The True Cost of Building an ADU
When you see an ADU price tag, what's actually included? Understanding the cost breakdown helps you budget accurately, compare options fairly, and avoid surprise expenses. Here's where your money goes when building an ADU in California.
Site Preparation (10-15% of Total)
Before any construction begins, your site needs to be prepared. This includes grading and excavation ($3,000-$10,000), foundation work ($8,000-$20,000), utility trenching for water, sewer, and electrical ($5,000-$15,000), and potentially tree removal or fence relocation ($1,000-$5,000). For prefab ADUs, site prep is simpler since the unit arrives largely assembled.
Permits & Design (5-10% of Total)
Permit fees in California range from $5,000 to $20,000 depending on your city. Plan preparation costs $3,000-$10,000 for custom designs, though prefab manufacturers like EVO include engineering plans in their pricing. Impact fees (school, park, traffic) can add $5,000-$15,000 in some jurisdictions, though many California cities have waived these for ADUs.
Structure & Framing (25-30% of Total)
The structural shell — framing, roofing, exterior walls, windows, and doors — typically represents the largest single cost category. Steel-frame construction (used by EVO) costs more per sq ft than wood framing but delivers superior fire resistance, pest resistance, and longevity. A steel-framed ADU structure costs approximately $40,000-$70,000 for a 500-800 sq ft unit.
Mechanical Systems (15-20% of Total)
Plumbing, electrical, and HVAC represent the behind-the-walls infrastructure that makes an ADU livable. Expect $10,000-$18,000 for plumbing (kitchen, bathroom, water heater), $8,000-$15,000 for electrical (panel, wiring, fixtures), and $5,000-$12,000 for HVAC (mini-split systems are most common in ADUs). All-electric ADUs, as required by California's building code, typically use heat pump water heaters and mini-split heat pumps.
Interior Finishes (15-20% of Total)
This is where your ADU goes from a shell to a home. Finishes include flooring ($4,000-$10,000), cabinetry and countertops ($6,000-$15,000), appliances ($3,000-$8,000), bathroom fixtures ($3,000-$7,000), paint and trim ($2,000-$5,000), and lighting ($1,500-$4,000). The quality of finishes dramatically affects both livability and rental income potential.
Prefab vs Site-Built: Where the Savings Are
Prefab ADUs achieve lower costs primarily through factory efficiency: bulk material purchasing (15-20% savings), reduced labor waste (factory workers specialize in specific tasks), weather-independent production (no rain delays), and quality control (fewer callbacks and fixes). The result: a prefab ADU typically costs 25-40% less than a comparable site-built unit.
Hidden Costs to Watch For
- Utility upgrades: If your main panel needs upgrading for the ADU, add $3,000-$8,000
- Sewer lateral: Connecting to the sewer main can cost $5,000-$15,000 if a new lateral is needed
- Landscaping restoration: Budget $2,000-$5,000 to repair your yard after construction
- Temporary parking: If converting a garage, you may need alternative parking solutions
- HOA considerations: Some HOAs restrict ADUs — verify before investing in plans