Accessory Dwelling Units

The Town of Windsor is offering an ADU Rebate Program!

The program is available through March 31, 2028 or until the funds are expended. Rebates are granted to eligible homeowners on a first-come, first-served basis. Click this link for details.

What is an Accessory Dwelling Unit (ADU)?

An accessory dwelling units (ADUs) are also known as granny or in-law units is a residential dwelling unit that is accessory to, and on the same parcel, as a primary residence. An ADU provides permanent and independent provisions for living, cooking, and sanitation. ADUs can come in many shapes and sizes, and they generally fall into one of the following categories:

Detached: Free-standing structure, such as a backyard cottage. This can include a legally existing garage or other accessory structure that is converted to an ADU.
Attached: Shares at least one wall with the primary residence and is an addition of space to the primary residence's structure.
Interior Conversion: Built from existing space in the primary residence (such as an ensuite, attached garage, or upstairs floor).

What is a Junior Accessory Dwelling Unit (JADU)?

A JADU is a type of ADU that cannot exceed 500 square feet in size and must be contained entirely within a single-unit residence or can be attached to a new or existing, detached ADU. A JADU may include its own bathroom or may share bathroom facilities with the adjoined residence and must have its own efficiency kitchen and separate entrance. An efficiency kitchen must include a cooking facility, storage cabinets, and a food preparation counter that are of a reasonable size in relation to the size of the JADU. The owner of the property is required to record a deed restriction, as provided in the Town’s Zoning Code.

Can I have an ADU on my property?

Accessory dwelling units may be located on a parcel zoned to allow single-unit or multi-unit residential uses and that contains an existing or proposed primary dwelling. In Windsor, residential properties are eligible to construct an ADU and a JADU, as allowed by State law. ADUs may also be placed on multifamily properties and in mixed-use zoning districts. See below FAQ “What can I build?” for further details.

What are the development standards, including setbacks, height, and size allowances?

The specific standards depend on the type of unit. The minimum setback requirement is typically four feet from the rear and side property lines. The maximum height is 16 feet, with exceptions as provided in the Zoning Code. The floor area is limited to 500 sq. ft. for JADUs, 800 sq. ft. for 1 bedroom/studio ADUs, and up to 1,200 sq. ft. for ADUs with 2 bedrooms or more. ADUs attached to the primary dwelling generally have architectural and design standards. A simplified reference guide can be accessed here. For a complete list of regulations, the development standards are located in the Town's Zoning Section for ADUs and JADUs.

Town Staff ADU Planning Hours

The Town’s ADU Planner is available for assistance remotely and at Town Hall during the following days and times:

Planning Counter in Town Hall, Building 400: Tuesdays 11:30 am -12 pm & 3 – 4 pm
Other counter hours available by appointment: Monday through Thursday, 8 am – 5 pm By phone, email, & Zoom: Monday through Thursday 7 am – 6 pm

Contact Information

To reach the Town's ADU Planner, call 707-838-5337 or email adu@townofwindsor.ca.gov.

Resources

Napa Sonoma ADU Center
Provides a wealth of resources for homeowners considering an ADU or JADU. Explore their website to find useful resources such as designs and floor plans, an ADU Handbook, helpful webinars, an ADU calculator, and stories from people that have built ADUs.
Homes for Sonoma
Nonprofit developer creating quality workforce housing options that support a safe and sustainable community, with an emphasis on ADUs.
HCD's Accessory Dwelling Unit Handbook
The State Department of Housing and Community Development provides a handbook that includes explanations and interpretations of California ADU Laws.
Casita Coalition's ADU Finance Guide
Nonprofit organization advancing housing solutions has prepared a financing guide for homeowners looking to build ADUs.

FAQs

What can I build?

The following units are allowed per California Government Code CHAPTER 13 for Accessory Dwelling Units [66310 - 66342], and JADUs.

On a Single-Family Lot

A property with an existing or proposed single-family home may be eligible for the following:

One ADU made through a conversion (Government Code 66323 Unit)
One Junior ADU (JADU) (Government Code 66323 Unit)
One new construction detached ADU (Government Code 66323 Unit)
One ADU created using Windsor’s Zoning Code Standards (Government Code 66314 Unit)

The following additional standards apply to the units listed above:

One ADU Conversion (Gov. Code 66323 Unit)
- Must be from existing space
- ADU must be a fully converted unit and cannot add additional space to an existing structure, except the unit may be expanded 150 sq. ft. from the existing structure for ingress and egress
- ADU can be within setbacks if the existing structure being converted is within the setback
One JADU (Gov. Code 66323 Unit)
- Can only be created from within an existing or proposed dwelling
- JADU is 500 sq. ft. maximum
- Can be created from an attached garage
- Cannot be detached
- JADUs cannot be expanded an additional 150 sq. ft. for ingress or egress, this is only for converted ADUs

One New Construction, Detached ADU (Gov. Code 66323 Unit)
- Can be within the front setback
- Cannot exceed 800 sq. ft. maximum
- See the Government Code for applicable height limits between 16 and 20 feet

One ADU Compliant with the Town’s Zoning Code Standards (Gov. Code 66314 Unit)
- This unit may be new construction or conversion and can be attached, detached, conversion, or conversion plus an addition
- These units utilize the Town’s Zoning Code Standards for ADUs such as size of structure, height, setbacks, floor area, architectural design, etc.

On Multi-Family Zoned Lots

If your property has an existing or proposed multi-family development:

Converted ADUs:
- Up to 25% of existing units (minimum of 1)
Detached ADUs:
- Up to 2 (if new multi-family)
- Up to 8 (if existing multi-family), not exceeding existing unit count
Attached ADUs:
- 1 new attached unit compliant with Gov. Code 66314

Are there any restricted uses of ADUs?

Per State law and Windsor’s Zoning Code, ADUs & JADUs cannot be used as a short-term rental.

For a JADU built without its own private bathroom, the owner is required to live on-site, in the main dwelling or JADU. For the complete list of restrictions that must be included in the deed, here is a link to the section in the Town's Zoning Code.

Does the Town have an Amnesty ADU Program for unpermitted units?

ADU amnesty refers to the process of legalization and issuance of permits for an ADU or JADU originally built without permitting approval. State law requires that jurisdictions issue permits for certain existing ADUs and JADUs originally built prior to January 1, 2020, and eliminate financial penalties to property owners who opt to legalize their unpermitted ADU or JADU. The full text of AB 2533 may be viewed at the following  link, which has been adopted in California Government Code Section 66311.7. Contact Planning staff for assistance with determining eligibility, adu@townofwindsor.ca.gov.

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