With the recent changes further lifting the restrictions on secondary dwelling units, in particular, Accessory Dwelling Units, it’s hard to keep track of what is allowed and what’s not.

The big change at the beginning of this year lifted the minimum lot size restriction for an ADU. In other words, regardless of the size of a lot, a single family residence can have a secondary unit up to 840 sq ft in size. However, there are still rules around setbacks which will sometimes place a limit on the size of an ADU on a smaller lot. While the lot size limitations have been lifted, properties must have adequate water and sewer or septic capacity in order to have an ADU or any other accessory living unit. 

Accessory Dwelling Unit versus Junior Accessory Dwelling Unit: What is a JADU?

The main differences between an ADU and Junior ADU (JADU) is that Junior ADUs are created out of existing living space such as a bedroom or garage, whereas an ADU is created through additions to the existing home or detached garage or through new construction on the site. 

Junior ADU

Here is a summary of the Sonoma County Junior ADU (JADU) requirements:

– Located in an existing single-family dwelling.

– Created within an existing, legal bedroom.

– Maximum size of 500 square feet.

– Has an “efficiency kitchen” or “wet bar”: A removable kitchen that has cooking appliances that do not require natural gas, propane, or electrical service greater than 120 volts; a limited food preparation counter; an under the counter refrigerator and a small sink. The entire kitchen counter length shall not exceed six feet

– Has a separate exterior entrance.

– Still has the interior door to the house.

– Private or shared bathroom

– No extra parking required

– May be rented for over 30 days, but no short-term rental is allowed.

– Must have adequate provision for water and sewerage

Accessory Dwelling Units:

An ADU is a small permanent home that may be established in addition to the main dwelling on a parcel zoned for residential or certain agricultural uses. ADUs may be attached or detached from the main dwelling. Here is a summary of the main Sonoma County ADU requirements that an ADU must meet in order to get a Sonoma County building permit:
– The maximum size for Accessory Dwelling Units (ADUs) is up to 1,200 square feet while the minimum size is 225 square feet.
– A full kitchen is required as well as a full bath
– As with Junior ADUs there must be adequate provision for water and septic to support an ADU
– ADUs must be at least 4 ft from side, front and rear setbacks. They must also conform to setback limits from septics and certain waterway.
-Some properties may be located in the “Z” (Accessory Dwelling Unit Exclusion) overlay zoning, which prohibits accessory dwelling units. However many of these properties have now been identified as properties where this restriction can be removed subject to approval. Here is a map of all properties identified

-May be rented for over 30 days, but no short-term rental is allowed.

City Rules for ADUs

The rules for ADUs within many cities are very similar to those for the county. For example in the City of Healdsburg the following rules apply:
-Any ADU up to 850 sq ft in size is not subject to the site coverage limits for the underlying zoning

-An ADU can be two stories and up to 25 ft high

-Front setbacks must adhere to the underlying zoning district but side and rear setbacks need to be just 4 ft.
-There is no requirement for additional parking spaces
-An ADU must have a separate entrance and access to the public right-of-way may be provided through the rear yard of the primary residence or a dedicated pathway. 


Other Legally Permitted Structures

However, in addition to secondary units, many properties in Sonoma County are permitted to support multiple buildings on a single residential site. Here is a summary of all the different permitted types

For example properties that have agricultural and resource zoning LIA (Land Intensive Agriculture), LEA (Land Extensive Agriculture), DA (Diverse Agriculture), RRD (Rural Resources and Development), AR (Agricultural Residential) and residential zoning RR (Rural Residential), R1 (Low Density Residential), R2 (Medium Density Residential), and R3 (High Density Residential) are allowed:

1) One Accessory Dwelling Unit
2) One Junior ADU
3) One Guesthouse
4) One Pool House
5) One Miscellaneous structure (Gym, Yoga studio, Art Studio etc)

Guest House
A guest house is another type of accessory structure that is a bedroom and living area with the option of a bathroom with different set of rules on size and facilities:
– The floor area of a guest house shall be a maximum of six hundred forty (640) square feet.
– There can be no food preparation facilities or food storage. This includes fridges, microwaves or food storage cupboards.
– A guest house can have a half bath or full bath or neither
– It cannot be located more than one hundred feet from the primary dwelling 
– A guest house cannot be leased, subleased, rented or sub-rented separately from the main dwelling.

– A guest house must be located closer to the primary dwelling on the subject lot than to a primary dwelling on any adjacent lot.

Pool House

A pool house is another additional structure that is permitted on a residential property or a property with agricultural zonings outlined above.

The key requirements of a pool house are as follows:

Only a half bathroom is allowed
If there is a shower, it has to have exterior access only
A pool house cannot have a bedroom
A pool house cannot have any indoor kitchen facilities but it can have an outdoor kitchen
The maximum size is 3000 square ft if it is purely a pool house and does not meet the specification for a guesthouse or secondary unit
There must be a pool!

If the pool house is less than 640 sq ft and there are no other accessory structures a Class III septic system will suffice

Miscellaneous Structure (Yoga, Studio, Gym)
How many properties have you seen which feature a yoga room, gym or art studio? The specifications for an additional structure of this type are identical to a pool house but without the requirement for a pool. Often the reason a structure is described like this is because it doesn’t meet the requirements for other type of accessory structures:
Cannot have a kitchen but it can have a “wet bar” (see junior ADU – small area designated for food prep but with no stove
It can contain a half bath but if it is has a shower, it must have exterior access only
If the miscellaneous structure is less than 640 sq ft and there are no other accessory structures a Class III septic system will suffice
The maximum size is 3000 square ft

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.