The core legal distinction
Under California state law, an ADU (Accessory Dwelling Unit) is a fully independent living unit — its own kitchen, bathroom, and entrance — that can be detached, attached, or converted from existing space. A JADU (Junior Accessory Dwelling Unit) is capped at 500 square feet, must be carved from within the existing footprint of a single-family home, and may share a bathroom with the primary residence. The JADU rules were designed to make a smaller, faster, lower-cost path to legal rental space available to homeowners who don't want to take on a full ADU project.
Cost and timeline differences
A JADU typically costs $80,000–$180,000 because it reuses existing structure, foundation, and most utilities. A detached ADU typically costs $250,000–$450,000+ because it's effectively a small custom home. JADUs also tend to permit faster — often 6–10 weeks of plan check versus 10–16 for a detached ADU.
Rental and occupancy rules
Both ADUs and JADUs can be rented long-term in California, but JADUs come with an owner-occupancy requirement — the owner must live in either the JADU or the primary house. Detached ADUs do not have a state-level owner-occupancy requirement, making them more flexible for investors and homeowners who may relocate.
Which one is right for you?
If your budget is tight, your timeline is short, and you have unused interior square footage, a JADU is often the smartest move. If you have backyard space, want maximum rental income, or plan to use the unit for multi-generational living, a detached ADU is usually the better long-term investment. AER Builders can walk your property and tell you honestly which path fits — sometimes the answer is even building both.