ADU 101
Let's start from the beginning


What is an ADU?
From space for your loved ones to building your retirement through renting, an ADU will grow with you and your family over time.
Accessory dwelling units (ADUs), also known as granny flats, casitas, in-law units, or backyard cottages, can come in many shapes and sizes and serve many different needs. ADUs are always a self-contained home that is often smaller than the main home and legally part of the same property. ADUs must have a kitchen, bathroom, and place to sleep, and can range from studios under 500 square feet to more than 1,000 square feet with multiple bedrooms.
ADU Types







Detached ADU
New freestanding structure separate from the main home, like a backyard cottage.
Pros
- Provides the most privacy between the main home and ADU
- Lots of flexibility in design
- Lower construction impact on the main home
- Higher potential rental income
Cons
- Usually the most expensive option
- Highest cost and more difficult to add new utility connections
- Takes up more space in your yard
- Solar panels are usually required

Attached ADU
Pros
- Lower cost for utility connections and upgrades
- Takes up less space in your yard
- May blend in better with the main home
- Easy access for a caregiver or loved one
- May have lower construction impact on neighbors
Cons
- Probably requires a custom design (can’t use a pre-designed plan or prefab unit)
- Less privacy between the main home and ADU (sounds and smells may be more noticeable and you may have to share outdoor space)
- Can be challenging to find a good place to connect to the main home
- Construction will likely impact the main home


Conversion ADU
Converted existing space in the main home or on the property (bedroom, basement, detached garage).
Pros
- Usually the lowest cost option, especially if converting existing living space
- Usually lower cost for utility connections and upgrades
- May have fewer or lower overall fees
- Takes up less yard space
- May blend in better with the main home
- May have lower construction impact on neighbors
Cons
- Can be challenging and costly if significant upgrades are necessary, such as bringing a garage up to code
- Less flexibility in design
- Potential loss of existing living space (bedrooms, etc.) or storage space (garage, basement, attic)
- Construction can impact the main home, especially if you convert existing living space
- May require upgrades to the whole house (talk to local staff if you want to convert more than 50% of the main home)

Junior ADU (JADU)
A type of conversion up to 500 square feet within the main home (existing or proposed). Size and other requirements may be different where you live. Check with local staff to confirm JADU rules.
Pros
- Usually the lowest cost option - design, construction, and maintenance
- Lowest cost for utility connections and upgrades
- Easy access for a caregiver or loved one
- Blends in better with the main home
- Lower construction impact on neighbors
- Doesn't require a full kitchen
Cons
- Requires the property owner to live in either the main home or the JADU and file a deed restriction
- Less privacy between the main home and ADU (sounds and smells may be more noticeable)
- Can be challenging to fit the necessities and design for comfort
- Lower potential rental income
- Construction will probably impact the main home

Multi-Family ADU
Any of the other types (except JADU) that is part of a multi-family building like a duplex, triplex, or apartments (existing or proposed). There are some important considerations to keep in mind for multi-family ADUs:
- Provides new housing options for community members usually at a more affordable rent
- Good option for underutilized parking, commercial or storage spaces
- Increased rental income and property value
- Larger construction loans may be available
- Some properties may be allowed to build multiple ADUs
- May increase property taxes
- Construction may impact existing tenants
- May reduce commercial space, shared outdoor space, parking, or storage

Not adus
RVs, tiny homes on wheels, yurts, and storage structures are usually not considered ADUs. Check with local staff to confirm what structures on your property may be permitted as a ADU.
Project Timing
Building an ADU is an investment of time as well as money. Most projects take 12-24 months to complete, with interior conversions being the fastest.

Project Cost
Generally, the total cost to build an ADU in San Mateo County ranges from $450-$600 per square foot. The San Mateo County ADU Calculator is a great place to start when developing a budget. It provides a rough estimate of costs and income and will help you understand how choices can impact your budget over time, all customized to real local numbers.
Construction Types

Site-Built/Traditional
A traditionally constructed ADU, built from scratch on your property (“stick-built”). This option typically allows for a lot of customization and smaller changes made throughout the construction process.

Prefabricated / modular / manufactured
These ADUs are partially or mostly built in a factory, then shipped to your site to be put together. Sometimes the company will include all services in their fee (“turn-key”), including help with permitting and all on-site construction tasks (e.g., laying the foundation, utility hookups, etc.). Other times you’ll need to hire additional professionals to help.
Why build an ADU?
Building an ADU is an investment in your property, your family, and your community.
ADUs often provide homes for the local workforce and small families who have difficulty finding local housing that meets their needs. For homeowners, there are many reasons to build an ADU — rental income, safe and convenient housing for relatives, flexibility over time, and many others.

A Home As You Age
Space for a caregiver or an accessible and comfortable option for aging adults.

Housing loved ones
Homes for adult children, aging relatives, or family with special needs.

Environmental Benefits
Small-scale living and reduced commutes for local workers.

Rental income
Money for retirement, savings, or a rainy day from either the ADU or the main home.

Safety
An extra set of eyes on the property day-to-day or when you’re away.

Community support and workforce housing
Housing for neighbors and support for the local economy.
Frequently Asked Questions
Find answers to the most frequently asked questions about ADU basics.
What do I do first?
This site walks you through each part of the ADU process, from gathering initial inspiration and learning what you can build through construction and becoming a landlord or moving in.
You can also use our Process-At-A-Glance resource for an overview of the process and some initial issues to consider as you get started.
Start with Getting Started and walk through the pages of this site one-by-one for a detailed guide on what to do next.
What’s the difference between an ADU and a JADU?
Accessory Dwelling Units (ADUs) come in many shapes and sizes but are always a self-contained home that is usually smaller than the main house and legally part of the same property. They must have a kitchen, bathroom, and place to sleep, and typically range from studios under 500 square feet to over 1,000-square-foot homes with multiple bedrooms.
Junior Accessory Dwelling Units (JADUs) are within the footprint of your home (or attached garage) and less than 500 square feet. They can share a bathroom with the main home and have an efficiency kitchen (sink, cooking appliance, fridge, and small counter). Construction costs for JADUs are typically much lower. In most cases, the property owner must live on site in either the main home or the JADU.
State law now allows homeowners to have both a JADU and a regular ADU on their property.
What is the difference between a site-built and a prefabricated or manufactured ADU?
Site-built/Traditional: A traditionally constructed ADU is designed and built specifically to your preferences and property and built on site (“stick-built”). This option allows for a lot of customization and smaller changes to be made throughout the construction process.
Prefabricated/panelized/modular: These ADUs are partially or mostly built in a factory, then shipped to your site to be put together. Sometimes the company will include all services in their fee (“turn-key”), including help with permitting and all on-site construction tasks (e.g., laying the foundation, utility hookups, etc.). Other times you’ll need to hire additional professionals to help.