ADU 101
Let’s start from the beginning
What is an ADU?
Accessory dwelling units (ADUs), also known as “granny flats,” “in-law units,” or “backyard cottages,” 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 can range from small studios to larger homes with multiple bedrooms.
Types of ADUs
ADUs do not always fit neatly into one category – like an attached ADU made from converted space, for instance.

Detached
New freestanding structure separate from the main home, like a backyard cottage

Attached
New structure (may include some converted space) sharing at least one wall with the main home

Conversion
Converted existing space in the main home or on the property (bedroom, basement, detached garage)

JADU
A smaller conversion ADU within the main home or attached garage
- Up to 500 square feet
- Efficiency or full kitchen
- Can share bathroom with main home (if accessible from JADU)
- Owner must live on the property

Multi-family
Any of the above (except JADU) that is part of a multi-family building like a duplex, triplex, or apartment building (existing or proposed)
Not an ADU
RVs, tiny homes on wheels, yurts, and storage structures are NOT accessory dwelling units
Types of ADU Construction
Prefabricated, Modular, Panelized

Prefab, modular, and panelized are partially or mostly built in a factory, then shipped to your site and assembled or placed on a foundation. Some companies provide a “turnkey” service that helps with permitting and all on-site construction, from laying the foundation to utility hookups. If not, you’ll need to hire additional professionals to help.
Using a prefab design can save time and make clearer what your completed project will look and feel like, but they also typically offer less customization, can be difficult to permit and finance, and often cost the same as site-built ADUs. People are often surprised at the cost, but transportation, crane, foundation, and sales tax can end up being 20-40% of the total cost. Some companies do offer a smaller price point.
To explore prefab or modular ADU options, you will want to get in touch with companies directly. They will visit, evaluate your property, and make recommendations. You can find prefab contractors by researching online.
Site-built

Site-built is a traditionally constructed ADU built from scratch on your property (AKA “stick-built”). This option allows for a lot of customization and smaller changes to be made throughout the construction process.
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.

Why Build an ADU?
Building small has a big impact on 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.
Here are some of the reasons why people build ADUs:

Housing Family
Close but independent living for adult children, aging relatives, or loved ones with special needs.

A Home As You Age
A nearby home for caregivers or for aging adults who want to rent their main home and live in their ADU.

Rental income
A steady source of additional income for retirement or a rainy day. Can come from the ADU or the main home.

Safety
An extra set of eyes and hands on your property can be helpful for safety, when you travel, upkeep, and more.

Property Investment
Not only will you receive rental income in the short term, but your sale price and property value can increase too.

Community Support
Provide housing to families, workers, and others who might struggle to find something local.
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 (“turnkey”), 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.
What is 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 large 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/or 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.
No room behind or next to your main home? You can build it in your front yard instead.
What do I do first?
The best place to start is by thinking about what you want, understanding your goals and concerns, and looking at other ADUs for inspiration. Once you have some ideas in mind, you can consider your budget and move on to Learning the Rules to figure out what you can build on your property.
Am I allowed to build an ADU?
In almost all cases, yes! ADUs and JADUs are allowed in all single-family and mixed-use zones. If residential buildings are allowed, ADUs are almost always allowed too (with limited exceptions for safety, traffic, and water). Confirm with local Planning staff – see our Contact page for contact information.
Let’s do this!
We have resources for every step of the way, from initial thoughts to move-in.
