The Four ADU Types — What California Law Recognizes
Before you talk to a contractor or call your city, understand which type of ADU your lot and budget actually support. The choice affects cost by $100,000+, timeline by 6–12 months, and rental income by $800–$1,500/month.
Every ADU project starts with the same question: which type should I build? The answer depends on four things — your lot configuration, your existing structure, your budget, and your goal (rental income vs. family housing vs. property value). This guide breaks down all four types with complete, honest detail so you can make that decision with real information.
Service area note: The ADU Pro® builds exclusively in Orange County, Los Angeles County, and the western cities of Riverside County — including Corona, Norco, Jurupa Valley, and Eastvale. All costs, timelines, and permit information in this guide reflect those specific markets as of 2025.
| ADU Type | Max Size | Typical Cost (All-In) | Timeline | Best For |
|---|---|---|---|---|
| Detached ADU | 1,200 sq ft | $240K–$490K | 12–18 months | Maximum value, privacy & rental income |
| Attached ADU | 1,200 sq ft | $185K–$380K | 10–16 months | Larger homes, no rear yard space |
| Junior ADU (JADU) | 500 sq ft | $80K–$165K | 6–9 months | Lowest cost, fastest path to rental income |
| Garage Conversion | Varies | $75K–$210K | 6–10 months | Existing structure, moderate budget |
The Detached ADU — A Freestanding Second Home on Your Lot
The most comprehensive, most expensive, and highest-value ADU type. A completely independent structure with its own foundation, walls, roof, kitchen, bathroom, and entrance.
A detached ADU is a freestanding structure — its own building, fully separate from the primary residence. In any neighborhood it looks and functions like a small independent house — with its own foundation, walls, roof, entrance, kitchen, and bathroom — that simply happens to share a lot with the main home.
This independence is what makes detached ADUs the highest-performing ADU type in every measurable way: rental income, property value contribution, tenant satisfaction, and privacy. It is also the most complex and expensive type to build, and the one that takes the longest from initial planning to certificate of occupancy.
What's Required for a Detached ADU
California state law requires detached ADUs to meet two core criteria: they must be on the same lot as a single-family residence (or in some cases a multifamily property), and the lot must be able to accommodate the setback requirements — 4 feet from the rear and both side property lines at minimum. Cities cannot require larger setbacks than this under Government Code §65852.2.
The detached ADU must have fully independent living facilities: a complete kitchen (full-sized or efficiency), a private bathroom, and a separate entrance that does not require walking through the primary home. It must comply with all applicable building codes including Title 24 energy efficiency, CalGreen sustainability requirements, and local building standards.
Detached ADU Costs — Complete Breakdown
The Attached ADU — A Fully Independent Addition to Your Home
An attached ADU shares at least one wall with the primary home but functions as a completely independent living unit with its own entrance, kitchen, and bathroom.
An attached ADU is built as an addition to the primary residence — it shares at least one wall with the main home, but is legally classified as a separate dwelling unit. The unit has its own independent entrance — you cannot pass through the main home to reach it — along with a private kitchen and bathroom. From the street it may resemble a room addition, but legally and functionally it is a completely separate home.
California limits attached ADUs to 50% of the primary home's living area, up to a maximum of 1,200 sq ft. So if your primary home is 1,800 sq ft, the maximum attached ADU is 900 sq ft. If your primary home is 2,800 sq ft or larger, you can build the full 1,200 sq ft maximum.
When an Attached ADU Makes Sense
Attached ADUs are a strong option when rear yard space is limited — when the lot doesn't have enough room for a detached ADU at 4-foot setbacks — or when the primary home has a wing, side yard, or ground floor space that can be converted or extended into an attached unit with minimal disruption to the main home's function and layout. They are also sometimes less expensive than detached ADUs because they can share some structural elements (a common wall, roofline connection, or foundation tie-in).
Attached ADUs share a wall with the primary home, which triggers California fire separation requirements. The shared wall must be constructed to a 1-hour fire-resistance rating — double drywall, proper framing, and blocking. This is not optional and is verified at both framing inspection and final inspection. The ADU Pro® designs all attached ADUs with proper fire separation from day one, which avoids correction notices and inspection failures.
