Planning and Designing Your ADU

Assess Your Property

Before diving into design, evaluate whether your property is suitable for an ADU. If a bedroom or family room inside the main house would do the job, compare that with an ADU in the ADU vs. home addition guide. Consider:

• Lot size and configuration
• Existing structures and their placement
• Access points for construction and future use
• Utility connections and potential upgrades needed
• Topography and drainage patterns
• Trees and landscaping that may be affected

State law requires cities to allow ADUs in zones that allow housing. A city may limit locations only for water, sewer, traffic flow, or public safety, and the shape of your lot can still limit the design (Gov. Code §66314).

Research Local Regulations

While California has statewide ADU laws, local jurisdictions may have additional requirements. Research:

• Zoning regulations specific to your area
• Size: a city cannot set a maximum below 850 sq ft of interior livable space, or 1,000 sq ft for an ADU with more than one bedroom. With no compliant ordinance, a new detached ADU may be up to 1,200 sq ft (Gov. Code §66314), and a city may allow more (Gov. Code §66325)
• Height restrictions and setback requirements. The statewide floors, including the 4-foot side and rear rule, are in the setback, height, and parking guide.
• Parking: the city cannot require replacement parking when a garage or parking space is converted or demolished for an ADU, and it cannot require ADU parking within a half-mile walk of public transit, among other cases (Gov. Code §§66314 and 66322)
• Design review processes if applicable
• Impact and connection fees

Contact your local planning department or check their website for ADU-specific guidance. If the choice is adding a unit or dividing the parcel, read ADU vs. SB 9 lot split.

Determine Your ADU Type

California allows several types of ADUs. Consider which is best for your property:

Detached ADU

A standalone structure separate from the main house. Offers maximum privacy but may require more extensive utility connections.

Attached ADU

Connected to the main house but with separate entrance. Often easier for utility connections and may be more cost-effective.

Conversion ADU

Created by converting existing space like a garage or basement. Often the most affordable option with simplified permitting in California.

Junior ADU (JADU)

Up to 500 sq ft, created within the walls of the main house. May have simplified requirements and lower costs.

On many single-family lots, state law allows both an ADU and a junior ADU, which can mean three homes on one lot (Gov. Code §§66323 and 66333). That combination is still what to plan on today. Starting January 1, 2027, AB 956 (Quirk-Silva), Chapter 791, Statutes of 2026 requires a city to ministerially approve two detached, new-construction ADUs with no more than 4-foot side and rear setbacks on a lot with a proposed or existing single-family home. A city may still cap each at 800 square feet of livable space and apply the height limits in Government Code section 66321(b)(4). The city does not have to ministerially approve a junior ADU on a lot where those two detached units were built. The ADU-and-junior-ADU combination above is not erased. Local ordinances have to follow the new detached count on January 1, 2027. Confirm with your city. The March 2026 HCD handbook does not cover this bill yet. See the 2026 law update.

Establish Your Budget

Develop a realistic budget for your ADU project. These figures are planning estimates, not an official state price and not a bid. They are the same ranges used on the cost guide:

• Junior ADU or interior conversion: $30,000–$80,000 planning estimate
• Garage conversion: $90,000–$180,000 planning estimate
• Attached ADU: $150,000–$300,000 planning estimate
• Detached ADU: $225,000–$450,000+ planning estimate

Your budget should include:

• Design and architectural fees
• Permitting costs
• Site preparation
• Construction costs
• Utility connections
• Finishes and fixtures
• Landscaping
• Contingency fund (typically 10-15%)

California financing options include home equity and construction loans. The CalHFA ADU Grant, which used to reimburse up to $40,000 in pre-development costs, had its latest round fully allocated on December 28, 2023 and is not open.

Design Your ADU

Work with professionals to design an ADU that meets your needs and complies with regulations:

• Hire an architect or designer with ADU experience
• Consider prefab or modular options for potential cost savings
• Focus on space-efficient design for smaller footprints
• Incorporate California-specific considerations like earthquake safety
• Plan for energy efficiency to meet California's Title 24 requirements
• Solar: a newly built detached ADU is generally subject to the Energy Code solar requirement, with exceptions, and the panels may go on the ADU or the main home. A conversion or an addition is not subject to that solar requirement (HCD ADU Handbook, March 2026)

Obtain Permits

Navigate the permitting process for your ADU:

• Submit plans to your local building department
• Prepare for plan check corrections and revisions
• Budget for permit fees (varies by jurisdiction)
• Understand the timeline (California law requires streamlined approval for compliant ADUs)

Under Government Code section 66317, a complete ADU application is approved or denied ministerially within 60 days when a home is already on the lot. The agency also has 15 business days to say whether the application is complete.

Select a Contractor

Choose a qualified contractor for your ADU project:

• Look for contractors with specific ADU experience
• Verify licenses and insurance
• Check references and past projects
• Get multiple bids to compare
• Ensure they're familiar with California building codes

Consider contractors who specialize in the type of ADU you're building (conversion, prefab, custom-built, etc.).