Prefab vs. Site-Built ADU: Which Should You Choose?

Prefab ADU marketing is excellent — fixed prices, factory precision, "installed in weeks." Site-built contractors counter that prefab is a box on a truck and the real costs hide in the fine print. Both sides are selling something. Having supervised construction for twenty years, here's the comparison I actually walk homeowners through.

What "prefab" really means

Prefab spans a range: full modular units craned in nearly complete, panelized kits assembled on site, and manufactured units on permanent foundations. What they share is that the unit is priced like a product — and the unit price is real. What the brochure underplays is everything the factory can't ship: your site.

The part nobody's brochure includes

Whether prefab or site-built, every ADU project pays for site work — and site work is where identical units land wildly different budgets:

• Foundation and grading (slopes cost real money)
• Utility trenching and connections — sewer, water, electrical, and a possible main-panel upgrade
• Crane access for modular units (tight lots can kill this option)
• Driveways, walkways, drainage, landscaping repair
• Permits, plan check, and fees — same process for both paths

On easy, flat lots with close utilities, site work is a modest share of the budget and prefab's fixed unit price shines. On sloped lots, tight access, or long utility runs, site costs can approach the unit cost itself — and prefab's price certainty evaporates precisely where you needed it.

Where each path wins

• Prefab wins on: schedule certainty for the shell (factory weather doesn't exist), price certainty for the unit itself, flat accessible lots, and owners who like the model-home shopping experience.
• Site-built wins on: difficult lots, custom layouts that fit your yard instead of a truck bed, matching the main house, maximizing every square foot toward the 750-or-less fee waiver, and — counterintuitively — sometimes total cost, because local crews compete on bids while factories publish list prices.
• Garage conversion beats both when the goal is minimum cost per rentable square foot — the shell already exists. See what a garage conversion really costs.

The questions that decide it

• Can a crane physically reach your backyard? (Drive by and look.)
• How far are your sewer and panel from the ADU site?
• Is your lot flat, or does "some grading" appear in every bid?
• Are you optimizing for speed, total cost, or rental income? Pick one — the floor plan decision follows from it.
• Have you leveled a prefab all-in quote (unit + site work + fees) against two site-built bids for the same program? This is the only comparison that means anything, and almost nobody does it.

My honest take

Prefab is a legitimate path, not a gimmick — and site-built is not automatically better craftsmanship. The mistake I see is choosing the path from marketing instead of from the lot. Run the feasibility first, get real all-in numbers for both, and let your actual site decide. That leveling exercise is standard scope in the ADU projects I manage — because a fair comparison requires making vendors bid the same project, which is harder than it sounds. New to the whole process? Start with the complete San Diego ADU guide.