ADU GUIDES · ARCADIA · SAN GABRIEL VALLEY
JADU vs ADU: What Arcadia Homeowners Should Know
California allows most single-family lots to carry both a junior ADU and a full ADU. They are not the same thing, they cost very different amounts, and the one that suits you depends on where the space already is.
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THE SHORT ANSWER
A JADU uses a smaller space within the home.
A junior ADU is capped at 500 square feet and is created out of space that already exists inside your home — a bedroom, a converted den, an attached garage bay. A full ADU can be a conversion or a new structure, has its own kitchen and bathroom, and is what most people mean when they say ADU. On a single-family lot in California, you are generally allowed one of each, so it is not always an either-or decision. Everything below covers the differences that actually change the outcome: size, cost, plumbing, and who can live where.
Where this applies
Arcadia and neighbouring San Gabriel Valley cities, single-family residential lots. Two-unit and multi-family properties run under different rules.
SIDE BY SIDE
The differences that matter.
On smaller screens, swipe across the table to compare all columns.
| Junior ADU (JADU) | ADU | |
|---|---|---|
| Maximum size | 500 sq ft | At least 850 sq ft for one bedroom, 1,000 sq ft for two or more, must be permitted by a city under state rules |
| Where it goes | Inside the existing home — converted bedroom, den or attached garage | Attached, detached, or a conversion of an existing structure |
| Kitchen | An efficiency kitchen is sufficient | A full kitchen is required |
| Bathroom | May share the bathroom of the main house | Must have its own bathroom |
| Utilities | Usually shares existing services | May need new runs, panel capacity and sewer connections |
| Project budget | Measured quantities, specifications and installation scope | Compare only after scope and floor area are fixed |
The size figures above are the state minimums a city must allow, not the maximums it may permit — some allow larger units. See the full ADU budget guide for Arcadia for the money side.
SPACE
Where the square footage comes from.
This is the practical dividing line. A JADU takes space you already have — which means you give up a room inside your own house. An ADU adds space that did not exist, either by converting a garage or building new. If you have a bedroom nobody uses and you do not want to give up yard, a JADU is often the least disruptive option available. If you do not have spare interior space, a JADU is not available to you at all and the conversation moves straight to conversions and new builds.
What a JADU conversion usually involves
Adding an efficiency kitchen, separating the unit with a door, providing an exterior entrance, and tidying the shared bathroom arrangement — all within existing walls.
The separation detail
A JADU needs to function as its own unit: its own entrance and a door between it and the rest of the house. That separation is the detail that most often gets drawn wrong on first pass.
The efficiency kitchen
Smaller than a full kitchen but still a real one. Compact cabinetry built to the actual available width — which is exactly the case where factory-made sizes earn their keep.
What nobody budgets for
Storage. Giving up a bedroom means finding a new home for what was in it, and a wardrobe built into the remaining rooms is usually part of the real scope.
COST
How existing space can reduce construction scope.
A reliable budget starts with the property and a defined scope. Existing structure, site access, utility capacity, floor area and premium finishes all affect the total. Compare itemized estimates for the same scope rather than a single cost-per-square-foot figure.
The caveat is that “cheapest” only holds while the work stays inside existing walls. The moment a JADU needs new plumbing runs or structural changes, it starts behaving like a small ADU and stops being the cheap option.
PLUMBING
Kitchen, bathroom, and the permitting difference.
A full ADU must have its own kitchen and bathroom. A JADU may have an efficiency kitchen and may share the main house bathroom. That single difference drives most of the cost gap, because a new bathroom means new supply, new waste, new venting and often a framing change — the most expensive part of any dwelling per square foot.
It also drives how the unit feels. A unit sharing a bathroom with the main house suits a family member or a live-in arrangement far better than a tenant, and that is worth being honest about before you choose.
OCCUPANCY
Verify the current owner-occupancy rule.
