Accessory Dwelling Units: The Basics
- Hayley Andersen George
- Aug 7
- 7 min read
Keep hearing that term thrown around and have no idea what it is or why it's such a big deal? An 'ADU' is an Accessory Dwelling Unit - the technical planning or zoning term for an in-laws unit, granny flat, mother-in-law's cottage, etc. They've been around forever, but the regulations around them have changed significantly over the last couple of years. We will break down our knowledge of these clever little value adders in our post below.
What we'll cover in this post:
The definitions of ADU and JADU (junior ADU) in the State of California
The types of configurations of ADUs (attached, detached, conversion, etc.)
Why you would want to build an ADU on your property
The State laws and local codes governing ADUs in Shasta County
What are the 'Pre-Approved' or 'Prototype' ADUs made available by building departments?
What are prefabricated ADUs?
Important fees to know before getting started
( 1 ) The definitions of an Accessory Dwelling Unit and other similar projects
ADU - An attached or detached residential dwelling unit that provides complete independent living facilities for one or more persons and is located on a lot with a proposed or existing primary residence. Accessory dwelling units shall include permanent provisions for living, sleeping, eating, cooking, and sanitation on the same parcel as the single-family or multifamily dwelling is or will be situated.
JADU - A new dwelling unit of less than 500 sf that is contained entirely within an existing single-family home and uses at least (1) existing bedroom and a new exterior entry. It may have its own sanitation facilities or share them with the main residence. Think existing bedroom suite that is converted to a separate unit with a new efficiency kitchen (definition below) and a new main entrance for the inhabitant.
Efficiency Dwelling Unit - This term is no longer used in the residential code, but it exists in the building code.
Guest House - These bonus units have been allowed to be constructed for a long time, and while they may have a bedroom or two, they do not contain a full kitchen and therefore are not considered a legal second dwelling unit on a single property. Essentially, they are more comfortable for short-term guest stays.
Pool house - These are accessory structures and do not provide sleeping quarters or a full kitchen
Accessory Structure - A structure whose use is accessory and incidental to that of the dwelling(s) and that is located on the same lot. An accessory structure is not used for dwelling - think detached garage or shop building.
Full Kitchen - A space for meal preparation including a sink, meal preparation space, refrigeration, and cooking equipment
Efficiency Kitchen - Includes a sink, cooking facility with appliances that do not require electrical services greater than 120V or gas, and a food prep counter with storage in relation to the size of the JADU
( 2 ) The Types & Configurations of ADUs
Maximum sizing - Generally, the maximum allowable size of an ADU will be 1,200 sf. Some jurisdictions have a secondary limit of 1/2 the square footage of the main residence, with a minimum of 600-800sf depending on the jurisdiction.
A New Construction ADU - This can be a detached, stand-alone unit, or a newly constructed unit attached to the main residence.
An ADU Conversion - Where an existing structure or space that was not used for dwelling is repurposed and re-permitted for use as an ADU
A Junior ADU or JADU - See above definition.
( 3 ) Why would you want to build an ADU on your property
We see two main reasons why people come to us for ADUs: one, a multi-generational family situation, and two, for rental income. Both are rooted in the generally high cost of homes and construction we see these days, and people trying to either save money or make money. ADUs are almost always seen as an investment opportunity.
The multi-generational home angle - Home prices are ridiculous post-COVID. One of the most common ways people are making it work is by bringing together multiple generations. We've seen the grandparents buying a house and building themselves an ADU so their kids/grandkids can live in the main house and everyone saves on home costs and childcare costs. We've seen families with teenage kids who want to be able to provide a place for them to be on their own as they grow, but still connected to the family. But probably most often we see families with aging parents who want to bring them into their homes to keep everyone close and ease worry and care needs. All are wonderful strategies that give us direction on how to best design the ADU features to meet those specific needs.
The rental income angle - This has (2) different applications that we've seen, but it is important to note that most agencies will not allow ADUs in any configuration to be used for short-term rentals (AirBnB, etc.). So, when we say rental, we mean a rental that is leased for greater than 30 days at a time.
1. Homeowner Rental: Homeowners who have the space on their lot and see it as potential income or to offset their own home costs.
2. Developer-Rental: With the current state laws, you could potentially build a new home with (1) ADU and (1) JADU. Which would be (3) separate rental incomes. This would be a large investment, and we'd have to examine the regulations carefully to make sure we are in line with all the square footage requirements, but the potential gain could be significant.
