The core JADU vs ADU difference is location and scale. A junior ADU (JADU) is up to 500 square feet carved out of a single-family home. It has an efficiency kitchen and may share a bathroom. A standard ADU is a complete, independent home, attached or detached. State law generally lets you build one at 850 to 1,200 square feet.
A JADU costs less and works well for a family member or a single tenant. An ADU gives more space, more privacy, and more rental flexibility. Many lots can have both.
What is a JADU under California law?
A JADU is a small unit of no more than 500 square feet built entirely inside a single-family home. It can be created in an existing house or in one being built, but never in a detached garage or backyard structure.
- State law defines a JADU as a unit "no more than 500 square feet of interior livable space in size" and "contained entirely within a single-family residence." [Source: Cal. Gov. Code 66313, 2026]
- A JADU needs a separate entrance and an efficiency kitchen: a cooking facility with appliances, a prep counter, and storage cabinets. Cities cannot demand more, and 220V circuits are not required. [Source: CA HCD ADU Handbook 2026]
- A JADU may have its own bathroom or share one with the main home. If it shares, it must have an interior door into the main living area. [Source: Cal. Gov. Code 66333, 2026]
Common JADU spaces include an extra bedroom suite, a den, or part of an attached garage.
How big can an ADU be compared with a JADU?
A JADU is capped at 500 square feet. An ADU is much larger, and state law sets floors that cities must allow, not just ceilings.
| Unit type | Size rule | Code section |
|---|---|---|
| JADU | Up to 500 sq ft, inside the home | Gov. Code 66313(d) |
| ADU, any local ordinance | Must allow at least 800 sq ft with 4-ft setbacks | Gov. Code 66321(b)(3) |
| ADU, studio or one bedroom | Local cap cannot be below 850 sq ft | Gov. Code 66321(b)(2) |
| ADU, two or more bedrooms | Local cap cannot be below 1,000 sq ft | Gov. Code 66321(b)(2) |
| Detached ADU, no compliant ordinance | Up to 1,200 sq ft | Gov. Code 66314(d)(4) |
| Attached ADU, no compliant ordinance | Up to 50% of the main home, at least 800 sq ft | Gov. Code 66314(d)(5) |
[Source: CA HCD ADU Handbook 2026] Many Peninsula cities allow larger ADUs than these minimums. Check your city's current ordinance, or see how an attached ADU compares with a detached ADU on your lot.
Does a JADU require owner occupancy in California?
Only if the JADU shares a bathroom with the main home. That rule changed recently, and many older articles still get it wrong.
- AB 1154, signed October 10, 2025, limits JADU owner-occupancy to units with shared sanitation. A JADU with its own bathroom cannot carry that requirement. [Source: LA County Planning, AB 1154 text, 2025]
- When owner-occupancy applies, the owner may live in either the main home or the JADU. [Source: Cal. Gov. Code 66333, 2026]
- For standard ADUs, cities cannot impose owner-occupancy at all, with narrow exceptions for units sold separately. Both unit types can be limited to rentals longer than 30 days. [Source: CA HCD ADU Handbook 2026]
A JADU also requires a recorded deed restriction. It bars selling the JADU separately from the home and locks in its size and features.
Can you have both a JADU and an ADU on one lot?
Yes. On a single-family lot, you can combine a JADU with one or more ADUs. Each unit must meet its own standards.
HCD says cities must allow three units together. They are one ADU from existing space, one JADU, and one new detached ADU up to 800 square feet. Cities must also allow at least one additional ADU under the general standards. [Source: CA HCD ADU Handbook 2026]
That flexibility is driving real demand. ADUs made up 48% of new housing units permitted in San Mateo County in 2024. [Source: San Mateo Daily Journal 2025] A common plan is a JADU now for a parent, then a detached ADU later for rental income.
How is a JADU permitted compared with an ADU?
Both are ministerial permits. That means no public hearing. The city must act on a complete application within 60 days when a home already exists on the lot.