California has revised owner-occupancy requirements for ADUs and JADUs more than once, and the rule has differed between the two unit types at different points. Rather than repeat a figure that may be out of date, the honest advice is this: ask your city directly, or check with the Contractors State License Board and the city’s planning counter, before assuming you can or cannot rent one, the other, or both.
We are a contractor, not a legal adviser. Nothing here is a legal interpretation; permit and occupancy questions belong to the City of Arcadia Building and Planning Departments (240 W Huntington Dr, Arcadia, CA 91007; Planning Division (626) 574-5423).
BOTH?
Can you have a JADU and an ADU?
On a typical single-family lot, California generally allows one ADU plus one JADU — they are counted separately rather than as two of the same thing. That combination is worth thinking about seriously: a JADU for family and a detached unit for rental income, or a JADU now and an ADU later.
Whether your specific lot qualifies depends on setbacks, coverage, utilities and the city’s own overlay rules. The reason to check it early is that building both later is more expensive than planning for both now, even if you only build one.
WHICH TO PICK
A short decision table.
| If this is your situation | Lean toward |
|---|---|
| You have a spare interior room and want the lowest entry cost | JADU |
| You need a unit with its own bathroom | ADU |
| You do not want to give up space inside your own house | ADU — conversion or detached |
| You want rental income from a completely separate dwelling | ADU |
| You may eventually want two units | Plan both, build the one you need first |
We measure what is possible on your lot before quoting design. Email 2403winrun@gmail.com, call +86 175 3881 2577, or send a project inquiry. Related: garage conversion vs detached ADU · how long the permit takes. · multigenerational ADUs
FAQ
The JADU questions that follow on from this comparison.
Can an attached garage become a JADU?
Yes — an attached garage bay is one of the most common JADU conversions, because it already sits inside the building envelope and usually has services close by. It reads as a JADU while it stays within the existing footprint and shares the main house bathroom; add its own bathroom or detach it and you are into full ADU territory. Where you want storage kept at the front, a partial conversion is sometimes possible if the depth allows it.
Does the 500 square foot cap include the bathroom and storage?
The cap applies to the unit as a whole, not to whatever rooms you decide to put in it — an efficiency kitchen, a bathroom and somewhere to put things all have to fit inside 500 square feet. That is why a JADU is laid out rather than decorated: a few inches of cabinetry depth decides whether the circulation works at all. Confirm how your city measures floor area before drawing to the limit.
Can I add a bathroom to the JADU later?
Often technically yes, but it changes what the unit is. New supply, waste and venting inside a finished house means opening floors and walls, and in older Arcadia houses the sewer lateral and panel capacity were rarely sized for a second set of fixtures. It also changes which occupancy rules apply, because whether the unit shares sanitation with the main house is part of how those rules are written. Decide the bathroom question at the start rather than planning to revisit it.
Can I rent a JADU out?
Renting is generally permitted; short-term letting is not. State law restricts ADUs and JADUs from being let as short-term rentals, and a city may require minimum lease terms longer than 30 days. If rental income is the plan, size the unit and its entrance for a tenant rather than for a family member — those are not the same drawing.
Does the owner-occupancy rule treat a JADU the same way as an ADU?
Not always, and it has changed more than once. Recent state legislation ties what a city may require to whether the unit shares sanitation facilities with the main house, so the answer depends on how your particular unit is configured. That is why we send people to the city rather than give them a number: ask with your actual layout in hand, and get the answer in writing. We are a contractor, not a legal adviser.
Clear scope. Clear accountability.
Service scope
KWA Home is not an architecture or design firm.
Licensed professionals
We do not offer architectural design to the public, we do not stamp drawings, and we do not perform structural engineering — plans and any structural calculations are prepared and stamped by California-licensed design professionals.
Project responsibility
What we do is coordinate that work, submit the permit and build it, under CSLB #1145730. The responsible contracting entity and scope are named in your agreement rather than left implied.