Overview of ADU types
California law recognizes several ADU paths. Each path has different advantages, constraints, and cost profiles. Understanding which types are available helps you plan strategically for your property.
The right choice depends on your property layout, available space, budget, timeline, goals (family housing vs rental income), and local rules. No single type is best for everyone—each serves different situations well.
Detached ADU
A detached ADU is a separate structure on the same lot as the primary home. It stands completely apart from the main residence.
Size
Up to 1,200 square feet for a multi-bedroom unit, or smaller for single-bedroom designs.
Design flexibility
Maximum design freedom. You design the unit from scratch within zoning constraints. Layout, size, and aesthetics are entirely customizable.
Cost
Highest cost among ADU types. New construction with utility connections from scratch is expensive.
Timeline
6–12+ months depending on permitting cycles and construction complexity. Full new construction timeline.
Best for
Homeowners who want maximum flexibility, full independence from the primary home, and are willing to invest more time and money. Ideal for rental income scenarios.
Attached ADU
An attached ADU is connected to the primary home. It shares a wall, foundation, or both with the main residence. It could be an addition, new wing, or upstairs unit.
Size
Limited by the primary home footprint and available walls. Often smaller than detached options but can still be 800–1,000+ sq ft.
Design flexibility
Moderate flexibility. You're adding to existing structure, so layout is constrained by the main home's footprint and existing walls.
Cost
Mid-range. More expensive than conversion but less than detached. Shared utilities may reduce some connection costs.
Timeline
6–10 months typically. Faster than detached new construction because utility connections are shorter.
Best for
Homeowners who want to expand their home while creating a separate unit. Works well for family housing scenarios or modest rental income.
Conversion ADU
A conversion ADU is created by converting existing space into a dwelling unit. This might be a basement, attic, bonus room, or any other underused structure on your property.
Size
Varies widely depending on what space you're converting. Typically 400–900 sq ft.
Design flexibility
Limited by existing space constraints. You work within the walls and structure you already have.
Cost
Highly variable. Can range from affordable to expensive depending on existing conditions and what upgrades are needed to meet code.
Timeline
3–8 months. Often faster than new construction because structural work is minimized. Surprises can extend the timeline.
Best for
Homeowners who have underused existing space and want to make use of it. Good for those with budget constraints who need a quicker option.
Garage conversion ADU
A garage conversion is a specific type of conversion ADU. Garages are common on many properties and often represent an affordable starting point for ADU development.
Why garage conversions are common
Garages are familiar structures many homeowners already have. Utilities may be nearby. Many homeowners explore this path first.
Parking requirements eased
California law has significantly reduced parking requirements for ADUs. In many areas near transit, no replacement parking is required. This removed the biggest historical barrier to garage conversions.
Typical size
400–600 sq ft. Limited by garage size.
Common challenges
Ceiling height, foundation adequacy, utility connections, and code surprises are frequent issues.
For detailed information about garage conversions, including costs, code requirements, and challenges, see the Garage Conversion Guide.
JADU (Junior ADU)
A JADU (junior accessory dwelling unit) is created within the existing walls of the primary home. It is the simplest and often lowest-cost ADU option.
Size limit
Maximum 500 square feet of interior livable space as of January 1, 2026.
Kitchen requirements
An efficiency kitchen or kitchenette is allowed. Full kitchen not required. This reduces complexity and cost.
Location
Must be created within existing walls of primary home. You cannot expand the primary home footprint to create a JADU.
Owner occupancy requirement
At least one unit (primary home or JADU) must be owner-occupied. This restriction does not apply to other ADU types.
Cost and timeline
Lowest cost option in many cases. Fast timeline, often 2–4 months. No new construction or major structural work needed.
Best for
Homeowners who want the simplest and lowest-cost path. Good for family housing or where you plan to owner-occupy one of the units.
For detailed information about JADUs, see the JADU Guide.
Side-by-side comparison
Here's how the main ADU types compare across key dimensions:
| Type | Typical Size | Typical Cost | Timeline | Design Flex |
|---|---|---|---|---|
| Detached ADU | 600–1,200 sq ft | $$$ | 6–12+ months | Full |
| Attached ADU | 600–1,000 sq ft | $$ | 6–10 months | Moderate |
| Conversion ADU | 400–900 sq ft | $ to $$ | 3–8 months | Limited |
| Garage Conversion | 400–600 sq ft | $ to $$ | 3–8 months | Limited |
| JADU | Up to 500 sq ft | $ | 2–4 months | Minimal |
Cost scale: $ = lowest, $$$ = highest (relative cost ranges vary by location and site conditions)
How to choose
Selecting the right ADU type for your situation requires considering several factors:
Budget
Limited budget? Consider JADU or conversion first. More resources? Detached offers maximum flexibility.
Property layout
Small lot with a garage? Garage conversion may be ideal. Larger lot with backyard space? Detached offers freedom. Existing underused space? Conversion works.
Timeline
Need it fast? JADU is quickest. Can wait? Detached new construction may offer better long-term flexibility.
Goals
Family housing for a relative? Attached or conversion may work. Rental income focus? Detached offers independence and market appeal.
Local rules
Check city guidelines. Some cities restrict certain types. Parking requirements vary. Some favor conversions; others prefer detached.
Existing conditions
Do you have an underused garage or basement? Is the foundation solid? Are utilities nearby? These practical realities narrow your options.
Recommended next steps
Once you understand the ADU types available to you, dive deeper into the options that make sense for your situation:
This page is for general educational purposes only. California ADU law, city implementation, code requirements, and cost estimates can vary widely by location. Always verify project-specific requirements with your planning department and qualified professionals.