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The 2025 California Residential Code, which takes effect on January 1, 2026, introduces important updates to the separation requirements for two-family dwellings. These changes are designed to improve fire safety, limit the transfer of smoke, and strengthen the protection of shared spaces between units. For design professionals, contractors, and code officials, understanding these provisions now will help ensure compliance and avoid problems during construction and inspection.
Section R302.3 makes it clear that all two-family dwellings must be separated in accordance with the new rules, even if a property line does not exist between the units. This closes a long-standing gap and ensures that the same level of protection applies regardless of ownership.
The code requires separation by vertical or horizontal assemblies, or a combination of both. These assemblies must have a minimum one-hour fire-resistance rating. The rating may be reduced to one-half hour if the building is fully equipped with an automatic sprinkler system installed under Section R309. Assemblies must be tested in accordance with ASTM E119 or UL 263, or designed through an approved analytical method under Section 703.2.2 of the California Building Code.
Continuity is emphasized in Section R302.3.3. Horizontal assemblies must extend tightly against exterior walls or vertical separation assemblies. Vertical assemblies must extend from the foundation to the roof sheathing or through horizontal assemblies. Where vertical assemblies terminate at a ceiling beneath an uninhabitable attic, the ceiling must be constructed with not less than five-eighths-inch Type X gypsum board, attic draft stopping must be provided as required by Section R302.12.1, and the supporting structural framing must be protected by not less than one-half-inch gypsum board or equivalent. This continuity ensures that fire and smoke separation is maintained without weak points.
Supporting construction must have a fire-resistance rating equal to or greater than the separation assembly itself as required by Section R302.3.4. This prevents the structure supporting the wall or floor-ceiling assembly from becoming the failure point.
Special provisions are included for vertically stacked dwelling units in Section R302.3.5. If automatic sprinklers are not provided, both horizontal and vertical assemblies must be built to limit smoke transfer. In addition, a notification appliance connected to the smoke alarms in one unit must be installed in the other unit. This ensures that occupants are alerted to fire conditions in the adjoining dwelling.
Shared accessory rooms such as storage rooms, laundry rooms, or carports must now be separated from dwelling units in accordance with Table R302.3.6. The required protection includes not less than one-half inch gypsum board on the accessory room side, five-eighths inch Type X gypsum board where habitable rooms are located above or below, and one-half inch gypsum board on supporting structures. Openings are restricted. No opening is permitted directly into a sleeping room, and other openings must be equipped with a one and three-eighths-inch-thick solid wood door, a steel door of equal thickness, or a fire door assembly with a 20-minute rating. All doors must be self-closing or automatically closing. Ducts penetrating the separation must be constructed of not less than number 26 gage sheet steel or other approved material with no openings into the shared accessory room. All other penetrations must comply with Section R302.11, Item 4.
Finally, Section R302.4 requires that penetrations of any fire-resistance-rated wall or floor-ceiling assembly must be properly protected. This includes ducts, pipes, wires, and other utilities that would otherwise compromise the integrity of the separation.
These provisions represent a meaningful step forward in protecting occupants of two-family dwellings. The new requirements strengthen fire-resistance, improve smoke control, and provide greater protection in shared accessory spaces. For contractors, design professionals, and code officials, early awareness of these changes will ensure smoother plan reviews, better construction practices, and more consistent enforcement once the code takes effect on January 1, 2026.
Section R302.3 makes it clear that all two-family dwellings must be separated in accordance with the new rules, even if a property line does not exist between the units. This closes a long-standing gap and ensures that the same level of protection applies regardless of ownership.
The code requires separation by vertical or horizontal assemblies, or a combination of both. These assemblies must have a minimum one-hour fire-resistance rating. The rating may be reduced to one-half hour if the building is fully equipped with an automatic sprinkler system installed under Section R309. Assemblies must be tested in accordance with ASTM E119 or UL 263, or designed through an approved analytical method under Section 703.2.2 of the California Building Code.
Continuity is emphasized in Section R302.3.3. Horizontal assemblies must extend tightly against exterior walls or vertical separation assemblies. Vertical assemblies must extend from the foundation to the roof sheathing or through horizontal assemblies. Where vertical assemblies terminate at a ceiling beneath an uninhabitable attic, the ceiling must be constructed with not less than five-eighths-inch Type X gypsum board, attic draft stopping must be provided as required by Section R302.12.1, and the supporting structural framing must be protected by not less than one-half-inch gypsum board or equivalent. This continuity ensures that fire and smoke separation is maintained without weak points.
Supporting construction must have a fire-resistance rating equal to or greater than the separation assembly itself as required by Section R302.3.4. This prevents the structure supporting the wall or floor-ceiling assembly from becoming the failure point.
Special provisions are included for vertically stacked dwelling units in Section R302.3.5. If automatic sprinklers are not provided, both horizontal and vertical assemblies must be built to limit smoke transfer. In addition, a notification appliance connected to the smoke alarms in one unit must be installed in the other unit. This ensures that occupants are alerted to fire conditions in the adjoining dwelling.
Shared accessory rooms such as storage rooms, laundry rooms, or carports must now be separated from dwelling units in accordance with Table R302.3.6. The required protection includes not less than one-half inch gypsum board on the accessory room side, five-eighths inch Type X gypsum board where habitable rooms are located above or below, and one-half inch gypsum board on supporting structures. Openings are restricted. No opening is permitted directly into a sleeping room, and other openings must be equipped with a one and three-eighths-inch-thick solid wood door, a steel door of equal thickness, or a fire door assembly with a 20-minute rating. All doors must be self-closing or automatically closing. Ducts penetrating the separation must be constructed of not less than number 26 gage sheet steel or other approved material with no openings into the shared accessory room. All other penetrations must comply with Section R302.11, Item 4.
Finally, Section R302.4 requires that penetrations of any fire-resistance-rated wall or floor-ceiling assembly must be properly protected. This includes ducts, pipes, wires, and other utilities that would otherwise compromise the integrity of the separation.
These provisions represent a meaningful step forward in protecting occupants of two-family dwellings. The new requirements strengthen fire-resistance, improve smoke control, and provide greater protection in shared accessory spaces. For contractors, design professionals, and code officials, early awareness of these changes will ensure smoother plan reviews, better construction practices, and more consistent enforcement once the code takes effect on January 1, 2026.
