Step 1: Choose your project type and scope
Prefab ADU installation often includes multiple levels of work. Your estimate changes dramatically depending on scope. For example:

• “Cosmetic” work is lighter and cheaper
• “Partial” is moderate
• “Full” includes major systems, structural adjustments, and more labor

Step 2: Enter square footage
Size is the foundation of your estimate. Larger ADUs raise:

• Materials
• Labor
• Mechanical system sizing
• Finish quantities (flooring, cabinets, tile, paint)
• Utility loads and connection requirements

Step 3: Select finish level
Finish level is one of the biggest “budget levers.” You’re basically choosing the lifestyle tier:

• Basic: clean, functional, simpler materials
• Mid-range: balanced upgrades, better durability
• High-end: premium surfaces, custom feel, higher fixture costs

Step 4: Pick your California region
Location affects pricing through labor rates, permit costs, and market multipliers. This is where generic national “averages” fall apart – California pricing is not one-size-fits-all.

Step 5: Add advanced factors (if needed)
Advanced inputs help refine your estimate when your site or plan isn’t straightforward. For example:

• Existing structure conditions (if tying into a garage conversion or attached build)
• Age/condition factors that affect demolition and hidden repairs
• Utility complexity and upgrade risk
• Contractor markup or labor rate adjustments

Step 6: Calculate and review the line-item breakdown
Instead of spitting out one number, the calculator breaks the cost into categories. This makes it easier to:

• spot “big movers”
• compare scenarios
• talk to contractors using the same language

Step 7: Review contingency and total
A built-in contingency (often around 10%) is there because real projects have real surprises – rocky soil, trenching issues, old lines, city requirements, or material lead times.