Building an ADU

Feasibility. The form PJR-149 Preliminary ADU Checklist outlines some of the important requirements to consider when deciding to build an ADU. Zoning, site conditions, required fees, and construction costs are a few things that can impact your ability to build an ADU. Please use your knowledge of the intended site and our parcel search tool to complete this preliminary checklist.

• PJR-149 – Preliminary ADU Checklist
• Parcel Search Tool & Permitted Zones

Development Standards. Sonoma County Municipal Code ultimately determines design and development standards for ADUs. Key items are summarized on our Development Standards for ADUs page and PJR-149 Preliminary ADU Checklist.

Estimate Project Budget. It is helpful to avoid having a fixed budget total in your head as you explore your options, because budgets are difficult to pin down this early in the process. That said, the ADU Center’s resources are a great place to start when developing a budget. It provides a rough estimate of costs and potential rental income and will help you understand how choices can impact your budget over time. Their Workbook and Exercises also have helpful information to get started, and our custom Feasibility Consults can help you get more specific.

Assess Financing Options. Many homeowners use a mix of options to finance their ADU, like their own savings and assets, funds from family, and/or loans. It is strongly recommended that you do not begin construction without your financing plan in place. Be sure to factor in potential rental income since it will be a source for repaying any loans.

• California Housing Finance Agency (CalHFA) Grant Program: The CalHFA operates a grant program to provide qualifying homeowners with reimbursement of costs, including site preparation, designs, permits, fees, and reports.

The ADU Center. The ADU Center is a valuable resource for broadly understanding the ADU development process. The Center has tools for budgeting and finance such as the ADU Calculator, a Fillable Budget Tracker, and a Vendor List for financial professionals.

The design of your ADU will be reliant on feasibility factors such as those referenced in PJR-149. Consider using a pre-reviewed ADU plan to save time, money, and effort in designing and permitting your ADU. Be sure to familiarize yourself with development standards for ADUs even if using pre-reviewed plans since certain project-specific standards will still need careful attention. Working with a professional through at least some of the process is recommended in either scenario. Owner-builders should review form CSS-026 Owner-Builder Information and Authorization.

• Option 1: General ADU
  • Customize all aspects of your ADU, ensuring consistency with ADU development standards. It is highly recommended you work with a licensed professional throughout this process if you are not one yourself.

• Option 2: Using ADU Ready Pre-reviewed Plans
  • Select an appropriate pre-reviewed ADU plan from our Pre-reviewed ADU Design Gallery and obtain a pre-reviewed ADU plan package from the listed vendor.
    • Project applicants are responsible for licensing fees and limitations on pre-reviewed plan use as required by plan vendors.
  • Integrate your selected pre-reviewed plan with remaining site- and project-specific standards. It is highly recommended you work with a licensed professional through this process. Some ADU development standards will be met within a pre-reviewed ADU plan package, but remaining site- and project-specific standards will need careful attention while integrating the pre-reviewed ADU design. See our ADU Ready page for more information.

Apply for permits. Most permit applications are submitted and processed electronically. Visit our online permitting website to submit a building permit or other development permit application, pay fees, schedule inspections, and more. Owner-builders accept full responsibility for all permits, standards, and fees applicable to their project.

• What permits do I need?
  • At a minimum, a building permit (BLD) is required to establish an ADU. However, multiple permits may be required depending on the project scope and site conditions. The most common additional permit types are for water, wastewater, grading, and encroachment.
  • ADUs in the coastal zone (CC zoning designation) require land use permit approval prior to submitting a building permit application. See Coastal Zone ADUs for more information.
• It is highly recommended and sometimes required that you work with a professional to obtain required permits. Owner-builders assume full responsibility for obtaining all necessary permits and should review form CSS-026 Owner-Builder Information and Authorization. Before applying for a building permit, review some of the considerations for each permit type described below.

• • • Water. Before applying for a building permit, find out whether there are any additional requirements related to water supply. Water requirements vary based on the type of water service and location.

• Will the ADU be served by a well in a water-scarce area (see above)? Contact a professional to conduct a dry weather well test ; (conducted between July 15 and October 1) demonstrating that the well meets minimum water yield requirements according to the County’s Dry Weather Well Testing policies. Find your parcel’s Groundwater Availability Area classification in the parcel report.
• If the well is located in a Class 4 Groundwater Availability Area or a Critical Habitat Area, the applicant must demonstrate compliance with Permit Sonoma Policy and Procedure 8-2-2 Guidelines for Net Zero Groundwater Use.
• Need a new well? Contact a professional for help preparing an application for a new well.
• Do you have public water service? Check with your provider to make sure they will provide service to the ADU.

