Junior ADU

Almost everyone who asks me about a junior ADU is really asking a comparison question. They have read that a JADU is cheaper, and they want to know what they give up.

The short answer is that you give up size, independence, and the ability to sell it separately, and you get to skip the foundation, the site work, and most of the timeline. Whether that trade is good depends almost entirely on who is going to live in it.

So this page is the comparison, then the rules, then the money. Cost first, because that is what the trade actually turns on.

What a Junior ADU Costs

Nationally, plan on roughly $20,000 to $100,000 all-in. That is the cheapest legal path to a rentable unit in the country, and it is the only ADU type where a real project can land under $50,000.

Size	National all-in range	California all-in range
150 sq ft	$15k to $25k	$27k to $45k
300 sq ft	$30k to $50k	$54k to $91k
400 sq ft	$40k to $66k	$73k to $121k
500 sq ft (the cap)	$50k to $83k	$91k to $151k

Source: ADU Wizard cost engine, standard finish, before city-level adjustment

The reason those numbers are so much lower than every other type is structural, not cosmetic. A JADU has no foundation, no site work, and no new utility connection, because it reuses a room that already has all three. What you are actually paying for is a kitchen, a door, and whatever the walls and the electrical panel need to make it legal.

The spread inside those ranges comes down to one question: does the room you are converting already sit next 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.

Our full breakdown of California ADU costs has the regional multipliers, and the latest builder cost data shows a wider spread than most published averages.

How It Compares to the Other Four Types

Type	Typical all-in	Timeline	Permit complexity
Junior ADU	$20k to $100k	2 to 5 months	Low
Garage conversion	$55k to $140k	3 to 6 months	Low
Attached ADU	$95k to $200k	5 to 10 months	Moderate
Above-garage ADU	$100k to $280k	7 to 12 months	High
Detached ADU	$120k to $360k	6 to 12 months	Moderate

Source: ADU Wizard cost data. Full side-by-side on the ADU types hub.

Nothing else on that table is close on either money or time. A JADU is roughly a third the cost of the next cheapest option and finishes in half the time, which is why it is the right answer far more often than its share of the search traffic suggests.

JADU vs ADU: The Decision Table

This is the comparison the search results bury three scrolls down, so here it is up front.

	Junior ADU	Full ADU
Max size	500 sq ft of interior livable space	850 sq ft (studio or 1 bed), 1,000 sq ft (2+ beds)
Where it can go	Entirely within the walls of the single-family residence	Detached, attached, or inside an existing structure
Kitchen	Efficiency kitchen (cooking appliance, prep counter, storage)	Full kitchen
Bathroom	Optional. May share the house’s bathroom	Required, private
Owner-occupancy	Required only if sanitation is shared, since January 2026	Not required
Deed restriction	Yes, recorded and running with the land	No
Sell separately	No	No, but it appraises as a second dwelling
Typical all-in	$20k to $100k	$95k to $360k depending on type
Typical timeline	2 to 5 months	5 to 12 months
Minimum rental term	Longer than 30 days	Longer than 30 days

Source: ADU Wizard, from California Government Code sections 66313, 66321 and 66333

The row that decides most projects is the bathroom row, and it decides it in a way people do not expect. Read on, because California changed what that row means at the start of this year.

Worth knowing before you choose either: in California you do not actually have to. State law requires ministerial approval of one accessory dwelling unit and one junior accessory dwelling unit per lot with a proposed or existing single-family dwelling. A JADU carved from the house and a detached ADU in the yard are not competing options. They are two units you are allowed to have at once.

The Rules That Define a JADU

A junior ADU is a narrower legal category than most people realize, and the definition does real work. California defines it as a unit of no more than 500 square feet of interior livable space contained entirely within a single-family residence.

Read both halves of that. The size cap is 500 square feet, and since the measuring stick moved to interior livable space, exterior walls no longer eat into it. The location rule is stricter than the size rule: entirely within the single-family residence.

That second half is what people get wrong. Your detached garage is not the residence. Converting it makes a garage conversion ADU, not a JADU, and that is usually better news, because a full ADU has no owner-occupancy question and no 500 foot ceiling.

The rest of the requirements come from the JADU statute, which local ordinances have to implement:

• Built within the walls of the proposed or existing single-family residence.
• A separate entrance from the main entrance to the residence.
• An efficiency kitchen, meaning a cooking appliance, a food preparation counter, and storage cabinets.
• A recorded deed restriction that runs with the land and prohibits selling the JADU separately from the house.
• Rentals for a term longer than 30 days, so no short-term letting.

A private bathroom is not on that list. A JADU is allowed to share the house’s sanitation, which is exactly why it can be so cheap. It is also, as of this year, the thing that determines whether you have to live there.

The Owner-Occupancy Rule Changed in 2026

This is the part most published JADU content still has wrong, and it is the single most useful thing on this page.

