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Z (Accessory Dwelling Unit Exclusion) Combining District Removal

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On September 17, 2019 the Sonoma County Board of Supervisors removed zoning restrictions that prohibit Accessory Dwelling Units (ADUs) on over 1,900 agricultural properties. An owner of this type of property will have the ability to apply for an Accessory Dwelling Unit and will be approved only if all applicable standards are met.

This policy adoption, which goes into effect October 17, 2019, introduces zoning text amendments to prevent potential impacts associated with Accessory Dwelling Units, and achieves General Plan objectives by eliminating unnecessary regulatory constraints to housing while continuing to protect and support agricultural and environmental resources.

Public Hearings

September 17, 2019

The Sonoma County Board of Supervisors met on September 17, 2019 and unanimously voted to approve the proposed changes to the Z (Accessory Dwelling Unit Exclusion) Combining District. View the staff report and other associated materials for the hearing.


May 7, 2019

The Planning Commission considered this item at a public hearing on May 2, 2019 and recommended that the Board of Supervisors approve the project. A complete staff report and associated materials from that hearing are available.

Background on the Z Combining District

Accessory Dwelling Units (ADUs) are a permitted use in all agricultural zones (LIA, LEA, DA). However, in 1993 the Board of Supervisors created the “Z Combining District” to limit ADUs where site constraints may prevent their development and use. The Z Combining District was also applied in agricultural areas to limit non-farming uses in an effort to preserve agricultural activity in these areas. ADUs in agricultural areas were also seen as superfluous because agricultural parcels could potentially develop farm specific housing types (e.g. agricultural employee housing).

Implementing the Housing Element

Since the Z Combining District was adopted, the County has been reviewing site-by-site applications to remove the Z District restriction on ADUs. Over 40 applications have been received and approved. The County has learned through the review of these applications that many agricultural areas that had site constraints no longer face those constraints (e.g. areas that had limited septic capacity may now be served by public sewer). In addition, many sites in agricultural areas do not qualify for farm-oriented housing types and the Z Combining District prevents working farms from providing a housing option to employees and family members. In response to these lessons, the County adopted a new policy in its Housing Element (Policy HE-3c) to “review Z Combining District restrictions on agricultural parcels of less than 10 acres county-wide, and consider removing the restrictions where appropriate.” 

Achieving Housing Objectives

The housing crisis necessitates bold and creative strategies to increase housing — especially affordable housing. In 1993, the Z Zone was applied on a broad scale to help preserve and protect agricultural lands and operations. Today, by reviewing the application of the Z District at a finer scale, we can identify parcels where ADUs may be appropriate, while protecting and supporting agricultural and environmental resources. This project would achieve Housing Element Objective HE-3.1 to “eliminate unneeded regulatory constraints to the production of affordable housing,” and Goal 3, to “promote production of affordable housing units.”

Parcels Eligible for Rezoning

To implement Policy HE-3c, staff developed criteria for site-specific evaluation to determine where Z District removal would be appropriate. 1,924 parcels meet all of the removal criteria:

  • the property is not located within a high or very high fire hazard severity zone;
  • the property is not within a critical biotic habitat area for the California Tiger Salamander;
  • an ADU on the parcel does not present the potential for groundwater contamination;
  • an ADU on the parcel will not unduly contribute to declining groundwater levels;
  • the property is not located in a Traffic Sensitive Combining Zone;
  • the property is not subject to a Land Conservation Act (Williamson Act) or other open space contract, or other recorded agricultural easements; and
  • the property is not located in the Coastal Zone

Ordinance Text Amendments

Permit Sonoma also identified two changes to county-wide ADU policy to reduce potential impacts:

  • To limit ADUs from over-developing agricultural parcels into residential uses, if a property was eligible to construct a farm-oriented housing unit, the ADU would count as one of the eligible farm-oriented units on the property. This prevents a property owner from constructing all eligible farm-related housing and an additional ADU on the site.
  • Current ADU standards prohibit ADUs from generating any additional water use in areas with low groundwater. Under a proposed text amendment, these same standards would also apply to areas where there are concerns that additional groundwater use could negatively impact sensitive fish and other stream dependent species.. 

Frequently Asked Questions (FAQs)

If the Z Combining District is removed from a property, is an ADU approved on that site?

No, not necessarily. An ADU permit is required to build an ADU anywhere in the county. To receive a permit, applicants must demonstrate consistency with the clear, objective standards established in the Zoning Code to address water, septic, access, and other potential issues related to the ADU. By removing the Z Combining District, a property owner will have the ability to apply for an ADU permit, which will be approved only if established standards are met.

Are ADUs compatible with rural land uses?

While the County seeks to focus growth primarily within Urban Service Areas, we recognize that ADUs in rural areas can provide valuable affordable housing opportunities in a time of significant need. ADUs in rural zones can help support small farms by providing supplemental income for farm families and may also reduce commute times and associated traffic and pollution by providing housing near rural jobs.

Are ADUs affordable?

The County surveys property owners with ADUs periodically. Surveys completed in 2006, 2013 and 2019 found that a majority of ADUs are offered at below-market rates that are affordable to moderate income households in Sonoma (ex. a 2-person moderate income household would earn $80,700; a moderate income family of four would earn $100,900.)

Can ADUs be used as hosted rentals or vacation rentals?

No. ADUs may only be rented for a minimum of 30 days.

Which parcels are eligible for Z Removal based on Permit Sonoma’s criteria?

View Map of Eligible Parcels

View More FAQs About ADUs and JADUs