Whether the goal is housing a parent, bringing in rental income, or giving an adult kid their own front door without actually moving them out, the ADU layout you pick shapes everything: cost, timeline, how the space lives, and how much of your yard disappears in the process.
In this article, we’ll share six ADU floor plan examples to give you a clearer picture of what your new space can be, depending on your needs.
| Floor Plan | Typical Sq Ft | Budget Range | Best For |
| Garage Conversion Studio | 300-576 | $100K-$180K | Rental income, lower cost |
| Junior ADU (JADU) | Up to 500 | $50K-$100K | Family members, no new foundation |
| Attached 1-Bed/1-Bath Addition | 400-600 | $130K-$175K | In-law suite, adult children |
| Detached Backyard Cottage | 500-700 | $150K-$220K | Full independence, long-term rental |
| Detached 2-Story 2-Bed | 700-800 | $185K-$250K | Multi-generational living, max sq ft |
| Open-Plan Studio + Outdoor Living | 450-600 | $150K-$200K | Southern California lifestyle |
1. Garage Conversion Studio
Budget: $100,000 to $180,000
Best for: Generating passive income
If you have a detached garage that has gradually turned into a storage unit with a roof, there is a reasonable chance the bones of your next rental are already sitting there. A
standard two-car garage typically runs between 400 and 576 sq ft, which is enough for a full bathroom, an efficiency kitchen, an open sleeping and living area, and a washer and dryer hookup.
What the budget actually covers is everything a garage lacks as a home: insulation, HVAC, plumbing, egress windows, and finished flooring.
2. Junior ADU (JADU)
Budget: $50,000 to $100,000
Best for: Aging parents or family members who want their own space
A JADU is carved out of square footage you already own: a large bedroom, an attached garage, or a section of the ground floor.
California caps them at 500 sq ft. They require a separate exterior entrance and an efficiency kitchen with cooking facilities, a food prep counter, and a sink.
One thing worth understanding before you commit to this layout is the bathroom. A JADU can have its own private bathroom or share one with the main home.
Under AB 1154, which took effect January 1, 2026, those two options carry different obligations:
- If the JADU shares a bathroom with the primary residence, the property owner must live on-site in either the JADU or the main home
- If the JADU has its own private bathroom, that owner-occupancy requirement no longer applies, meaning the property can be fully rented out
For families housing a parent who wants their own space but doesn’t mind sharing a bathroom with nearby relatives, the shared configuration works fine. For homeowners planning to rent the JADU while living elsewhere, a private bathroom is not optional.
3. Attached 1-Bed/1-Bath Addition
Budget: $130,000 to $175,000
Best for: Adult children returning home, in-law suites, any arrangement that needs a separate entrance
An attached ADU shares one wall with the main house and runs entirely independently from it. Private entrance, full kitchen, dedicated bedroom, separate bathroom. It functions like an apartment that is connected to yours.
The shared wall is the financial lever here. You avoid the full foundation and structural cost of a detached build while still getting a properly functional separate living space. Utility connections are simpler too, since they run off systems already in place at the primary residence.
For adult children moving back after college or aging parents who still want to handle their own groceries, this layout threads a needle that neither a JADU nor a fully detached cottage quite does.
4. Detached Backyard Cottage
Budget: $150,000 to $220,000
Best for: Long-term tenants or family who want their own home
This is the floor plan most homeowners picture when the topic of ADUs comes up: a standalone structure in the backyard with its own roofline, front door, kitchen, bathroom, bedroom, and laundry hookup. No shared anything.
What separates a good detached cottage layout from a forgettable one is natural light. A single-story unit with high ceilings, a skylight over the kitchen, and south-facing windows feels dramatically larger than its measured sq ft.
The exterior can either match the main house in materials and roofline for visual harmony, or take a completely different direction if the two structures are far enough apart that consistency does not matter.
5. Detached 2-Story 2-Bedroom ADU
Budget: $185,000 to $250,000
Best for: Multi-generational households, maximizing property value, long-term rental income
Two bedrooms without covering the entire backyard means building up rather than out. A two-story detached ADU keeps the ground-floor footprint compact, parks the main living area and kitchen on the first level, and places the bedrooms upstairs.