Well permits should be submitted and in “Issued” status prior to applying for a building permit for the ADU. Well yield studies should be completed and submitted to the Well & Septic Division prior to applying for a building permit.

• Before applying for a building permit, find out whether there are any additional requirements related to wastewater disposal.
  • Do you have capacity in your current septic systemor will you do a "bedroom swap" to create capacity for the new unit? Contact a consultant to analyze the capacity and quality of your current system, prepare a findings report, or prepare an application for a bedroom swap.
  • Will you need to build a new septic system? Contact a consultant for professional help to design and apply for a permit for a new system.
  • Do you have sewer service? Make sure your sewer provider will provide service to the new unit and contact a professional for help preparing an application for a sewer permitif one is required.

If sewer or septic permits are needed, these permits should be submitted and in “Issued” status prior to applying for a building permit for the ADU.

• • • Building. All newly constructed ADUs require at least a building permit (BLD), but depending on the scope of construction, a grading permit, and/or encroachment permit may also be required. Once preliminary permits and requirements such as water and wastewater have been established, submit a complete building permit (BLD) application package. Plans must be signed by the responsible designer.

• Option 1 General ADUs
  • Follow building and permit submittal requirements to complete the application.
  • Complete applications will generally be approved or denied within 60 days.
• Option 2 Using ADU Ready Pre-reviewed Plans
  • Use ADU Ready Project Application Checklist (PDF coming soon) to gather appropriate materials.
  • Complete applications will generally be approved or denied within 30 days.

Pay initial fees for routing and review. Your application will begin with the Intake division for review for completeness. Once deemed complete, initial review fees are invoiced. After payment is made, all affected divisions will review your application. Initial fees for reviewing an ADU project will depend on the full project scope but typically include the following:

• Building Review Fees
  • Site Review
  • Building Plan Check
    • Option 1 General ADUs: Standard Building Plan Check
    • Option 2 Pre-reviewed ADUs: Reduced Building Plan Check - 2 Hours Minimum
      • Plan check fees are not applicable to pre-reviewed ADU components and only apply to site-specific elements unless modifications from the original pre-reviewed design are proposed.
    • CALGreen Residential Plan Review
      • Option 1 General ADUs: Standard CALGreen Residential Plan Review
      • Option 2 Pre-reviewed ADUs: Reduced CALGreen Residential Plan Review
        • This will be a 50% reduction from the typical rate established in the Permit Sonoma fee schedule.
      • Other Review Fees
        • Zoning Review
        • Fire Review
        • Well & Septic Review
        • Engineering Review
        • Grading Review

Once initial fees are paid, Permit Sonoma will begin review.

• Site review. A site evaluation is conducted by a Permit Sonoma building inspector to determine site conditions that could affect proposed new buildings, additions to buildings and/or grading. Site conditions that may require further review and/or affect the design of the proposed construction include, but are not limited to, flood zones, steep slopes, slides, soft or expansive soils, overhead power lines, wind exposure, seismic zones and areas of poor drainage, etc. After the site evaluation is completed, the building inspector will determine whether additional reports or studies are required.
• Building Plans Examiner review. The building plans examiner shall review submitted plans.
  • If pre-reviewed plans are used, the plans examiner will review all limitations of use on stock plans in relation to the project specific design.
    • If all limitations of use, including all relevant structural criteria and other items identified in item have been adhered to in the BLD application, the plans examiner will approve the application.
    • If there are location specific design criteria outside of the limitations, the plans examiner will deny the BLD application.
    • If there are any items missing for approval, the plans examiner will identify these items and enter a Plan Check Comments Sent entry to allow resubmittal of any missing sheets, details, information, or supporting documents.
  • Review by other divisions. All affected divisions shall review the project and either approve the application for issuance or provide issues to be addressed through the digital plan room. Other divisions to review the project include Planning, Fire, Well & Septic, and Engineering.
    • Any issues will be provided to the applicant via an email link.
    • Once all divisions have reviewed and either approved or provided issues to be addressed on an application, the applicant is able to resubmit.