The old rule was flat: a JADU meant the owner had to live on the property, in the house or in the unit. That requirement is why a lot of investors wrote JADUs off entirely.

Assembly Bill 1154 amended the JADU statute effective January 1, 2026, and owner-occupancy is now conditional. A local agency requires it only if the junior ADU has shared sanitation facilities with the existing structure. The statute goes on to say owner-occupancy shall not be required if the JADU has separate sanitation facilities, or if the owner is another governmental agency, land trust, or housing organization.

So the bathroom is no longer a comfort decision. It is the decision.

Share the bathroom and you have the cheapest possible unit, and you have to live there. Give the JADU its own bathroom, spend the extra money, and the owner-occupancy requirement comes off. That reframes the whole type for anyone who might want to move out later, and it is worth pricing both versions before you draw one.

Let me be careful here, because this is new law and cities implement on their own schedule. The state requirement is what it is, but your city’s ordinance may still carry the old blanket language until it updates. Ask your planning counter directly what they are applying today, and get the answer in writing.

Who Should Build a Junior ADU

Build a JADU if you have a bedroom you do not use, the room sits near existing plumbing, and the occupant is family or a long-term tenant who does not need a fully separate home.

Skip it if any of these is true.

• You want the unit to appraise as a second dwelling, which a JADU does not do cleanly.
• You need more than 500 square feet, which is a hard cap, not a local one.
• You plan to move out and keep it rented, unless you are giving it separate sanitation.
• The only convertible room is nowhere near the existing plumbing stack.
• You want to sell the unit separately one day, which the recorded deed restriction permanently prevents.

That said, the case for a JADU is stronger than its reputation. Losing a bedroom sounds worse than it is when the alternative is a $300,000 detached build you keep postponing.

Junior ADU Gotchas I See

The deed restriction is permanent and it is recorded. It runs with the land, so it survives your ownership. Nobody reads it until closing, and then it becomes a conversation with a buyer’s agent who has never seen one.

Losing a bedroom shows up in the listing. A four-bedroom house that becomes a three-bedroom house plus a JADU is not obviously worth more to every buyer. In a rental market it is. In a family neighborhood, sometimes not. Think about your street.

The separate entrance is a real construction item. It sounds like hanging a door. On an exterior wall it is a header, a landing, a path, and sometimes a setback question. It is the most commonly underpriced line on a JADU.

Efficiency kitchen does not mean whatever you want. The statute names the parts: a cooking appliance, a food prep counter, storage. Some cities read that narrowly and will bounce a plan that shows a full range where the ordinance expects a compact unit. Ask before you spec it.

One JADU plus one ADU is a strategy, not a loophole. People treat this as a choice between types. On a single-family lot in California it is not. If you can afford both eventually, sequencing the JADU first funds the second one.

Frequently Asked Questions

What Is the Difference Between a JADU and an ADU?

A junior ADU is capped at 500 square feet of interior livable space and has to sit entirely within the walls of the single-family residence, with its own entrance and an efficiency kitchen, and it may share the house’s bathroom. A full ADU can be detached or attached, runs up to 850 or 1,000 square feet depending on bedroom count, and needs its own bathroom. The JADU is cheaper and faster, at $20,000 to $100,000 against $95,000 and up, and it carries a recorded deed restriction that a full ADU does not.

Does a Junior ADU Require Owner-Occupancy?

Only if it shares sanitation facilities with the house. AB 1154 amended the statute effective January 1, 2026, so owner-occupancy applies to shared-bathroom JADUs and is not required where the junior ADU has separate sanitation facilities, or where the owner is a governmental agency, land trust, or housing organization. That is a meaningful change if you might move out and keep the unit rented. Confirm what your city is applying today, because local ordinances update on their own timeline.

How Much Does a Junior ADU Cost?

Roughly $20,000 to $100,000 all-in nationally, or about $27,000 to $151,000 in California depending on size and city. A 300 square foot JADU, which is the common case, prices around $30,000 to $50,000 nationally. Proximity to existing plumbing moves the number more than square footage does, so a bedroom backing onto a bathroom wall is the cheap version of this project.

Can You Have Both a JADU and an ADU?

Yes, in California. State law requires ministerial approval of one accessory dwelling unit and one junior accessory dwelling unit per lot with a proposed or existing single-family dwelling, so a JADU carved from the house and a detached unit in the yard can coexist. Outside California this varies by state and city, so check the local ordinance rather than assuming.

How Big Can a Junior ADU Be?

500 square feet of interior livable space, and that is a state cap rather than a local one, so no city can grant you more under the JADU category. Since the measurement moved to interior livable space, exterior walls no longer count against it. If you need more room than that, you are looking at an attached ADU or a garage conversion instead.

If you have a spare bedroom near a bathroom wall and a family member who needs somewhere to live, I would build the JADU and not overthink it. If you are building for a tenant and you might move out, spend the extra on separate sanitation and buy your way out of the owner-occupancy rule while you are already in there.