ADU vs JADU: What's Actually Different

Homeowners usually ask me “ADU or JADU?” as if it were a style choice. It is not. The two are different legal categories with different size caps, different kitchens, different owner rules, and, in California, they are not even competing options, because you can often build both.

Here is how I explain the difference on a site visit, with the costs pulled from our engine rather than a guess.

ADU vs JADU at a Glance

	Junior ADU (JADU)	ADU
Size limit	500 sq ft of interior livable space, a state cap	Your city must allow at least 850 sq ft (studio or 1 bed) or 1,000 sq ft (2+ beds)
Where it goes	Entirely inside an existing or proposed single-family home	Detached, attached, or inside an existing structure like a garage
Kitchen	Efficiency kitchen: cooking appliance, prep counter, storage	Full kitchen
Bathroom	Optional; may share the house’s	Its own, required
Owner on site	Only if it shares a bathroom (since January 1, 2026)	Not required
Deed restriction	Yes, recorded	No
Rentals	Longer than 30 days	A city may require 30 days or more
Typical cost (national)	About $20,000 to $66,500	About $73,200 to $360,000
Typical timeline	2 to 5 months	3 to 12 months

Source: California Gov. Code 66313, 66315, 66321 and 66333; costs from the ADU Wizard cost engine; timelines from our ADU types guides

What an ADU Is

An ADU is a complete second home on a lot with a house: its own entrance, kitchen, bathroom and sleeping space. It can be a detached ADU in the backyard, an attached ADU that shares a wall with the house, or a conversion of a garage or other existing space.

The size rules are floors, not ceilings. In California a city has to allow a detached unit of at least 800 square feet, and it cannot set its cap below 850 square feet for a studio or one bedroom, or 1,000 square feet for two or more. Many cities allow more. The “1,200 square foot ADU” you see quoted everywhere is a common local number, not state law.

What a JADU Is

A junior ADU is a unit of no more than 500 square feet of interior livable space, contained entirely within a single-family home (Gov. Code 66313). In practice it is a bedroom, or a bedroom plus a bathroom, turned into a small unit with its own outside door and an efficiency kitchen.

The JADU statute, Gov. Code 66333, spells out the rest. The efficiency kitchen means a cooking facility with appliances, a food prep counter and storage cabinets. If the JADU has no bathroom of its own, it needs interior access to the main living area. Rentals have to run longer than 30 days. And a deed restriction gets recorded, which mainly means you can never sell the JADU separately from the house.

The rule that changed is owner occupancy. Under AB 1154, effective January 1, 2026, a city can require you to live on the property only if the JADU shares a bathroom with the house, and even then you can live in either the house or the JADU. Give the JADU its own bathroom and the requirement comes off. Honestly, that one change made JADUs worth a second look for a lot of people who had written them off.

What Each One Costs

These are the cost engine’s numbers at standard finish, nationally and at California’s state average. Your city moves them, sometimes a lot.

The reason a JADU is so much cheaper is structural. There is no foundation, no site work and usually no new utility connection, because the room already has all three. What you are paying for is a kitchenette, a door, and whatever the walls and the panel need to make it legal.

The spread inside the JADU number comes down to plumbing. A bedroom that backs onto an existing bathroom wall is the cheap version. A bedroom on the far corner of the house is not, and adding a bathroom to escape the owner rule moves you toward the top of the range. Our California ADU cost breakdown has the city-level detail, and the ADU Wizard data hub shows how wide real bids run.

Can You Build Both?

In California, usually yes, and this is the part almost every comparison gets wrong by treating it as one line. Here is what state law lets you combine on a lot with a single-family home, under Gov. Code 66323:

Combination	What it allows	When
One ADU + one JADU, inside existing or proposed space	An ADU and a JADU within the house or an existing accessory structure, like a garage conversion plus a bedroom JADU	Now
One detached new-construction ADU	Up to 800 sq ft, with a 16 foot height floor (18 near major transit)	Now
Both of the above together	A detached unit, plus an ADU and a JADU within existing or proposed space, on one lot; SB 543 confirmed these combine	Now
Two detached new-construction ADUs	AB 956 raises the detached count from one to two, but then the city does not have to approve a JADU ministerially on that lot	From January 1, 2027

Source: Gov. Code 66323 as amended by SB 543 and AB 956 (Chapter 791, Statutes of 2026)

So the real question after 2027 is not ADU or JADU. It is whether you want two detached units, or one detached unit plus a JADU and a conversion. I walk through every change like this one in our California ADU law tracker, and the California ADU permit guide covers the process for each.

One caution. If you are also thinking about a lot split, the rules flip: a city does not have to permit an ADU or a JADU on a parcel that uses both SB 9’s lot split and its two-unit authority. Our page on SB 9 lot splits vs ADUs lays out that tradeoff.

Which One Fits Your Goal

Your goal	What I would build	Why
House a parent or adult child, lowest cost	JADU	Cheapest, fastest, and you are living there anyway
Rent to a stranger long term	Detached or garage conversion ADU	Tenants pay for separation; a JADU shares walls and maybe a bathroom
Rent now, maybe move out later	JADU with its own bathroom, or a full ADU	A shared-bath JADU ties you to the property
Maximum value on resale	Detached ADU	It appraises as a second dwelling; a JADU usually does not
Most units on one lot	Both: a JADU plus a detached ADU	State law guarantees the combination today

That said, the goal I hear most is “some income without a huge loan.” For that, a JADU first and a detached unit later is the sequence I like, because the JADU’s rent can help carry the bigger build. If you are working out how to pay for the second one, our ADU financing guides cover the loan side, and whether an ADU increases property value covers the resale side.

Frequently Asked Questions

What Is the Main Difference Between an ADU and a JADU?

A JADU is capped at 500 square feet and has to sit entirely inside a single-family home, with an efficiency kitchen and an optional shared bathroom. An ADU is a complete second home that can be detached, attached or a conversion, with a full kitchen and its own bathroom, and your city has to allow it at 850 to 1,000 square feet or more.

Is a JADU Cheaper Than an ADU?

Yes, by a lot. Our cost engine puts a typical 300 square foot JADU at about $39,900 nationally, against about $73,200 for a 400 square foot garage conversion and $240,000 for an 800 square foot detached ADU. The JADU skips the foundation, site work and most utility work.

Can I Have an ADU and a JADU on the Same Lot?

In California, yes. State law lets a single-family lot have an ADU and a JADU within existing or proposed space, plus a detached new-construction ADU. From January 1, 2027, AB 956 allows two detached ADUs instead, but then a city does not have to approve a JADU on that lot ministerially.

Do I Have to Live on the Property If I Build a JADU?

Only if the JADU shares a bathroom with the house. Since January 1, 2026, under AB 1154, a city cannot require owner occupancy for a JADU with its own bathroom, and where it can require it, you may live in either the house or the JADU.

Does a JADU Need Its Own Bathroom?

No. A JADU may share the main house’s bathroom, but then it needs interior access to the main living area, and the owner occupancy rule applies. Adding a bathroom costs more and removes that rule.

If I were choosing today with a spare bedroom near a bathroom wall and a backyard I might build in later, I would do the JADU now and price the detached unit next. Start with the junior ADU guide for the rules in full.