The ADU construction process generally includes:

1. Feasibility Study & Design: Assessing your property for ADU suitability, determining the best location, and designing the unit to meet your needs and local regulations.
2. Permitting: Submitting detailed plans to your local planning and building departments for approval. California law mandates a 60-day approval timeline for ADU permits once a complete application is submitted.
3. Financing: Securing the necessary funds for construction.
4. Site Preparation: Clearing the site, preparing the ground, and laying the foundation.
5. Construction: Building the ADU, which can involve traditional stick-built methods, prefabricated panels, or modular construction.
6. Utility Connections: Connecting the ADU to water, sewer/septic, electricity, and gas lines.
7. Finishing & Inspections: Completing interior and exterior finishes and undergoing final inspections to obtain a Certificate of Occupancy.

State law sets general guidelines:

• Detached ADUs: Generally up to 1,200 square feet.
• Attached ADUs: Cannot be larger than 850 sq ft for a studio or one-bedroom, or 1,000 sq ft for two or more bedrooms, and generally cannot exceed 50% of the primary residence's size.
• Height: Typically up to 16 feet for detached ADUs, with some exceptions allowing up to 25 feet or 35 feet depending on proximity to the main dwelling or local zoning. Local jurisdictions may have slightly different interpretations or additional restrictions, so always confirm with your city/county.

In many areas of California, if your property is within half a mile of public transit, additional parking is NOT required for an ADU. Otherwise, local ordinances may still require one parking space per ADU, depending on the location and property layout.

The timeline can vary based on the complexity of the project, local permitting efficiency, and chosen construction method. Generally, from initial design to move-in, an ADU project can take anywhere from 6 to 18 months, with permitting often being the longest phase. Utilizing pre-approved plans or modular construction can significantly reduce the overall timeline.

The overall time to build a modular ADU in California can vary, but it's generally significantly faster than traditional stick-built ADUs, primarily because much of the construction happens off-site in a factory while site work occurs simultaneously.

Here's a breakdown of the typical timeline:

• Overall Project (from design to move-in): While traditional ADUs can take 9-18 months, modular ADUs can often be completed in 6 to 9 months, with some claims as short as 3-5 months or even as little as 1 month once the site is ready and permits are issued (though this is less common for the entire process).

Here are the key phases and their estimated durations:

1. Consultation and Design:
   • Timeframe: Approximately 6 weeks to 4 months.
   • What happens: Discussing your needs, budget, site assessment, and finalizing the ADU design. Customization can extend this phase.
2. Contracts and Permitting:
   • Timeframe: Roughly 2 to 6 months.
   • What happens: Finalizing plans, securing financing, and submitting plans to state and local authorities for approval. While modular homes benefit from state-approved plans for the structure, local permits for site work and connections are still required. California law aims for a 60-day permit approval once a complete application is submitted, but local workload can affect this.
3. Manufacturing (Off-site):
   • Timeframe: This happens concurrently with site preparation, typically a few weeks to a few months, depending on the manufacturer and the ADU's complexity.
   • What happens: Your ADU modules are built in a climate-controlled factory, ensuring efficiency and quality control, unaffected by weather delays.
4. Site Preparation and Foundation:
   • Timeframe: Typically 2 to 4 weeks.
   • What happens: Clearing the land, grading, excavation, installing the foundation (slab, crawl space, or basement), and bringing utilities (water, sewer, electricity, gas) to the ADU site.
5. Delivery and On-site Installation ("Set Day"):
   • Timeframe: The actual "set" of the modules by crane can be done in as little as 1 to 3 days.
   • What happens: The finished modules are transported to your property, lifted by crane onto the prepared foundation, and securely joined together.
6. On-site Finishing and Utility Connections:
   • Timeframe: Approximately 2 weeks to 2 months (after the initial set).
   • What happens: Connecting the ADU to all utilities, completing exterior finishes (siding, roofing, trim at seams), and interior finishes (drywall, paint, flooring at seams, final fixtures).
7. Final Inspections and Occupancy Permit:
   • Timeframe: Usually 1 to 2 weeks.
   • What happens: Local building inspectors perform final checks to ensure everything meets code, leading to the issuance of the Certificate of Occupancy, allowing you to legally use the ADU.

Factors that can affect the timeline:

• Customization: More complex or highly customized designs will take longer in the design and permitting phases.
• Local Permitting Office Backlog: Even with state mandates, local building departments can experience delays.
• Site Complexity: Challenging terrain, difficult access for cranes, or extensive existing demolition can extend site preparation time.
• Utility Line Extensions: If utility lines need to be extended a significant distance, this can add time and cost.
• Weather: While modular construction minimizes weather delays during the build, severe weather can impact site work and transportation.
• Contractor Experience: An experienced ADU builder with streamlined processes can help keep the project on track.

By choosing a modular ADU, you're primarily gaining efficiency in the construction phase itself, significantly reducing the overall project duration compared to starting from scratch on-site.