Learn the basics of accessory dwelling units and find out if building one is right for you.
What Is an ADU? A Guide for Bay Area Homeowners
If you're starting to consider building an Accessory Dwelling Unit (ADU) on your property, you're not alone. ADUs have gained significant popularity in recent years as homeowners look for flexible, high-value ways to add living space, generate rental income, or support multigenerational living. Whether it's for extra family space, a guest house, or a rental unit, ADUs are a great way to maximize the potential of your property.
But what exactly is an ADU? Is it the same thing as a companion unit, granny flat, casita, guest house, or in-law suite? What types of ADUs can you build, and how do you go about getting started?
In this guide, we’ll walk you through everything you need to know about ADU housing, including:
But what exactly is an ADU? Is it the same thing as a companion unit, granny flat, casita, guest house, or in-law suite? What types of ADUs can you build, and how do you go about getting started?
In this guide, we’ll walk you through everything you need to know about ADU housing, including:
What Is an ADU?
An ADU, or Accessory Dwelling Unit, is a self-contained living space built on the same property as a single-family home. ADUs have their own entrance, kitchen, bathroom, and sleeping area, which means they function as completely independent homes. You might hear them called granny flats, in-law units, backyard cottages, casitas, or guest houses.
ADUs can be attached to your main house, built as a separate backyard structure, or created by converting existing space like a garage. They range from compact studios under 400 square feet to full two-bedroom units over 1,000 square feet.
California has become one of the most ADU-friendly states in the country, with laws that streamline permitting, reduce fees, and limit cities' ability to block projects. The result is a construction boom across the Bay Area, with homeowners adding ADUs for rental income, family housing, home offices, and more.
ADUs can be attached to your main house, built as a separate backyard structure, or created by converting existing space like a garage. They range from compact studios under 400 square feet to full two-bedroom units over 1,000 square feet.
California has become one of the most ADU-friendly states in the country, with laws that streamline permitting, reduce fees, and limit cities' ability to block projects. The result is a construction boom across the Bay Area, with homeowners adding ADUs for rental income, family housing, home offices, and more.
ADU Terminology
Accessory Dwelling Unit (ADU)
The official term for a secondary residential unit on a single-family property with its own kitchen, bathroom, and entrance.
Granny Flat / In-Law Unit
Informal terms for an ADU, typically referring to units used for family members.
Guest House
A detached structure for guests. Must have a full kitchen and meet building codes to qualify as a legal ADU.
Junior ADU (JADU)
A smaller unit (500 sq ft or less) built within the existing home. Requires a separate entrance and efficiency kitchen but can share a bathroom with the main house.
Detached ADU
A standalone structure built separately from the main house.
Junior ADU (JADU)
A smaller ADU (500 square feet or less) built within the existing footprint of a primary residence. JADUs must have a separate entrance and an efficiency kitchen, but they can share a bathroom with the main house.
Detached ADU
A standalone structure built separately from the main house, usually in the backyard.
Attached ADU
An ADU that shares at least one wall with the primary residence.
Conversion ADU
An ADU created by converting existing space like a garage, basement, or attic.
Types of ADUs We Can Build
There are several ways to add an ADU to your property. The best option depends on your lot size, your budget, how you plan to use the space, and the regulations in your city. Here's an overview of the main types.
Detached ADUs
A standalone structure built separately from your main home, usually in the backyard. Offers the most privacy and flexibility, making it ideal for rentals or independent family housing. California allows detached ADUs up to 1,200 square feet. Higher cost than conversions, and you'll need enough lot space to meet setback requirements.
Attached ADU
Connected to your primary residence, either as an addition or conversion of attached space like a garage. Can be more affordable than detached builds since you share some structural elements and utilities. Less privacy due to shared walls, but good for families who want to stay close.
Conversion ADU
One of the most cost-effective options since the basic structure already exists. You save on foundation, framing, and roofing. California no longer requires you to replace lost parking spaces. Limited by the existing footprint (typically 400 to 500 square feet for a two-car garage).
Junior ADU (JADU)
A smaller unit (500 square feet or less) created within your existing home by converting a bedroom, basement, or other space. Most affordable option since you're repurposing existing space. California allows you to build both a JADU and a detached ADU on the same property.
Why Bay Area Homeowners Are Building ADUs
How Many ADUs Can You Build?
