The short version: a JADU is created inside your existing house, is capped at 500 square feet, may share a bathroom with the main home, and only needs a kitchenette rather than a full kitchen. A standard ADU can be detached or attached, can be up to 1,200 square feet, and must be fully self-contained with its own bathroom and complete kitchen.
The practical difference is cost and independence. A JADU is the least expensive path to a second unit because you are working within existing walls. A standard ADU costs considerably more but functions as a genuinely separate home. Here is how to tell which fits your situation.
What a JADU Is
A junior accessory dwelling unit is a small independent living space created within the walls of an existing single-family home — typically a converted bedroom, a portion of the ground floor, or an attached garage that is part of the main structure.
Defining characteristics:
- Maximum 500 square feet
- Must be created within the existing home footprint
- Requires its own exterior entrance
- Requires an efficiency kitchen or kitchenette
- May share a bathroom with the primary residence, or have its own
- May have an interior connecting door to the main house
The efficiency kitchen requirement is what keeps costs down. A JADU kitchenette generally needs a sink, cooking appliance, food preparation counter, and storage — not a full residential kitchen with a range hood and the associated ventilation and gas work.
What a Standard ADU Is
A standard ADU is a complete, fully independent dwelling. It can be detached in the yard, attached to the house, or converted from existing space including a detached garage.
Defining characteristics:
- Detached units generally allowed up to 1,200 square feet
- Attached units up to 50 percent of the primary home living area, with cities unable to restrict units of 800 square feet or smaller
- Must have a full kitchen
- Must have its own bathroom
- Must have independent access
- Must function completely independently of the main house
Cost and Complexity
JADUs are the least expensive second unit available, for straightforward reasons. There is no foundation, no new framing, no roof, and utilities are already present in the walls you are working within. The main scope is a new exterior door, the kitchenette, any bathroom work, fire separation, and finishes.
Standard ADUs cost substantially more. A detached unit is new construction in every respect. Even a garage conversion involves slab work, structural reinforcement, insulation to meet Title 24, and utility connections.
The tradeoff is what you get. A JADU is a small space inside your house. A detached ADU is a separate home.
Fees
Both benefit from state fee protections, and JADUs benefit most.
Impact fees cannot be charged on ADUs under 750 square feet, which covers every JADU by definition. Recent legislation additionally exempts units under 500 square feet from school impact fees — again covering all JADUs.
Standard permit fees still apply to both and vary by city.
Can You Build Both?
Yes. On most single-family lots, state law allows both a JADU and a standard ADU on the same property.
This is a genuinely useful combination. A JADU inside the house can serve an aging parent who benefits from proximity, while a detached ADU in the yard houses a tenant or an adult child who wants independence. Two units, two very different purposes.
Specific combinations depend on lot conditions and current statute, so confirm what applies to your property.
Which One Fits
A JADU tends to fit when:
- Budget is the primary constraint
- Your house has space that is genuinely underused
- The occupant is family who benefit from being close
- Your lot has no room for a detached structure
- You want to preserve yard entirely
- You want the option to reabsorb the space later
A standard ADU tends to fit when:
- Rental income is a primary goal
- Real privacy matters for both households
- You need more than 500 square feet
- You have yard space to use
- You want the unit to add meaningful property value
- The occupant needs genuine independence
A Note on Rental Expectations
JADUs rent for less than standard ADUs, and the gap is wider than the square footage difference alone suggests. A shared bathroom, a kitchenette rather than a full kitchen, and an interior connection to the main house all reduce what a tenant will pay.
If rental income is the main objective, run the numbers on both paths before assuming the cheaper build is the better investment.
Frequently Asked Questions
Can a JADU be detached?
No. By definition it must be created within the walls of the existing single-family home. A detached structure is a standard ADU.
Does a JADU need its own bathroom?
Not necessarily. It may share with the main residence, though most homeowners include one for privacy and rental appeal.
Can I convert a JADU into a full ADU later?
Sometimes, depending on the configuration and whether the space can be expanded or fully separated. It requires new permits and is not always practical, so plan for what you actually want.
Do both require permits?
Yes. Both are dwelling units and both require building permits. There is no legal path to an unpermitted unit of either kind.
Which adds more property value?
A standard ADU, generally, particularly a detached one. Appraisers increasingly use an income approach, and standard ADUs generate more rent.
Deciding Which Fits Your Home
Design by M&M is a Bay Area design-build company working with homeowners in Palo Alto, San Francisco, and surrounding Peninsula and South Bay communities. We evaluate your existing layout, lot conditions, and utility capacity before design begins, so the choice between a JADU and a standard ADU rests on what your property can support.
If you are considering either, a consultation is the place to start. Contact Design by M&M to discuss your options.