Attached ADU Costs vs. Detached
Attached ADUs typically cost 10–25% less than a detached ADU of similar square footage because they eliminate some site work costs (no separate foundation pour as large, shared wall reduces framing cost, potential to tap existing utilities without full new service runs). However, the savings can be offset by the need for interior remodeling to the primary home's affected areas, and by fire separation requirements that add framing and drywall cost.
The Junior ADU (JADU) — The Fastest, Most Affordable Path to an ADU
Created entirely within the existing footprint of the primary home. Limited to 500 sq ft. The lowest cost, fastest permit, and most accessible ADU type available under California law.
A Junior ADU is unique among ADU types in one critical way: it must be created entirely within the existing footprint of the primary residence. No new exterior walls, no new foundation poured, no addition built. You are carving a self-contained unit out of space that already exists within your home's four walls — typically a large master suite with exterior access, an interior attached garage, an underused bedroom wing, or a bonus room.
This constraint is also what makes JADUs so much faster and cheaper than any other ADU type. The structural shell already exists. You're not paying for foundation, framing, roofing, or exterior finish on a new structure. Working with what's already there, you are adding or modifying an entrance, installing a kitchen, upgrading electrical, reconfiguring plumbing for the bathroom, and finishing the interior to habitable residential standards.
JADU Legal Requirements Under California Law
California Government Code §65852.22 governs JADUs specifically and sets out clear requirements. The JADU must: be contained within the existing space of the primary dwelling or an attached garage; not exceed 500 square feet in size; have a separate exterior entry door; include an efficiency kitchen with a sink, cooking appliance, food prep area, and refrigerator space; and be permitted ministerially (no discretionary review). JADUs may share bathroom facilities with the primary home — this is explicitly permitted by state law — though many JADUs include their own bathroom for maximum functionality and rental appeal.
California's moratorium on owner-occupancy requirements expired January 1, 2025. Local agencies may now require owner-occupancy as a condition of JADU approval — meaning the homeowner must live on the property. This does not apply to standard ADUs. Before planning a JADU, verify whether your specific city currently enforces an owner-occupancy requirement. The ADU Pro® checks current local ordinances for every project in our service area before the design process begins.
What Can and Cannot Be a JADU
Understanding what space qualifies as JADU territory is important because it determines your options before you design anything. The most common JADU conversions we handle in Orange County and LA County:
- Interior attached garage: A two-car garage attached to the home can typically yield 400–500 sq ft of JADU space. This is the most common JADU conversion we build. The garage door opening is framed in, a new entrance door is added, and the space is fully conditioned and finished.
- Ground-floor master suite: Some homes have a ground-floor master bedroom with sliding glass door access to the rear yard. With the right layout, this can be converted to a JADU with minimal structural work — a kitchenette is added, the sliding door serves as the separate entrance, and the existing bathroom serves both the JADU and main home.
- Accessory room or bonus space: Casitas, bonus rooms, or sunrooms that are part of the existing footprint can often be converted to JADUs if they can be provided with a separate entrance and a kitchenette.
- What doesn't qualify: A standalone detached garage is not eligible for JADU designation — that would be a garage conversion ADU (Type 04). JADUs must be within the attached structure of the primary home.
We regularly see homeowners come to us after being quoted $45,000–$55,000 for a JADU garage conversion. These bids almost always omit one or more of: plumbing for bathroom and kitchen, electrical panel upgrade, egress window (required for sleeping area), fire separation improvements between JADU and main home, permit fees, utility connection fees, or engineering. A complete, properly permitted JADU in Orange County or LA County rarely comes in below $75,000 when all costs are included. Any bid significantly below this warrants a very detailed line-by-line review before you sign.
The Garage Conversion ADU — Turning Dead Space Into Rental Income
Converting a detached or attached garage into a permitted, habitable ADU. Lower cost than new construction because the structural shell exists — but the full permit and construction process still applies.
A garage conversion ADU takes an existing attached or detached garage and transforms it into a fully habitable, permitted ADU. The structural shell — concrete slab floor, framed walls, roof — already exists, which is why garage conversions cost significantly less per square foot than new construction. But everything inside the shell must be brought up to residential habitability standards, which means substantially more work than most homeowners expect.
