A Junior ADU is the smallest ADU: up to 500 square feet, inside the house you already have.

A Junior ADU is the smallest kind of accessory dwelling unit: California Government Code §66313(d) defines it as "a unit that is no more than 500 square feet of interior livable space in size and contained entirely within a single-family residence." It is built inside the walls of the house you already have, typically an unused bedroom or wing, and it keeps an efficiency kitchen and its own entrance. In Orange County a Junior ADU typically runs $135K – $220K+ over 4–6+ mo.

Size and location. A JADU is capped by statute at 500 square feet of interior livable space and must sit "within the walls of the proposed or existing single-family residence" (§66333(d)); a detached ADU is a standalone building, typically 500 – 1,200 sqft at $245K – $440K+. A JADU may share a bathroom with the house; an ADU has its own. A JADU carries an owner-occupancy rule when the bath is shared and a deed restriction against separate sale; a standard ADU does not carry the owner-occupancy rule.

Five hundred square feet of interior livable space, and it has to be inside the existing or proposed single-family home. The statute reads: "no more than 500 square feet of interior livable space in size and contained entirely within a single-family residence" (§66313(d)). Avorino's Junior ADUs typically land at 300 – 500 sqft.

A kitchen, yes, and it is required: the ordinance must "Require the permitted junior accessory dwelling unit to include an efficiency kitchen," meaning "A cooking facility with appliances" and "A food preparation counter and storage cabinets that are of reasonable size in relation to the size of the junior accessory dwelling unit" (§66333(f)). A bathroom is optional: a JADU "may include separate sanitation facilities, or may share sanitation facilities with the existing structure" (§66313(d)).

Only if the JADU shares a bathroom with the house. Government Code §66333(b) requires owner-occupancy "If the junior accessory dwelling unit has shared sanitation facilities with the existing structure," and adds that "Owner-occupancy shall not be required if the junior accessory dwelling unit has separate sanitation facilities, or if the owner is another governmental agency, land trust, or housing organization." When it applies, "The owner may reside in either the remaining portion of the structure or the newly created junior accessory dwelling unit." Cities record the rule as a deed restriction that runs with the land.

Yes. Government Code §66323(a)(1) requires cities to ministerially approve "One accessory dwelling unit and one junior accessory dwelling unit per lot with a proposed or existing single-family dwelling" when the conditions in that section are met, and §66323(a)(2) covers "One detached, new construction, accessory dwelling unit" on the same lot. In practice that is a JADU inside the house plus a detached ADU in the yard, each on its own permit.

Typically $135K – $220K+, over 4–6+ mo, because no new foundation or exterior structure is involved. That sits below a garage conversion at typically $85K – $170K+ and well below a detached ADU at typically $245K – $440K+. The Avorino cost estimator prices your city, square footage and finish level in about a minute.

Rent it, yes, on terms of more than 30 days: the ordinance must "Require that a rental of a junior accessory dwelling unit be for a term longer than 30 days" (§66333(g)). Sell it separately, no: the recorded deed restriction must include "A prohibition on the sale of the junior accessory dwelling unit separate from the sale of the single-family residence" (§66333(c)(1)).