sq ft | HB 1337 (2023) |
| Colorado | Yes (by 2025) | Varies | HB 24-1152 |
| Vermont | Yes | 900 sq ft | Act 47 (2023) |
| Montana | Yes (cities >5K) | Varies | SB 528 (2023) |
| Texas | City-dependent | Varies | No state mandate |
How to Build an ADU: The Process
Building an ADU involves several key steps. While the exact process varies by jurisdiction, here's the general timeline:
Feasibility Check (1–2 weeks)
Verify your property's zoning, lot size, setback requirements, and utility capacity. Most cities have an ADU checklist you can review before investing in design.
Design & Plans (4–8 weeks)
Hire an architect or use pre-approved plans. Designs must comply with local building codes, Title 24 energy requirements, and ADA accessibility standards if applicable.
Permit Application (4–12 weeks)
Submit plans to your city's building department. Under California law, cities must approve or deny within 60 days through ministerial review — no planning commission hearing required.
Construction (4–10 months)
Hire a licensed general contractor or owner-build if permitted. Construction includes foundation, framing, MEP (mechanical, electrical, plumbing), and finish work.
Inspections & Occupancy (2–4 weeks)
Pass all required inspections (foundation, framing, electrical, plumbing, final) and receive your certificate of occupancy. Then you're ready to move someone in or start renting.
Compare Builder Quotes & ROI
Master the costs before you build. Get a free ROI analysis and connect with vetted ADU lenders & contractors in your area.
Common ADU Terminology
The ADU world has its own vocabulary. Here are the most important terms to know:
| Term | Meaning |
|---|---|
| Granny Flat | Informal term for an ADU, originally built for aging family members |
| In-Law Suite | Attached living quarters for relatives, often a precursor to modern ADUs |
| Casita | Spanish term for a small house, commonly used in the Southwest |
| Backyard Cottage | Term used in Seattle and the Pacific Northwest for detached ADUs |
| Laneway House | Canadian term for an ADU accessed from a rear lane or alley |
| Ohana Unit | Hawaiian term meaning 'family' — Hawaii's version of an ADU |
| Setback | Minimum distance required between your ADU and the property line |
| Ministerial Review | Non-discretionary permit review based on objective standards only |
Frequently Asked Questions
What does ADU stand for?
ADU stands for Accessory Dwelling Unit — a secondary housing unit built on the same lot as an existing single-family home. ADUs are also commonly called granny flats, in-law suites, backyard cottages, or casitas.
How big can an ADU be?
In California, detached ADUs can be up to 1,200 square feet. Attached ADUs are typically limited to 50% of the primary home's living area or 1,200 sq ft, whichever is less. Junior ADUs (JADUs) are capped at 500 sq ft. Size limits vary by state and municipality.
How much does an ADU cost to build?
ADU construction costs typically range from $150,000 to $400,000 for a full detached unit, depending on size, location, and finishes. Garage conversions cost $80,000–$180,000, while JADUs can be built for $50,000–$100,000.
Do ADUs increase property value?
Yes. Studies consistently show ADUs increase property value by 20–35%, often adding more value than the construction cost. A $200,000 ADU build can add $250,000–$350,000 in appraised value in strong markets.
Can I build an ADU on my property?
In California and many other states, homeowners have a by-right entitlement to build at least one ADU on any residential lot. Check your local zoning code, but state laws have removed most of the traditional barriers — no special permits, hearings, or neighbor approval required.
Related Glossary Terms
Ready to Start Your ADU Project?
Join hundreds of homeowners who used our guides to navigate their ADU build successfully.
✓ Free comprehensive guide ✓ Vetted contractor list ✓ No credit card needed