Prefab ADU Cost

The total installed cost of a prefab ADU typically ranges from $150,000 to $300,000, which is significantly higher than the base unit price.

Base prices for small prefab units (200-400 sq ft) start between $40,000 and $120,000, but this excludes site work and delivery.

Site preparation and foundation work generally account for 10% to 20% of the total budget, often costing $15,000 to $35,000.

Utility connections (water, sewer, electrical) are a major variable expense, typically adding $15,000 to $40,000 to the project.

Crane rental and delivery logistics for placing the prefab unit usually cost between $5,000 and $15,000.

The rest of it, in plain terms

• Permitting and impact fees vary by municipality but generally range from $5,000 to $15,000.
• The typical timeline for a prefab ADU project is 3 to 6 months, compared to 6 to 12 months for a custom stick-built unit.
• California state law (Gov. Code § 65852.2) mandates that local jurisdictions allow ADUs up to 800 sq ft with 4-foot setbacks.
• Prefab ADUs are generally 15% to 30% cheaper than comparable custom stick-built ADUs, which cost $250,000 to $500,000.
• Modular prefab ADUs must comply with local and state building codes (like the CBC), unlike manufactured homes which follow the federal HUD code.

Figures below are researched market numbers for budgeting and comparison. They are not quotes. Final pricing depends on your site, your jurisdiction, finish selections, foundation and utility distance.

• Total Installed Cost: $150,000 to $300,000 (Source: Custom Home Design and Build)
• Small ADU Base Price (200-400 sq ft): $40,000 to $120,000 (Source: Prefahb.co)
• One-Bedroom ADU Base Price (300-600 sq ft): $80,000 to $180,000 (Source: Prefahb.co)
• Large ADU Base Price (600+ sq ft): $150,000 to $300,000+ (Source: Prefahb.co)
• Site Preparation and Foundation: $15,000 to $35,000 (Source: Custom Home Design and Build)
• Utility Connections: $15,000 to $40,000 (Source: Custom Home Design and Build)
• Crane and Delivery: $5,000 to $15,000 (Source: Custom Home Design and Build)
• Permits and Fees: $5,000 to $15,000 (Source: Custom Home Design and Build)
• Custom Stick-Built ADU Cost (for comparison): $250,000 to $500,000 (Source: Custom Home Design and Build)

ADU rules are set locally and change often. These are the provisions that most commonly control the outcome. Verify against your own jurisdiction before you design anything.

• California Government Code Section 65852.2: Mandates local jurisdictions to allow ADUs up to 800 square feet, 16 feet in height, with no more than 4-foot side and rear yard setbacks.
• California Government Code Section 65852.27: Requires all jurisdictions to have pre-approved ADU plans available to the public.
• California Government Code Section 66314: Prohibits denying an ADU application due to existing building code violations on the primary unit unless they present a health and safety risk.
• HUD Code (24 CFR Part 3280): Federal standard for manufactured homes, which preempts local building codes, though modular and site-built ADUs must follow local/state codes (e.g., California Building Code).

Prefab ADU advertising quotes the unit price, which is the cost of the building leaving the factory. The number you actually need is delivered and installed and permitted, and the difference between those two figures is routinely thirty to sixty percent of the unit price. Neither number is dishonest; they answer different questions, and only one of them is the question you are asking.

The components between unit price and finished cost are freight, crane and set, foundation, utility trenching and connection, site work and grading, permits and impact fees, and the engineering and energy documentation your jurisdiction requires. Of these, utility distance and site access have the widest variance, and both are knowable in advance if someone walks the site.

When you compare suppliers, normalize every quote to delivered-and-installed on your parcel before comparing anything. A supplier quoting a low unit price and excluding set and foundation will look cheaper than one quoting turnkey, right up until the invoices arrive. Ask each one for the same scope, in writing, including what they are not doing.

From unit price to finished cost

Most pages on this subject put the cost behind a contact form. The reasoning is understandable and the result is that you cannot budget, cannot compare, and cannot tell whether a bid is reasonable until you have already given up your phone number to three companies.

We publish the figures instead. They are researched market ranges rather than quotes, and site conditions will move them more than any other factor, but a range you can plan against beats a call-back you have to wait for. The constraints that decide feasibility — setbacks, height, utility distance, site access — are stated up front for the same reason. If your project is not viable, that is worth knowing on the first visit rather than the third phone call.

Factory production windows move with queue depth, material lead times and seasonal demand. We quote the current window rather than a fixed promise, because anyone promising a specific delivery date months out without seeing the factory schedule is guessing. Permitting is usually the longer pole: in ministerial states the review clock is defined by statute, while in discretionary jurisdictions it can run considerably longer.

The sequence that controls your schedule is site plan, permit submittal, plan check and corrections, permit issuance, factory slot, delivery and set, then utility connection and final inspection. Factory build time overlaps permitting only if you commit to the slot before permits are issued, which carries its own risk.