A window or door large enough to allow emergency exit from a habitable room — required by California Building Code in every sleeping area.
Minimum 5.7 sqft openable area, 24" minimum height, 20" minimum width, and ≤44" sill height. Critical concern in basement and garage-conversion ADUs because cutting new openings in concrete or below grade is expensive ($3k-$8k per opening, plus engineering).
Part of the BackyardADU California ADU Glossary — plain-English definitions of the regulatory, construction, and financing terms you'll encounter during a California ADU project.