- The 60-day deadline applies to ADUs and JADUs alike. [Source: CA HCD ADU Handbook 2026]
- For water, sewer, and power, a JADU "shall not be considered a separate or new dwelling unit." So no new connection fee applies. [Source: Cal. Gov. Code 66338, 2026]
- HCD told Los Altos Hills in December 2025 that its 2020 ordinance may be outdated. It also noted no impact fees may be charged on a JADU of 500 square feet or less. [Source: CA HCD 2025]
The local point matters. When a city ordinance falls out of step with state law, state rules apply directly. Our San Mateo ADU permit guide covers each review step.
JADU vs ADU cost: which makes more sense?
A JADU almost always costs less, because it reuses existing walls, roof, and utility service. An ADU costs more, but it creates a full, independent home with far more value and rental flexibility.
A fully custom ADU in San Mateo and Santa Clara Counties typically runs about $300,000 to $450,000+. JADU budgets vary widely with what already exists, such as plumbing location and electrical capacity. Fee rules also favor smaller units. ADUs of 750 square feet or less are exempt from impact fees. [Source: CA HCD ADU Handbook 2026]
| Your goal | Better fit | Why |
|---|---|---|
| Space for a parent or adult child, lowest cost | JADU | Uses existing rooms, can share a bathroom |
| Long-term rental income | ADU | Full kitchen, own bath, more privacy |
| Maximum future resale value | ADU | Adds a complete, independent home |
| Keep the backyard open | JADU or attached ADU | No new footprint, or a small one |
Converting a garage is a middle path. An attached garage inside the home can become a JADU, while a larger conversion becomes a full ADU. See our garage conversion page and our guide to garage conversion parking rules. For rent estimates, read ADU rental income in the Bay Area.
How ADU San Mateo helps
ADU San Mateo designs and builds fully custom ADUs across San Mateo and Santa Clara Counties. That includes attached ADUs, detached ADUs, and garage conversions. Founder Elie AlChaer is a licensed California general contractor (CSLB #1145539). He oversaw more than $2B in commercial and luxury construction in the Bay Area and Dubai before starting the firm.
Our feasibility review compares a JADU, an attached ADU, and a conversion on your actual floor plan and lot. We check plumbing routes, panel capacity, and your city's current rules. Then we give you a clear recommendation. Learn about our team, see our process or read answers on our FAQ page.
JADU vs ADU FAQs
Can I rent out a JADU?
Yes. California law allows JADUs to be rented, and many owners rent them to a single tenant. Your city can require rental terms longer than 30 days, which rules out nightly vacation rentals. If the JADU shares a bathroom with the main home, the owner must live on site. That can be in the main house or the JADU. If the JADU has its own bathroom, owner-occupancy cannot be required under the 2025 change. A JADU also cannot be sold separately from the house, because a recorded deed restriction ties the two together. Check local rules before signing a lease.
Can a JADU be built in a detached garage?
No. A JADU must sit entirely within the walls of a single-family home, whether existing or proposed. A detached garage, shed, or pool house cannot become a JADU. Those spaces can become a full ADU instead, as a conversion of an existing accessory structure. An attached garage is different. Because it shares the home's walls, part or all of it can become a JADU, subject to the 500 square foot cap. HCD has said cities cannot require parking for a JADU, including one carved from an attached garage. That removes a common objection.
Is a JADU or an ADU better for resale value?
In most cases, a full ADU adds more value. It is a complete, independent home with its own kitchen and bathroom, and it can be rented without owner-occupancy rules. A JADU adds flexible space, but it stays legally tied to the main house. Buyers often see it as a bonus suite rather than a second home. That said, a JADU costs far less to build, so the return on each dollar can still be strong. If resale is your main goal, compare projected value against total project cost. A local appraiser or agent can help.
Do I need an architect for a JADU?
Usually you need permit drawings, but not always a full architectural team. A JADU often reuses existing rooms, so plans focus on the floor layout, kitchen, entrance, fire separation, and any plumbing or electrical changes. A design-build contractor can prepare these alongside construction pricing, which keeps the budget realistic. Larger projects that add square footage benefit from deeper design work. We also work with outside designers through our architect partnership program when a homeowner already has one.
The bottom line
Choose a JADU when you want a lower-cost suite inside your home for family or a single tenant. Choose an ADU when you want a full second home with more space, privacy, and long-term value. On many lots, you can do both.
Want a clear answer for your property? Book a free feasibility consultation. There is no obligation, and we reply within one business day.