"ADU" is a broad term that covers several very different types of construction. Before you can plan a budget or timeline, you need to know which type of ADU your property can support — and which one best fits your goals. Here's a plain-English breakdown of all five.

1. Detached ADU

A detached ADU is a fully separate structure built on your property, typically in the backyard. It has its own foundation, walls, roof, and utilities. It shares no walls with the main house.

Best for:

• Homeowners who want maximum privacy and separation
• Rental income — detached units command the highest rents
• Properties with enough backyard space after setbacks

Typical size: 400–1,200 sq ft (state law caps ADUs at 1,200 sq ft)

Typical cost in SoCal: $150,000–$280,000

Timeline: 10–14 months from design to certificate of occupancy

Key consideration: You need enough usable lot area after required setbacks (typically 4 feet from side and rear property lines). On lots under 5,000 sq ft, this can be tight.

2. Attached ADU

An attached ADU is an addition to the main house — it shares at least one wall with the primary structure but has its own entrance, kitchen, and bathroom. Think of it as a self-contained apartment added onto the side or rear of your home.

Best for:

• Properties where backyard space is limited
• Homeowners who want the ADU connected to the main house (easier for aging parents)
• Situations where a shared wall reduces construction cost

Typical size: 400–1,000 sq ft

Typical cost in SoCal: $120,000–$220,000

Timeline: 8–12 months

Key consideration: The addition must meet current building codes, which may require upgrading parts of the existing structure (electrical panel, foundation, etc.) to current standards.

3. Garage Conversion ADU

A garage conversion transforms an existing attached or detached garage into a livable unit. The shell already exists — you're adding insulation, drywall, plumbing, electrical, HVAC, and finishes.

Best for:

• Budget-conscious homeowners
• Properties with limited backyard space
• Fastest path to a permitted ADU
• Homeowners who don't need the garage

Typical size: 240–600 sq ft (limited by existing garage size)

Typical cost in SoCal: $80,000–$140,000

Timeline: 4–8 months

Key consideration: You lose your garage. A single-car garage (240–300 sq ft) is tight for a full ADU — a two-car garage (400–500 sq ft) is much more workable. Ceiling height can also be a constraint in older garages.

4. Junior ADU (JADU)

A Junior ADU is a smaller unit created entirely within the existing footprint of the main house — typically by converting a bedroom, den, or part of the living space. JADUs are capped at 500 sq ft by state law.

A JADU must have:

• Its own separate entrance (can be shared with the main house, but must have a separate exterior door)
• An efficiency kitchen (doesn't need a full range — a cooktop and sink are sufficient)
• A bathroom (can be shared with the main house in some cases)

Best for:

• Homeowners with a large home and underused space
• The lowest-cost ADU option
• Properties where lot constraints prevent other ADU types
• Multigenerational living where some sharing is acceptable

Typical size: Up to 500 sq ft

Typical cost in SoCal: $40,000–$90,000

Timeline: 3–6 months

Key consideration: The owner must live on the property (in either the main house or the JADU) — JADUs cannot be rented if the owner doesn't occupy the property. This is a state law requirement.

5. Above-Garage ADU

An above-garage ADU adds a new living unit on top of an existing detached garage. This is a less common option but works well on specific properties.

Best for:

• Properties where the backyard is fully used or has access issues
• Homeowners who want to keep the garage functional
• Lots where a detached ADU in the yard isn't feasible

Typical size: 400–700 sq ft

Typical cost in SoCal: $160,000–$260,000

Timeline: 10–14 months

Key consideration: The existing garage structure must be evaluated by a structural engineer to determine if it can support the added load. Many older garages need significant reinforcement, which adds cost.

Which ADU Type Is Right for You?

The honest answer is: it depends on your property. The best ADU type for a 6,000 sq ft lot in Pasadena is different from the best option for a 4,500 sq ft lot in Torrance or a large lot in Temecula.

The factors that matter most:

• Lot size and usable backyard space (after setbacks)
• Existing structures (garage size, condition, location)
• Your goals (rental income, family housing, resale value)
• Budget
• Timeline

Our free ADU Site Analysis looks at all of these factors for your specific property and recommends the ADU type — and size — that makes the most sense. It's the fastest way to go from "I'm thinking about an ADU" to "here's exactly what I should build and what it will cost."