Code-minimum STC 45 sounds reasonable on paper and feels terrible in real life. This guide shows the wall, floor, and ceiling assemblies we use on attached ADUs, basement AADUs, and multi-gen builds — with what they cost per square foot and where they fail.
When to use: Meets IBC 1207 minimum for unrelated dwellings. Acceptable when the AADU is for family, not paying tenants.
When to use: Recommended baseline for any rentable AADU sharing wall with main house.
When to use: Bedrooms backing onto living rooms; long-term rental ADUs where complaints kill renewals.
When to use: Recording rooms, home theaters, or AADUs adjoining a primary bedroom.
Single-family use only. Fails the comfort test for upstairs/downstairs tenants.
Basement AADU with bedrooms above — standard rentable spec in our last 6 builds.
When the upper unit has hardwood/tile and the lower unit has a bedroom directly below.
| Scenario | Wall target | Floor target | Notes |
|---|---|---|---|
| Attached AADU, family use only | STC 45 (code) | STC 45 / IIC 45 | Meets IBC 1207; not recommended if rented. |
| Attached AADU, long-term rental | STC 52+ | STC 54 / IIC 52 | Our standard spec for rentable AADUs. |
| Basement AADU below primary bedroom | STC 55+ | STC 62 / IIC 60 | Floor system is the gating element here, not walls. |
| DADU + main house (not connected) | STC 30 (exterior envelope) | n/a | Soundproofing problem is HVAC + window placement, not assemblies. |
| Multi-gen: parent suite next to teen room | STC 55+ | STC 54 / IIC 52 | Acoustic doors (STC 35+) are the weak link — solid-core + perimeter seal. |
A wall is only as soundproof as its weakest penetration. STC ratings test a perfect lab assembly — these are the real-world bypasses that drop a STC-55 wall to a STC-30 system.
Problem · Drops a STC-55 wall to STC-25 system overall.
Fix · Solid-core door + perimeter weatherstrip + drop seal at threshold (STC 32–38).
Problem · Direct air path between units; can lose 8–12 STC.
Fix · Offset boxes by 16" + putty pads on every box on demising wall.
Problem · Each penetration drops ceiling STC.
Fix · Use IC-rated airtight cans with putty hoods, or surface-mount LEDs.
Problem · Structure-borne noise bypasses any ceiling treatment.
Fix · Use a structural break, double-joist with gap, or floating-floor system above.
Problem · Telephony — talk in one room, hear it in another.
Fix · Separate ductwork per unit; use lined ducts and 90-deg turns at the boundary.
Problem · Flush noise transmits even through STC-55 assembly.
Fix · Cast iron drain stack + insulate wall cavity around it + furr out one side.
Send your floor plan and how the ADU will be used. We'll recommend a STC + IIC target and quote the assembly add — usually $5–$12/sf for a rentable-quality build.
We spec STC 50+ on every rental ADU. Send your wall framing plan (or just the address) and we'll redline the assembly to hit the target.
Plans or address + intended rental use. PDF in 48h.
◇ No sales script · No spam · Your details stay with Leo