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 #
For now, the Plans Gallery is focused on detached ADUs. But there are many types of ADUs, and they don’t always fit neatly into one category:
Detached New freestanding structure separate from the main home (site-built or prefab/manufactured)
ATTACHED New structure (may include converted space) sharing at least one wall with 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 house (if it’s accessible from the JADU)
- Owner must live on the property
MULTI-FAMILY A detached, attached, or conversion ADU on a multi-family property (duplex, triplex, apartment building)
WHAT ISN’T AN ADU? RVs, tiny homes on wheels, yurts, and storage structures are generally NOT considered accessory dwelling units.
Construction Types #
Site-built ADUs, also called stick-built, are built from scratch using traditional construction methods. This option allows for a lot of customization and smaller changes to be made throughout the construction process.
Prefabricated ADUs, also called Prefab, Manufactured, Modular, or sometimes Panelized, are built in a factory, then shipped to the site to be assembled or placed on a foundation. These units typically offer less flexibility and customization options, but often have a more streamlined process and fewer hidden costs and risks. We require all prefabricated ADUs in the Plans Gallery to be certified by either state or federal government. As a result, they will always appear as pre-approved (link) on the Plans Gallery.
Project Timing #
Building an ADU is an investment of time as well as money. Most projects take 12-24 months to complete.
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.
FAQs #
The following are written for properties in California – each state has their own rules concerning ADU development.
Am I allowed to build an ADU? #
In almost all cases, yes! ADUs and JADUs are allowed in any single-family or multi-family zone. If residential buildings are allowed, ADUs are almost always allowed too (with limited exceptions for safety, traffic, and water).
California state law now allows a single-family property to have a JADU, an internal conversion or attached ADU, and a detached ADU. Some places allow for more. Multifamily properties – like duplexes, triplexes, and apartment buildings – can have two detached ADUs or up to 25 percent of the number of units, depending on your property. Talk to local staff for more information if interested in building ADUs on a multifamily property.
How large can my ADU be? #
Generally, ADUs can range from 150 square foot efficiency units up to 1,000 square feet but this depends on local rules for your property – you will need to contact local Planning or Building staff in your area to confirm.
According to California state law, you can at least build up to an 800 square feet ADU, as long as rear and side setbacks are at least 4 feet and it is not above 16 feet tall. Otherwise, size limits depend on your property. No room behind or next to your main home? You can build it in your front yard instead.
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.
California 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 if my property is in a historic zone/coastal zone/high-fire severity area/flood zone/etc? #
Ask your local Planning or Building Department for guidance about building an ADU in special zones. It may require a simple additional application, or it could significantly limit what you can build on your property – so make sure you ask before you purchase any plans.
What do I do first? #
The best place to start is by thinking about what you want, and understanding your goals and concerns. The Plans Gallery can be useful at this stage to see what options are available and to get inspired. Once you have some ideas in mind, we recommend setting a rough budget, which includes identifying your funding source or sources. From there, we recommend saving some of your favorite designs in the Gallery and contacting those designers, which can help you start to understand what you are looking for in a professional to help you complete the project.
