The core difference
A Junior ADU (JADU) is created from existing space within your main house — a converted bedroom, basement, or interior area — capped at 500 square feet, generally with a smaller "efficiency kitchen" rather than a full one. A full ADU (whether detached, attached, or a garage conversion) is a larger, fully independent unit with a complete kitchen and its own utility connections, generally allowed up to 850–1,200 square feet depending on your city and configuration.
Side-by-side comparison
| Factor | Junior ADU (JADU) | Full ADU |
|---|---|---|
| Typical Cost | $70,000 – $140,000 | $115,000 – $420,000+ (varies by type) |
| Typical Timeline | 2–5 months | 4–15 months (varies by type) |
| Max Size | 500 sq ft | 850–1,200 sq ft, depending on city and type |
| Kitchen | Efficiency kitchen generally allowed | Full kitchen |
| Separate Entrance | Not always required | Generally expected/required |
| Owner-Occupancy | More commonly required in some cities | Generally not required statewide |
Figures reflect typical ranges across our Southern California service area — see our full ADU cost guide for how these are calculated.
When a JADU tends to make more sense
- Budget is the main constraint. A JADU is generally the least expensive way to add a rentable or livable unit to your property.
- You have limited lot space but usable interior square footage, like an underused bedroom or basement.
- Speed matters. Converting existing interior space generally moves faster than new construction or even a garage conversion.
- You want family close by rather than fully separate — since a JADU can sometimes share an entrance with the main house, it suits a tighter multigenerational arrangement well.
When a full ADU tends to make more sense
- You want maximum privacy and independence between the unit and your main house, for a tenant or family member who wants their own space.
- Rental income potential is a priority — a larger, fully independent unit with a real kitchen is generally more attractive to a wider range of tenants.
- Your lot or existing structures support it. An existing garage or available backyard space makes a full ADU realistic without necessarily costing dramatically more than a heavily-finished JADU.
- You're building for long-term value, not just the fastest, cheapest option — see our ADU property value guide for how independence and size tend to factor into that.
You can build both, on many lots
California law generally allows a JADU and a full ADU to be combined on the same qualifying single-family lot — see our guide on how many ADUs one property can have. Some homeowners start with the lower-cost JADU and add a full ADU later, or build both at once if the budget and lot support it.
Frequently asked questions
Generally yes, since it converts existing interior space rather than requiring new construction or a full kitchen — but a heavily upgraded JADU could still cost more than a very basic garage conversion in some cases. See our ADU cost guide for typical ranges by type.
Often yes, though some cities apply an owner-occupancy condition to JADUs that doesn't apply to full ADUs — confirm your city's current rule before assuming either way. See our ADU rental income guide for more on rental considerations.
Not exactly — a JADU is a distinct category with its own size cap and kitchen rules, so "upgrading" it generally means building a separate full ADU elsewhere on your lot rather than converting the JADU itself into a larger unit.