Cost and planning guide

Prefab vs Custom ADU in the Bay Area

Prefab works when the lot, access, model, and city path fit. Custom works when the site or homeowner goals need flexibility. The best answer is boringly practical: choose the path that survives your property.

Reviewed 2026-05-15

Guide

What homeowners need to know.

Prefab strengths

Factory production, repeatable models, and clearer product specs can help if the site is compatible.

Custom strengths

Custom plans handle awkward yards, garage conversions, slopes, design constraints, and higher-control finish goals.

The site decides more than marketing does

Crane access, utilities, setbacks, trees, grade, foundation, and city process can make or break either route.

Cost table

Use ranges until scope is real.

Item Planning range Why it moves
Prefab path $250k-$500k+ Total project cost depends on site work and permitting.
Custom path $300k-$600k+ More flexibility, more design/project management variables.
Garage conversion $120k-$300k+ Different category; compare separately.

Mistakes

Avoid these expensive shortcuts.

  • Comparing prefab base unit price to custom all-in project price.
  • Choosing prefab before checking site access.
  • Choosing custom without budget guardrails.

FAQ

Fast answers.

Is prefab or custom better?

Prefab is better when the lot and model fit cleanly. Custom is better when constraints or design goals need flexibility.

Sources

Official and provider sources checked.

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