Are you a homeowner in California looking to increase your property value and generate rental income? Building an Accessory Dwelling Unit (ADU) might be the perfect solution for you. ADUs are secondary units that can be built on residential properties, offering flexibility and functionality. However, navigating the steps and requirements of building an ADU in California can be challenging. In this comprehensive guide, we will walk you through the process of successfully building an ADU in California, from understanding zoning laws and permits to designing and constructing your ADU. Get ready to unlock the full potential of your property with our expert tips and insights!

Understanding ADUs: What You Need to Know

Before diving into the process of building an ADU, let’s start by understanding what an ADU is and its benefits. ADU stands for “Accessory Dwelling Unit” and refers to a secondary dwelling unit on a residential property that exists alongside the primary dwelling. ADUs are versatile spaces that can be used as rentals, guest homes, or even personal workspaces. They provide homeowners with an opportunity to increase their living space and generate additional income.

ADUs go by various names, including granny flats, companion units, casitas, guest houses, granny cottages, secondary living units, mother-in-law apartments, second dwelling units, in-law suites, and carriage houses. These terms are often used interchangeably, but they all refer to the same concept of a secondary dwelling unit on a residential property.

Who Can Build an ADU in California?

According to California legislation, any owner of a residential property with an existing dwelling may build an ADU as long as the structure complies with state and local regulations. However, there are specific requirements and restrictions to be aware of. Property owners are generally allowed to build one detached accessory dwelling unit per lot, and the size and height limitations may vary depending on the city or jurisdiction.

For example, in California, detached ADUs are typically limited to a maximum size of 1,200 square feet and a maximum height of 16 feet. However, some cities may have different regulations and allow for larger ADUs. It is crucial to check your city’s specific legislation on ADU builds to ensure compliance on both state and local levels.

Building an ADU involves several steps, from initial feasibility and design to obtaining permits, construction, and final touches. Let’s explore each step in detail:

1. Feasibility & Design

The first step in building an ADU is conducting a feasibility study to determine if the project is viable and within budget. This study examines property setbacks, zoning regulations, topography, easements, and any other factors that might impact the ADU build . It is essential to have a clear understanding of the anticipated budget and any necessary site work before proceeding.

During the feasibility stage, you will also finalize the design of your ADU. Consider your personal budget, available space, and intended use of the ADU when determining the floor plan. Whether you want additional rooms for hobbies or a larger living space for entertaining, work with a contractor to create the best layout for your unit. Once the design is finalized, the specifications will be incorporated into construction documents for submission to your city.

2. Permitting & Revisions

Obtaining permits for your ADU project is a crucial step that involves submitting construction documents to your city for review. These documents outline all the details of your ADU, including its location, layout, and compliance with local regulations. It is common for cities to request revisions before issuing the permit. Your contractor will work closely with you to make any necessary changes and ensure compliance with the city’s requirements.

Throughout the permitting process, you will also be responsible for paying plan check and permitting fees, which vary depending on the size of your unit. Some jurisdictions may offer fee waivers for smaller ADUs. Once the fees are paid and the revisions are made, you will be ready to proceed with the construction.

3. Mobilization

After obtaining the necessary permits, construction mobilization begins. Within two weeks, the ADU’s footprint is located on the property using stakes, allowing you to visualize the unit in relation to existing features. During this phase, any necessary site work, such as foliage removal or hole filling, will be completed. Additionally, temporary toilet facilities may be installed on-site. Your project manager will review the approved documents with you and discuss the next steps.

4. Site Work

Site work involves preparing the property for the construction of the ADU. This includes creating a stable base for the foundation, installing retaining walls if needed, and running utilities such as septic, water, gas, and electricity. Grading is also performed to ensure the site is level. If your property is on a slope, additional work may be required to ensure the ADU remains level once constructed.

5. Trenching & Foundation

Trenching and foundation work are essential for the stability and functionality of the ADU. Trenches are dug around the footprint of the ADU to lay the foundation and run utility lines. The foundation is built using wooden planks and filled with concrete. This step sets the stage for the framing of the ADU.

6. Framing

Framing brings the structure of the ADU to life. Wooden planks are used to construct the rough frame of the unit, both for the interior and exterior. This step requires precision and attention to detail to ensure the ADU is built according to the approved design.

7. Rough MEP

Rough MEP (Mechanical, Electrical, and Plumbing) involves integrating utility lines into the ADU. Wires, electrical cables, and gas lines are installed within the framed structure. These lines will later be concealed within the walls, providing essential services to the ADU.

8. Exterior Finished / “Water Tight”

Exterior finishes, such as roofing, stucco, siding, windows, and doors, are installed to make the ADU “water tight.” While the exterior finishes may not be fully completed at this stage, the elements necessary for weatherproofing are put in place. This allows interior work to commence while the exterior finishes are finalized.

9. Insulation & Drywall

Insulation plays a crucial role in maintaining energy efficiency and comfort within the ADU. Both interior and exterior insulation are installed to seal heat inside the home and create a barrier against external temperatures. Drywall is then installed, transforming the ADU’s interior into a livable space.

10. Interior Finished

The interior finishes bring the ADU to life and reflect your design choices. Plumbing fixtures, such as valves and inserts, are installed before the walls are painted. Flooring, cabinets, doors, baseboards, and closets are all part of the interior finishes. Countertops and appliances are also installed, completing the major changes within the ADU. Cleaning and touch-ups are performed before the final inspection by the city.

11. Final Touches

During the final phase of the project, all the remaining details are taken care of. A thorough cleaning is performed, ensuring that all fixtures are properly installed and in working order. A final walkthrough is conducted, and the city conducts a final inspection. Once the ADU passes inspection, a “Certificate of Occupancy” is awarded, marking the completion of the project. Landscaping and any additional exterior work can be completed after the ADU build is finished.

Remember, each ADU project is unique, and the timeline and specific requirements may vary. Working with a professional contractor experienced in ADU construction can greatly simplify the process and ensure a successful build.


Building an Accessory Dwelling Unit (ADU) in California offers homeowners an opportunity to increase property value and generate rental income. By following the steps outlined in this guide, you can successfully navigate the process of building an ADU, from feasibility and design to obtaining permits, construction, and final touches. Remember to consult local regulations and work with an experienced contractor to ensure compliance and a smooth build. Unlock the full potential of your property with an ADU and create a versatile and functional space that meets your needs and enhances your living experience.

ADU construction is a complex process that requires careful planning and attention to detail. It is important to understand the local regulations and guidelines for building an ADU, as well as the necessary steps to take for a successful build. With proper preparation and research, you can begin the journey of creating an accessory dwelling unit that meets your needs and enhances your living experience. From feasibility studies and design plans to obtaining permits, constructing the ADU, and finishing touches, this guide has provided you with the information needed to make your ADU project a success.

Contact us today to start creating your dream ADU!

Cameron Meredith

As Vice President of ADU West Coast, Cameron heads our overall operations. Cameron oversees seeking new projects and opportunities throughout Los Angeles and Orange County. He has been involved with various development/construction projects throughout his career. One of them being a 47-acre greenhouse commercial development project in Adelanto, CA.