Prefab and traditional ADU builds both have real advantages. Here’s an honest comparison of cost, timeline, quality, and the scenarios where each approach makes more sense.

Cost — Advantage: Prefab

Prefab total project cost: $28,000–$65,000. Traditional site-built in California: $120,000–$250,000+. Prefab cost estimates are also more predictable — factory production removes most labor-driven variables that cause traditional builds to run over budget.

Timeline — Advantage: Prefab (significantly)

Prefab: 8–14 weeks from signed contract to move-in. Traditional site-built: 9–18 months from design to certificate of occupancy. A 6–9 month difference represents real money — specifically the rental income you’re not collecting while the project drags on. Faster installation means you begin generating income, housing family, or using the space months sooner.

Quality — Roughly Equal, Prefab Has Lower Variance

Factory production means tighter construction tolerances, consistent materials, and independent HCD inspection. Traditional site-built can achieve exceptional quality but requires active oversight — quality varies significantly by contractor.

When Each Makes Sense

Prefab fits best when your budget is under $60K, you want to rent within 6 months, your lot is a standard configuration, and you want minimal disruption — a faster, more predictable path to additional living space or rental income.