Correct issues (if required). Once all relevant divisions such as Planning, Fire, and Well & Septic have reviewed your application, each will either approve the application or create plan check issues that must be addressed to ensure compliance with all adopted codes and ordinances. After this has happened, a resubmittal of corrected plans and supporting documents can occur. Requests by all divisions must be met with an updated application submittal demonstrating compliance before your building permit can be approved.

Once your application has been approved, final fees associated with your ADU will be assessed. These must be paid before your building permit can be issued. Some fees are a flat rate while others may be assigned based on the valuation of the project, square footage, or other measurements. Typical final fees are assessed for the categories listed below, though additional fees may apply depending on project scope. See ADU Fees and Permit Sonoma Fee Schedules for more information.

• Building Permit
• Address Assignment
• Electrical
• Mechanical
• Plumbing
• Fire Prevention
• Photovoltaic System – If included in project scope
• Water/Sewer Connection – If included in project scope
• Inspections
• Impacts & Mitigation
  • Fire Impact Fees – Certain fire districts
  • Parks Mitigation Fees – If the ADU is greater than 750 square feet
  • Traffic Impact Fees – If the ADU is greater than 750 square feet
  • Schools – If the ADU is greater than 500 square feet

Receive your building permit.
Once all appropriate divisions have approved your application and all fees have been paid, your building permit will be issued, and you may begin construction.

Revisions. You can apply for revisions to building permits after the building permit has been issued.

• All revisions proposed shall be prepared by the licensed design professional in responsible charge of the design.
• Revisions to an issued permit require submittal and review through the standard building permit revision process.

Hire a contractor. If you are not a licensed contractor or already working with a design/build firm, it is highly recommended that you hire one for the construction phase of your ADU. You can ask your architect or design team to recommend a contractor they’ve previously worked with or reach out to your community for recommendations. You will use drawings from your design team to get bids from contractors. Be sure you are specific about what you want each bid to include (license details, insurance information, examples of past work, etc.). Your design team may be able to help you with this.

• Owner-builders. If choosing to construct your ADU yourself, please review form CSS-026 Owner-Builder Information and Authorization.

 Begin construction and schedule inspections within a year from BLD permit issuance, or the issued permit shall expire. Once your ADU is under construction, . Guidance on scheduling inspections may be found on our webpage: Schedule an Inspection.

Pass Inspections. Your ADU must pass all inspections to receive a certificate of occupancy. Inspections for an ADU may include but are not limited to:

• Grading
• Compaction
• Site Utilities
• Foundation
• Interior Elements
• Exterior Elements
• Fire Prevention
• CALGreen – Performed by third party special inspectors, not Permit Sonoma staff 

 Obtain a Certificate of Occupancy. Make any required adjustments and pass all final inspections to receive a certificate of occupancy prior to inhabiting your ADU.

• If using a manufactured home (mobile home): HCD Form 433(A) and HCD Form 433(B) must be completed by the applicant and recorded with the Sonoma County Recorder’s Office after installation and inspections before a certificate of occupancy can be issued.
  • Applicants are responsible for recordation fees and HCD transportable unit fees.

Who will occupy your ADU?

Friends, aging parents, adult children, and other family members are common ADU residents, though you may choose to expand your pool of possible tenants.

Renting your ADU?

• Preliminary steps. If choosing to rent our your ADU, prepare to rent your unit by obtaining insurance, setting up utilities, and developing a plan for handling finances.
• Understand rental laws. Be sure to understand the rules and responsibilities of becoming a landlord.
  • California Tenants Guide
  • California Landlord Resources
• Set the rent. Consider offering your ADU as an affordable unit. Incentives may apply for ADUs with recorded Affordable Housing Agreements. See Article 89 – Affordable Housing Program Requirements and Incentives for more information.
• Rental Duration. Pursuant to County Code Sec. 26-88-060 Accessory Dwelling Units, ADUs may not be rented for periods of less than 30 days.

Ownership.

Pursuant to County Code Sec. 26-88-060, your ADU may be rented but not sold or otherwise conveyed separate from your primary residence except as specifically provided for by state law.

• PJR-149 Preliminary ADU Checklist (PDF)
• ADU Allowability Map
• ADU Development Standards
• ADU Ready: Pre-reviewed ADU Plans
• ADU Ready Project Application Procedure and Checklist (PDF Coming Soon)
• CSS-026 Owner-Builder Information and Authorization (PDF)
• Schedule an Inspection
•  Frequently Asked Questions

Contact Permit Sonoma divisions directly with questions.

Find Division Contact Information