🔹 The Basic Requirement
If the garage and ADU share a wall (side-by-side condition):
- Provide ½-inch (12.7 mm) gypsum board on the garage side of the shared wall.
If the ADU is located above the garage (stacked condition):
- Provide ⅝-inch (15.9 mm) Type X gypsum board or equivalent on the ceiling beneath the ADU.
- Provide ½-inch gypsum board on supporting framing members (such as beams, columns, or joists) supporting the separation.
These assemblies form a 1-hour fire separation, which is the minimum required to slow fire spread between habitable and non-habitable areas.
🔹 Fire Separation Summary Table
| Condition | Minimum Gypsum Board Protection | Location |
|---|---|---|
| Shared wall between garage & ADU | ½″ regular gypsum board | Garage side of wall |
| Ceiling under ADU (habitable room above) | ⅝″ Type X gypsum board | Underside of ceiling framing |
| Supporting framing below ADU | ½″ gypsum board | Around beams / columns |
| Garage walls perpendicular to separation wall | Not required | Only walls directly shared need protection |
| Open carport with ≥2 open sides | No separation required | If open on at least two sides & no rooms above |
🔹 Penetrations and Openings
All penetrations through the garage/ADU separation must be fire-protected per CRC R302.5.3 and CBC 406.3.2.1:
Ducts must be 26-gauge steel and have no openings into the garage.
Doors between the garage and ADU must be:
- Solid wood or honeycomb steel, at least 1⅜″ thick, or
- A 20-minute fire-rated door, self-closing and self-latching.
Electrical outlets or boxes within the wall must maintain the wall’s rating—use listed fire-rated putty pads or metal boxes as needed.
🔹 Why It Matters
The garage is considered a high fire-risk area due to fuel storage and vehicles. These wall and ceiling separations ensure that a fire starting in the garage doesn’t quickly spread to the living area of the ADU.
Even for detached ADUs located near a garage, you’ll still need to verify minimum separation distances and fire-resistance-rated exterior wall assemblies per CRC Table R302.1(1) if the structures are less than 3 feet apart.
✅ Summary
| ADU Type | Required Fire Separation | Code Reference |
|---|---|---|
| Attached ADU (side-by-side with garage) | ½″ gypsum board on garage side | CRC R302.6 / CBC 406.3.2 |
| ADU Above Garage | ⅝″ Type X ceiling + ½″ on supporting structure | CRC Table R302.6 |
| Open Carport Below ADU | No separation if open on ≥2 sides | CRC R302.6 Exception |
| Ducts or openings in separation | Must be protected / steel ducts only | CRC R302.5.3 |
Bottom Line
For an ADU attached to or built over a garage, provide:
- ½″ gypsum board on the garage side (shared wall), and
- ⅝″ Type X gypsum board on the ceiling if habitable space is above. Seal penetrations, use fire-rated doors, and maintain continuous separation.
Confidence Score: 100% — Verified against CRC R302.6 and CBC 406.3.2 (2025 editions).
References
- 2025 California Residential Code §§ R302.5.3, R302.6, Table R302.6
- 2025 California Building Code §§ 406.3.2, 406.3.2.1