The most important legal protection for garage conversions in California: under SB 13, cities cannot require replacement parking when an existing garage is converted to an ADU. This was one of the most commonly used barriers to garage conversions before 2020. It no longer applies — your city cannot tell you that you must rebuild parking elsewhere on the lot as a condition of converting your garage.
Detached vs. Attached Garage Conversions
The ADU classification of a garage conversion depends on whether the garage is attached to or detached from the primary home. An attached garage converted to a dwelling unit is typically classified as a JADU (if under 500 sq ft and accessible from within the home) or an attached ADU (if it constitutes a separate dwelling unit). A detached garage converted to a dwelling unit is classified as a detached ADU — the most favorable classification, as it allows up to 1,200 sq ft and carries no owner-occupancy risk.
A detached two-car garage in Orange County or LA County typically measures 20 ft × 20 ft = 400 sq ft, or 20 ft × 24 ft = 480 sq ft. Converting this to a permitted ADU at $150–$200/sq ft yields a cost of $60,000–$96,000 in hard construction costs — significantly below what any other path to a legal ADU delivers. Add $20,000–$35,000 for design, permits, and utility connections, and you're looking at $80,000–$130,000 for a legal, permitted, rentable ADU. In the right Orange County city, that ADU rents for $1,600–$2,100/month. The ROI math is very strong.
What Garage Conversion Actually Involves
The most common misconception about garage conversions is that they're simple because "the structure already exists." The structure does exist — but the structure is a garage, and a garage is not a home. Converting it into a habitable living space requires significant work in almost every category.
All Four ADU Types — Complete Side-by-Side Comparison
Every decision factor across all four types, in one place. Use this to narrow your choice before the site assessment.
Which ADU Type Is Right for Your Situation?
The right type depends on your lot, your budget, and your goal. These scenario cards cover the most common situations we see across Orange County, LA County, and western Riverside County.
Yes — California state law permits one standard ADU and one JADU on the same single-family lot simultaneously. The right property configuration can yield two additional rental units from a single parcel. See FAQ #11 below for full details, or call (877) 398-8002 to find out if your lot qualifies.
How California Law Treats Each ADU Type
State law creates a framework that applies to all four types, with specific rules for each. Here's what every homeowner in our service area needs to understand.
California's ADU legislation — primarily Government Code §65852.2 for standard ADUs and Government Code §65852.22 for JADUs — applies uniformly across Orange County, LA County, and Riverside County. Local cities can adopt ordinances that are more permissive than state law but cannot be more restrictive than the state floor.
Ministerial Approval — All Four Types
All four ADU types receive ministerial approval under California law, meaning they must be approved if they comply with applicable objective standards — without a public hearing, design review committee, or discretionary decision by a planning commission. Your neighbors cannot formally object. Your HOA's architectural preferences don't override the permit. The building department must act on a complete application within 60 days.
Setbacks
The 4-foot minimum setback from rear and side property lines applies to detached ADUs and detached garage conversions. Attached ADUs follow the primary home's existing setbacks (since they share a wall with the main structure). JADUs, by definition, are within the existing home footprint and therefore follow the primary home's existing setbacks — which are already established and legal.
Parking
California law prohibits cities from requiring ADU parking for units: within a half-mile of public transit, within an architecturally or historically significant historic district, or in any case where the ADU is part of the primary residence or accessory structure. Most critically, no replacement parking can be required when an existing garage is converted to an ADU. This is one of the most powerful protections in California ADU law — and one of the most commonly violated by cities that haven't updated their practices.
Fees
ADUs under 750 sq ft are exempt from impact fees under SB 13. For ADUs 750 sq ft and larger, impact fees must be proportional to the ADU's square footage relative to the primary home — they cannot be charged at the full single-family rate. School impact fees in the LAUSD area run approximately $4.08 per sq ft for ADUs over 500 sq ft; other districts have varying rates. Utility connection and capacity fees are set by each utility district and vary significantly — budget $8,000–$28,000 for these in Orange County and LA County for a fully metered detached ADU.