Cornejo’s Builders completed exactly this kind of project in Alhambra: a 735 sq ft two-story ADU with a full kitchen, bathroom, living room, central heating, and washer-and-dryer hookups, at a total project cost of $250,000. The architectural plans alone came to $15,000 of that total.
Two-story ADUs require more structural engineering, especially in earthquake-prone areas. Height limits and the placement of second-floor windows relative to neighboring properties also require careful attention during the design phase.
Our blog on bi-level home design in California explores how vertical layouts are handled on Southern California lots.
6. Open-Plan Studio with Outdoor Living
Budget: $150,000 to $200,000
Best for: Homeowners who want a space that takes advantage of the weather
Building an ADU in a region with an average of around 270 sunny days per year and keeping it fully enclosed is a design choice that leaves a lot on the table. This floor plan treats indoor and outdoor space as a single continuous area.
Sliding or folding glass doors connect the main living area to a private patio or courtyard, and the outdoor zone is built to actually function:
- A built-in outdoor kitchen with a grill, counter space, and covered storage
- A pergola or shade structure that makes the patio usable through midday heat
- String lighting and integrated seating so the space works for evenings too
- A planting buffer between the ADU and the main home for visual separation
The interior stays deliberately lean: kitchen, dining, and sleeping area in one connected open-plan layout, bathroom and closet tucked toward the back.
The outdoor area compensates for the compact sq. ft., and the whole setup works well for short-term rentals in high-demand neighborhoods, where local rules allow it.
Frequently Asked Questions
What is the maximum size for an ADU in California?
For detached ADUs, state law sets a maximum of 1,200 sq ft. For attached ADUs, the limit is either 1,200 sq ft or 50% of the main home’s square footage, whichever is smaller. Junior ADUs are capped at 500 sq ft. Some cities may allow larger detached ADUs beyond the state standard, while local rules can also reduce the maximum size to as little as 850 sq ft for a one-bedroom unit. Confirming limits with your city’s planning department before finalizing plans is the right first step.
Can I build both a JADU and a detached ADU on the same lot?
Yes. California state law allows a single-family property to have both a JADU and a detached ADU on the same lot, provided the property meets setback and zoning conditions. If the JADU shares a bathroom with the main home, the property owner must live on-site in either the JADU or the primary residence.
How long does it take to build an ADU in Southern California?
Most ADU projects in Southern California run between 6 and 9 months from initial consultation to final inspection. Simpler builds like a straightforward garage conversion can come in faster. Every project is different, and after an on-site visit, Cornejo’s Builders will give you a specific timeframe for your project.
Do all ADU types require permits?
Every ADU in California requires a permit, including garage conversions and JADUs. No exceptions. The permitting process has been streamlined considerably by recent state legislation, and many cities now offer pre-approved ADU plan libraries that significantly speed up the review timeline.
Will adding an ADU increase my property taxes?
A new ADU triggers a supplemental assessment specifically of the value of the new construction. Your existing home’s assessed value is not changed. California’s Proposition 13 protections remain in place for the original structure, and the ADU is assessed separately based on its construction value when it is completed.
What is the difference between a JADU and a standard ADU?
A JADU is created entirely within the existing walls of your primary residence, is capped at 500 sq ft, and requires only an efficiency kitchen rather than a full kitchen. It can share a bathroom with the main home, though doing so triggers an owner-occupancy requirement. A standard ADU is fully self-contained with its own full kitchen and private bathroom, and can be attached, detached, or converted from an existing structure like a garage.
Can a JADU be rented out on Airbnb or short-term rental platforms?
No. Under AB 1154, effective January 1, 2026, all JADUs in California are restricted to rental terms exceeding 30 days. Short-term rental of a JADU does not comply with state law regardless of local STR rules.
Let Someone Else Handle the Complicated Part
Picking a floor plan takes an afternoon. Getting it built, permitted, designed to code, connected to utilities, and finished on a real timeline is a different undertaking entirely.
Cornejo’s Builders manages the full process from design through construction, handling permits, architectural plans, plumbing, electrical, and everything in between, across Downey, Alhambra, Whittier, and the rest of Southern California.
If you want to figure out which ADU layout makes sense for your specific property and goals, take a look at our ADU contractor page for more, or call us at (562) 319-3178 or message us here.