An Accessory Dwelling Unit (ADU) is a secondary housing unit on a single-family residential lot. Also known as granny flats, in-law suites, backyard cottages, or casitas, ADUs are one of the fastest-growing housing trends in America. With ADU permits up 63% since 2019 and new laws making them easier to build, there has never been a better time to consider adding an ADU to your property.
Types of ADUs
ADUs come in several forms, each with different costs, timelines, and requirements:
| Type | Description | Typical Cost | Timeline |
|---|---|---|---|
| Detached ADU | Standalone structure in the backyard | $150K-$350K | 8-14 months |
| Attached ADU | Addition to the primary home | $100K-$250K | 6-10 months |
| Garage Conversion | Converting an existing garage | $60K-$180K | 4-8 months |
| Basement ADU | Converting basement space | $50K-$150K | 3-6 months |
| JADU | Junior ADU within existing home (max 500 sq ft) | $30K-$80K | 2-4 months |
| Prefab ADU | Factory-built, delivered to site | $80K-$250K | 4-8 months |
Learn more about each type in our detailed ADU Types Guide.
Benefits of Building an ADU
Rental Income
ADUs generate $1,000-$4,000+/month in rental income. Many homeowners fully offset their mortgage with ADU rent alone.
Property Value
An ADU can add $100,000-$300,000+ to your property value — often more than the cost to build.
Family Housing
Provide independent living for aging parents, adult children, or extended family while keeping them close.
How Much Does an ADU Cost?
ADU costs vary significantly by location, type, and size. Here are general ranges:
National averages: $80,000 to $350,000+ for a complete ADU project including design, permits, and construction. Cost per square foot ranges from $150 to $500+ depending on your market and finishes.
Key cost factors include your city's permit fees, local labor costs, site conditions (slope, access, utilities), and level of finishes. Garage conversions and JADUs are the most affordable options. Use our ADU Cost Calculator for a personalized estimate.
Finance Your ADU Project
Most ADU projects are funded through HELOCs, construction loans, or cash-out refinancing. Compare rates from top lenders in minutes.
Compare ADU Financing Options →ADU Regulations by State
ADU regulations vary dramatically by state and city. California, Oregon, and Washington have the most progressive statewide laws. Key things to check:
- Allowed zones — Most states now require ADUs in single-family zones
- Size limits — Typically 800-1,200 sq ft maximum
- Setbacks — Usually 4-5 ft from property lines
- Height limits — Commonly 16-25 ft
- Parking — Many states have eliminated parking requirements
- Owner occupancy — Increasingly not required
Find the exact rules for your city using our city-by-city database, or read about California ADU laws.
How to Build an ADU: Step by Step
Research Your City's Rules
Check your city's ADU zoning rules, setbacks, size limits, and permit requirements. Start with our city pages.
Set Your Budget
Determine total budget including design, permits, construction, and a 10-15% contingency. Use our cost calculator.
Secure Financing
Compare HELOCs, construction loans, and ADU-specific lenders. See our financing guide for details.
Design Your ADU
Hire an architect or use pre-approved plans. Check if your city offers standard plan sets to save time and money.
Get Permits
Submit plans to your city's building department. Most cities must process ADU permits within 60 days. Read our permit guide.
Build
Construction typically takes 3-12 months depending on ADU type. Find vetted contractors in your area.
Final Inspections
Pass all required inspections and receive your certificate of occupancy. Plan for 1-3 weeks.
Rent & Earn
List your ADU and start earning income. Consider long-term rental vs. short-term depending on your city's rules.
Find ADU Rules for Your City
Look up the exact zoning rules, setbacks, and permit requirements for your city.
Search Your City →