JADU vs ADU

The difference between a JADU and an ADU comes down to scale, construction method, and rules. A junior ADU (JADU) is created within the existing walls of a single-family home — typically capped at a small size (often up to 500 square feet), built from space that already exists, and sometimes allowed to share a bathroom with the main house. A full accessory dwelling unit (ADU) is generally larger, fully self-contained, and may be an attached addition or a standalone structure.

Cost is the most visible contrast. Because a JADU reuses existing interior space, it is usually the cheapest unit to create. A full ADU — especially a detached new-construction one — costs more because it adds floor area or a whole new building. The trade-off is space and independence: an ADU offers a larger, more private, fully separate home, while a JADU is a compact unit tied more closely to the main house.

Rules differ too. JADUs often carry owner-occupancy requirements and an efficiency-kitchen standard, while full ADUs have their own size, setback, and parking rules that vary by jurisdiction. Within full ADUs, the next decision is form — see detached vs attached ADU.

For a homeowner, the choice usually hinges on budget, how much usable interior space already exists, and how much separation the future occupant needs. Both paths require a building permit. The specific size caps, occupancy rules, and fees vary by state and city and can change, so verify current requirements with your local jurisdiction before deciding.