( 4 ) The State Laws and local codes governing ADUs in Redding and Shasta County
Many laws and bills have been passed regarding ADUs over the last decade. Many of these laws supersede any local regulations that cities and counties (agencies) may have previously had in place to regulate ADUs more strictly. Some agencies have updated their code language to match, but some have not, so it is important to know what is allowed by state law. We won't go over every detail here, but we'll list out the big-ticket regulations to know.
Where ADUs are allowed - Generally, ADUs must be allowed in ALL residential zones and generally should not be subject to 'discretionary approval'. Meaning if you live in a residential zoned area, you have a right to build at least (1) ADU without having to apply for any extra planning permits or go through any hearings or reviews.
Lot Requirements - Generally, a local jurisdiction cannot impose lot coverage, floor-area ratio, or standard setbacks on ADUs. ADUs generally only have to have a minimum setback from the property lines of 4 feet for both the side and rear of the building. Many residential zones have setbacks of 10 to 30 ft, so this regulation saves a lot of space.
This also applies to ADU conversions. Many old garages were built closer to the property lines than current zoning permits - often before any setbacks were in place. State regulations say that garages or other accessory structures can be converted to ADUs as long as they meet the 4ft setback.
ADU height - The state laws give thresholds for maximum heights that may override arbitrary or onerous height limitations listed by agencies.
Parking - There are a number of exemptions given in the state laws for providing new parking spaces. Many cities and counties used to require (1) off-street parking space per bedroom or ADU, but the state law lists a number of conditions that may allow ADUs to be built with no new off-street parking, and if you convert your garage or carport into an ADU, you are not necessarily required to provide that off-street space elsewhere.
Owner Occupancy - State law has largely eliminated the ability of cities to require owner-occupancy for either the ADU or the main residence. This opens the door for local developer - rental markets.
Impact Fees - ADUs under 750sf shall not have impact fees. ADUs over 750 sf may have impact fees (and school fees), but they must be proportional to the primary dwelling's impact.
Fire Sprinklers - While fire sprinklers are required in all new residences (since 2008), for ADUs, the regulations are different. If the main residence does not have fire sprinklers, then the ADU is not required to have fire sprinklers.
**We've recently seen the Shasta County Fire Marshal require fire sprinklers in new ADU garages even where they are not required in the residence *
( 5 ) What are 'Pre-Approved' or 'Prototype' ADU Plans?
This is a new concept that a lot of cities and counties across the country are trying out, for better or worse. These are plans that have been purchased by the agency and pre-reviewed for code compliance, enabling the agency to offer the plans to its citizens for free, with just the permit fees due. The process makes architectural plans more accessible to the public by enabling the homeowner to skip the design and construction document process and fees, which can be a significant barrier to entry. As of this writing, the process has only been applied to Accessory Dwelling Plans, and they are available locally from the City of Redding and Shasta County.
( 6 ) What are Prefabricated ADUs?
As ADUs become more and more popular, many companies are popping up claiming to be able to provide pre-fabricated ADUs faster and cheaper than custom, stick-built. These are built in a factory and delivered in one or more modules that are joined together on-site. Their quality and price ranges span the whole spectrum - from what most people think of as mobile or modular homes to custom ultra-modern units to kits that owners can put together. If this is the path you are thinking about, check out our post on Pre-fab projects to get a better idea of what this strategy requires that may be left out of marketing materials.
( 7 ) Important fees to know before getting started
The 750 SF threshold - ADUs over 750 SF will incur school fees on the full area of the unit, which locally is about $5 per square foot. ADUs under 750 SF do not have to pay school fees.
Expect to be required to pay permitting fees somewhere from 2% to 4% of your construction cost when picking up your approved permit. School fees often have to be paid to the school board in advance of picking up the permit, and you will need to bring the receipt to the permit center with you as proof the fees are paid.
Terms:
ADU - Accessory Dwelling Unit, sometimes referred to as an In-laws Unit or a Mother-in-law Cottage, etc.
JADU - Junior Accessory Dwelling Unit - An ADU that is less than 500 sf, may be either attached or detached to/from the main residence.
AHJ - Authority Having Jurisdiction - the government agency or building department that will issue your building permit
Links & Sources:
The City of Redding ADU Handout/Guide - https://www.cityofredding.org/home/showpublisheddocument/26108/638035094897700000
Shasta County Announcement for Pre-Approved ADU Program - https://www.shastacounty.gov/planning/page/accessory-dwelling-units
State of California ADU Regulation Guides - https://www.hcd.ca.gov/building-standards/adu/handbook