Under current California law, most single-family homeowners can build:
- One detached ADU (up to 1,200 square feet)
- One Junior ADU within the existing home (up to 500 square feet)
- One conversion ADU from existing non-habitable space
This means a single property could potentially have four living units: the main house, a detached ADU, a JADU, and a converted garage. Exact rules vary by city and lot size. The best way to find out what's possible on your property is to schedule a free consultation.
- One detached ADU (up to 1,200 square feet)
- One Junior ADU within the existing home (up to 500 square feet)
- One conversion ADU from existing non-habitable space
This means a single property could potentially have four living units: the main house, a detached ADU, a JADU, and a converted garage. Exact rules vary by city and lot size. The best way to find out what's possible on your property is to schedule a free consultation.
How to Start Building an ADU
Step 1: Evaluate Your Property
Determine what's possible based on your lot size, setbacks, utility access, and local regulations. A professional site evaluation identifies opportunities and challenges early.
Step 2: Design Your ADU
Choose between a custom design or pre-designed floor plan. Work with a designer to create construction drawings that maximize your space while meeting code.
Step 3: Obtain Permits
Submit plans to your local building department for approval. California requires cities to review ADU permits within 60 days, though actual timelines vary.
Step 4: Build
Construction includes site prep, foundation, framing, electrical, plumbing, finishes, and inspections. Most builds take four to seven months.
Step 5: Move In or Rent Out
After passing final inspections, your ADU is ready to use however you planned.
Determine what's possible based on your lot size, setbacks, utility access, and local regulations. A professional site evaluation identifies opportunities and challenges early.
Step 2: Design Your ADU
Choose between a custom design or pre-designed floor plan. Work with a designer to create construction drawings that maximize your space while meeting code.
Step 3: Obtain Permits
Submit plans to your local building department for approval. California requires cities to review ADU permits within 60 days, though actual timelines vary.
Step 4: Build
Construction includes site prep, foundation, framing, electrical, plumbing, finishes, and inspections. Most builds take four to seven months.
Step 5: Move In or Rent Out
After passing final inspections, your ADU is ready to use however you planned.
Costs and Timeline at a Glance
Typical Costs
- Garage conversions: $100,000 to $200,000
- Attached ADUs: $150,000 to $300,000
- Detached ADUs: $200,000 to $400,000+
Costs depend on size, finishes, and site conditions. We provide detailed estimates before any work begins. See our ADU Costs page for a full breakdown.
Typical Timeline
- Design: 1 to 2 months
- Permitting: 2 to 4 months
- Construction: 4 to 7 months
Total project time is usually 10 to 18 months from consultation to move-in.
View all ADU Costs- Garage conversions: $100,000 to $200,000
- Attached ADUs: $150,000 to $300,000
- Detached ADUs: $200,000 to $400,000+
Costs depend on size, finishes, and site conditions. We provide detailed estimates before any work begins. See our ADU Costs page for a full breakdown.
Typical Timeline
- Design: 1 to 2 months
- Permitting: 2 to 4 months
- Construction: 4 to 7 months
Total project time is usually 10 to 18 months from consultation to move-in.
Common Challenges and How to Avoid Them
Permitting Delays
Every city has different rules. Work with a team that knows your local process and can submit accurate, complete plans the first time.
Design Constraints
Lot size, setbacks, and slope can limit your options. Get a professional feasibility study before committing to a design.
Budget Surprises
Many homeowners underestimate costs. Get detailed pricing early that includes permits, utilities, and site work.
Contractor Issues
Work with a design-build firm that handles the entire process. When one team owns the project start to finish, problems don't fall through the cracks.
Every city has different rules. Work with a team that knows your local process and can submit accurate, complete plans the first time.
Design Constraints
Lot size, setbacks, and slope can limit your options. Get a professional feasibility study before committing to a design.
Budget Surprises
Many homeowners underestimate costs. Get detailed pricing early that includes permits, utilities, and site work.
Contractor Issues
Work with a design-build firm that handles the entire process. When one team owns the project start to finish, problems don't fall through the cracks.
Ready to Learn More?
Schedule a free consultation with our team to find out what's possible on your property. No pressure, no obligation.
Contact us
Get in touch with our team to start your ADU project. Whether you have questions about what's possible on your property or you're ready to begin the design process, we're here to help. Fill out the form and tell us about your goals.
We'll review your property, put together a cost estimate, and walk you through what comes next. Consultations are always free with no obligation.
We'll review your property, put together a cost estimate, and walk you through what comes next. Consultations are always free with no obligation.
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