ADU Types Compared: Detached, Attached, Garage, JADU and Prefab
A junior ADU or garage conversion is the least expensive way in ($110,000–$250,000+ in Contra Costa); a detached ADU gives the most privacy at $250,000–$450,000+. Prefab is a way of building a detached unit, not a different set of rules.
Side by side
The five options
| Detached | Attached | Garage conversion | Junior ADU | Prefab | |
|---|---|---|---|---|---|
| What it is | New separate building | Addition joined to the home | Existing garage made livable | Space inside the home, ≤500 sq ft | Factory unit on a local foundation |
| State size limit | Up to 1,200 sq ft; 800 guaranteed | Up to 50% of the home | Size of the structure | 500 sq ft | As detached |
| Setbacks | 4 ft max side/rear | 4 ft max side/rear | None for existing structure | None (inside home) | 4 ft max side/rear |
| Cost (Contra Costa) | $250,000–$450,000+ | $220,000–$380,000+ | $150,000–$250,000+ | $110,000–$180,000+ | $300,000–$450,000+ (800 sq ft, Bay Area, all-in) |
| Typical timeline | Longest; often about a year | Long; structural tie-in | Shorter | Shortest | Shorter build on site; same permits |
| Sewer (Central San) | Ties into house lateral downstream of cleanout | No separate line | Depends on location | No separate line | As detached |
| New utility fee | Proportional if 750+ sq ft | Proportional if 750+ sq ft | None for existing space | None | As detached |
| Owner occupancy | Not required | Not required | Not required | Only if sanitation is shared | Not required |
| Best for | Rental, family, privacy | Aging parents close by | Budget, speed | Adult child, caregiver | Standard layouts, easy access |
Sizes and rules: Gov. Code §66314, §66321, §66324, §66333. Costs: Mazzamuto Construction (Contra Costa) and IMKAT (Bay Area prefab), 2026. Sewer: Central San.
Not sure which type fits your lot?Get a Quoteor call (925) 709-5438
Plans
What each type looks like on paper
Detached ADUs → · Garage conversions → · Prefab ADUs → · Cost by type →
FAQ
Choosing a type
What are the drawbacks of an ADU?
High cost (roughly $110,000–$450,000+ depending on type), a long timeline, lost yard or garage space, construction disruption, possible panel or sewer upgrades, and higher property tax on the new construction. You also can't sell the ADU separately from the house.
What is the difference between an ADU and a JADU?
A junior ADU is up to 500 sq ft carved out of the existing home (including an attached garage), with at least an efficiency kitchen; it may share a bathroom with the house. An ADU can be detached, attached or converted and has its own full kitchen and bath. Owner occupancy can apply to a JADU only when it shares sanitation with the main home.
Which ADU type adds the most value?
It depends on the lot and the market; this site doesn't give investment advice. Detached units offer the most privacy, conversions the lowest cost per finished unit. Ask a local appraiser or agent for comparable sales.
Next step
Ready to price your ADU?
Share your lot and goals. A local ADU builder can review feasibility and give you a quote.