ADU Drywall and Separation Walls in San Diego: What the Code Actually Requires
Fire separation, sound separation, and the cover inspection — three drywall requirements that determine whether your ADU passes or fails. Here's what they are and how to meet them.
Quick Answer
Attached ADUs in San Diego require: (1) a 1-hour fire-rated separation wall between the ADU and the main residence, (2) STC 50 sound separation on shared walls and IIC 50 on shared floor-ceiling assemblies, and (3) a cover inspection before those walls are closed. Detached ADUs have different requirements. This guide covers both.
ADUs are one of the most permit-intensive residential projects in San Diego. The drywall work is rarely the complicated part — but it's one of the most common sources of failed inspections. That's usually because the substrate, assembly type, or inspection sequence wasn't planned ahead of time.
This guide covers the specific drywall requirements that apply to ADU projects in San Diego County and the City of San Diego — fire separation, sound separation, inspection sequence, and the assembly types that satisfy each one.
The Four Drywall Requirements That Apply to Most ADUs
These apply to attached ADUs. Detached ADUs have fewer requirements — see the comparison table below.
Fire-Rated Separation
The wall between an attached ADU and the main dwelling must be a 1-hour fire-rated assembly — typically 5/8" Type X drywall on both sides.
CRC R302.3 / R302.6
Sound Separation (STC 50)
Shared walls between dwelling units must meet STC 50. Usually requires RC channel, batt insulation, and double drywall, or an equivalent rated assembly.
CRC R302.13
Cover Inspection
Framing, fireblocking, substrate, and the separation assembly must be inspected before drywall covers them. Mandatory stop — no exceptions.
CRC R109.1
Fireblocking
All concealed cavities, stud bays, and horizontal framing in the separation assembly must be fireblocked before the wall is closed.
CRC R302.11
Attached vs. Detached vs. Above-Garage: What Changes
The ADU type determines which separation requirements apply.
| ADU Type | 1-Hr Fire Wall? | STC 50? | IIC 50? | Notes |
|---|---|---|---|---|
| Attached ADU (side-by-side) | Required | Required | If shared floor | Most common config in San Diego |
| ADU above garage / main house | Required | Required | Required | Floor-ceiling assembly must meet IIC 50 |
| Junior ADU (JADU) within existing space | Verify with AHJ | Verify with AHJ | Usually N/A | Requirements vary by configuration |
| Detached ADU (separate structure) | Not required | Not required | Not required | Must still meet habitability requirements |
Don't rely on this table as your permit set. San Diego County and the City of San Diego have local amendments that may affect your specific project. These requirements reflect general CRC provisions. Confirm the actual requirement for your project type with your building department or a licensed designer before framing.
The 1-Hour Fire-Rated Separation Wall: What It Actually Takes
A fire-rated wall is not just a board type — it's a tested assembly.
CRC R302.3 requires that walls separating attached dwelling units be fire-resistance rated at 1 hour. This doesn't mean "use 5/8 Type X" and call it done. It means the entire wall assembly — framing, insulation, board thickness and type, fastener pattern, and joint treatment — must match a tested and rated assembly.
The most common residential approach is a UL-listed or Gypsum Association (GA-600) listed assembly: typically 2x4 or 2x6 framing, one or two layers of 5/8" Type X on each side, with joints staggered and fastened per the rated assembly specs. The inspector will ask which listed assembly you're using — it should be documented on your permit plans.
The fire-rated separation must also be continuous — it can't stop at the ceiling and leave the attic space open between units. The separation must extend through the attic or the attic itself must be divided by a fire-rated assembly. This is a common ADU permit failure point.
Pre-Drywall Checklist for ADU Separation Walls
Every item here needs to be complete before you call for the cover inspection.
Framing matches the rated assembly specification
Stud spacing, plate configuration, and blocking conform to the listed assembly (GA-600, UL, or approved equivalent) identified on your permit plans.
Fireblocking complete at all required locations
Top and bottom of stud bays, horizontal concealed spaces, and penetrations through the wall are fireblocked per CRC R302.11.
RC channel installed correctly (if used for STC)
Channel is fastened at one flange only, screw lengths won't short-circuit the decoupling, perimeter gaps maintained. No rigid contact at floor, ceiling, or perpendicular walls.
Insulation in place in all cavities
Batt insulation fills the stud cavity. Required for both the fire assembly and sound performance.
Penetrations sealed or sleeved
All plumbing, electrical, or mechanical penetrations through the separation wall are firestopped per CRC R302.4 before the inspector arrives.
Assembly is documented on the approved plans
The listed assembly type (GA number or UL design) is on your permit plans and matches what was installed. Inspector will verify.
Code References & Resources Used in This Article
Building an ADU in San Diego?
SGP Drywall specializes in ADU separation walls, fire-rated assemblies, and RC channel installations across San Diego County. We coordinate with your GC and inspector to make sure the cover inspection passes the first time.