Ever since January 1st 2017 when California relaxed the rules around permitting for Accessory Dwelling Units (ADUs) to  require local governments to approve ADUs based on some pre-defined parameters, there has been a lot of discussion and focus on ADUs. As more and more manufacturers are manufacturing ADUs, the discussion has centered on ADUs as a way to alleviate some of the pressures of the housing market and also as a way for people to care for their aging relatives.

While the rule changes are fairly extensive a couple of the key rule changes reduced the number of parking spaces required down to one per bedroom and the change in regulation also precludes a local government from banning them entirely.

In a previous post, I covered some of the companies which have launched products targeting this growing sector of the housing market.

In this post I want to cover off some of the more frequently asked questions for people who are looking to put an ADU on an existing property or property they plan to purchase.


Sonoma County ADUs

What is an ADU?

An ADU is a small permanent home that may be established in addition to the main dwelling on a parcel zoned for residential use. ADUs may be attached or detached from the main dwelling.

What is the maximum size for an ADU in Sonoma County?

The rules for the size of an ADU vary depending on whether the property is governed by County regulations or City regulations.

       For a property that is on public sewer, as long as the parcel size is greater than 5000 square feet, an ADU of up to 1200 sq ft is permitted

       For a property on a septic system but on public water and less than 2 acres, an ADU is limited to 640 sq ft and then, only if the original property has one bedroom.

       If a property is on 2 or more acres, an ADU of up to 1200 sq ft is permitted.

How long does it take to get a permit?

It is hard to be specific. However, the new California state law governing ADUs limits the maximum review period to 120 days. 

Does an ADU have to be a separate building?

No. An existing part of a house may be converted to be an ADU, however to be classified as an ADU it must have a separate exterior entrance 

Who Do I contact To Find Out If I can Have One?

Sonoma County has now set up a customer service system for people applying for ADUs in unincorporated Sonoma county. You can contact the planning department at planner@sonoma-county-org or on +1 707 565 1900 option 5. An appointment can be made on Tuesdays and Thursdays between 9am and 12 noon to meet with a planner. 

Do I need to provide additional parking to have an ADU?

As a general rule, Yes. One off-street parking space is generally required for an accessory dwelling unit. The parking does not typically need to be covered.

How Do I Apply For a Permit?

You would need to complete a permit application form (see below) as well as make sure that you have a site plan, floor plans, elevations, a completed application, the permit fee and an indemnification agreement. In addition, you will need to obtain any access permits required, proof of sufficient water and proof of a septic system capable of supporting the ADU.

Can I rent out an ADU as a vacation rental?

Regardless of whether the primary residence is permitted to operate as a vacation rental, an ADU is not permitted to be rented as a vacation rental. It can be rented for more than 30 days. 

Are There Limitations to the Materials an ADU can be constructed from?

An ADU has to be architecturally compatible with the primary residence. For example, it should be clad in the same siding material and be the same color.

New Rules For City of Healdsburg

What is the maximum Size in the City Of Healdsburg?

Healdsburg has just relaxed the rules for ADUs changing a couple of key regulations.

Previously the maximum size of an ADU was limited to 850 sq ft, the new maximum size is now 1,200 sq ft.

It used to be the case that an ADU could not exceed 45% of the floor area of the single family residence (excluding garages). This site coverage exception has now been removed for ADUs of 850 sq ft or less.

If a garage is being converted, replacement parking needs to be provided but it can be either covered or uncovered and can either be on an existing driveway or a new paved driveway.

The city also reduced the impact fees for units up to 850 sq ft to zero and for units bigger than this but smaller than 1,200 feet, the fees have been reduced by 50%. Both of these are significant. For example the total impact fees for a unit of less than 850 sq ft were $27,516 (including streets and traffic, parks, drainage, electricity, water system, sewer system), now they are zero!

In Summary

There is no doubt that both at a state, county and city level there has been a relaxing of the rules when it comes to putting an accessory structure on your property and also making it more affordable. 

With an aging population and a housing crisis that probably isn’t going to abate any time soon, this definitely opens up new opportunities for property owners to both care for family and/or generate income in a way that was never previously possible.

Additional Resources

Make an appointment to discuss an application:

Sonoma County Customer Service Information Sheet 

Estimated Fee Schedule 

ADU Planning Application Form 

Healdsburg city ADU brochure 

Healdsburg City ADU Ordinance


This content is not the product of the National Association of REALTORS®, and may not reflect NAR's viewpoint or position on these topics and NAR does not verify the accuracy of the content.