Building an ADU: Costs, Requirements & Design Essentials

Understanding ADU regulations in California
Building an ADU begins with determining whether your property is suitable for an ADU. Factors of zoning regulations include the size, height, and location of the ADU. As of 2025, the State of California continues to pass new ADU laws lifting restrictions and making it easier and more affordable to build Accessory Dwelling Units. Once you confirm that an ADU is possible on your property, the design can be completed and submitted to the city for approval. After getting the necessary permits, construction can begin.

Define your ADU Floor Plan Goals
Once you have a clear understanding of the regulations, you need to define your goals for your ADU. Do you want to rent or use it as a guest house? Will you need a full kitchen or just a kitchenette? Do you want to create a private outdoor space or share it with the main house? A clear idea of your goals will help you design a space that meets your needs and budget.

ADU Design and Permits
The next step is to design your ADU and obtain the necessary permits. We can connect you with an architect or designer that aligns with your style or select your own. The final design should include all the necessary living spaces, such as a bedroom, a bathroom, a kitchen, and a living area. Building codes must be addressed in your plan, accounting for insulation, ventilation, electrical, plumbing, and structural requirements. Once your ADU design is ready, you must submit it to the permitting department and obtain a building permit.

ADU Construction & Timeline
The construction phase involves site preparation, foundation laying, framing, roofing, and installation of windows and doors. Following this, electricity, plumbing, and HVAC systems are installed. The final phases encompass interior finishing, including insulation, drywall, flooring, and painting, and exterior work, such as landscaping and driveway construction. Most ADU projects follow a general timeline range between 6-12 months, depending on permit approvals and weather conditions.

How Much Does It Cost to Build an ADU? 
The cost of building an ADU varies depending on the unit’s size, location, and design. The ADU cost breakdown in the South Bay ranges from $300,000 to $700,000 depending on the size and site preparation specifics of your ADU. Building an ADU can be an excellent investment to customize your lifestyle with more livable space, increase your property value, or provide an avenue for additional income, especially if you plan to rent out the unit.