If you are building an attached or detached accessory dwelling unit (ADU) in Lacey under Washington's HB 1337 housing laws, building codes require a 1-hour fire-resistance rated drywall assembly (5/8" Type X) between the ADU and the main house, moisture-resistant board in kitchens and baths, and sound isolation rated to STC 50. Solis Interiors hangs, fire-tapes, and finishes ADU drywall across Lacey, helping homeowners and general contractors pass their City of Lacey pre-rock fire inspection the first time. In 2026, complete ADU drywall packages in Thurston County range from $3,400 to $7,800.
Why ADUs Are Booming in Lacey
Living right here in Hawks Prairie, we see new backyard cottages and garage conversions going up every week along Carpenter Road, College Street, and throughout the Lacey Urban Growth Area.
Washington State's HB 1337 made it much easier for Lacey homeowners to build up to two ADUs on their property to create rental income or space for family.
Framing the walls is only half the battle. When you convert a garage or build a standalone mother-in-law suite, the City of Lacey building inspector looks closely at your drywall—especially your fire barriers, soundproofing, and plumbing penetrations.
What drywall fire separation is required between an attached ADU and the main home in Lacey?
An attached ADU must be separated from the primary home by a 1-hour fire-resistance rated wall and ceiling assembly under IRC Section R302.3 and City of Lacey Municipal Code (LMC 16.51), which requires 5/8-inch ASTM C1396 Type X fire-rated drywall hung horizontally with staggered joints and full fire-taping across all shared walls and ceilings.
That is the code on paper. On an actual jobsite, passing the pre-rock inspection comes down to the details:
- Intumescent Fire-Stop Caulking: Every electrical wire penetration, plumbing stack, and HVAC register cutting through the 5/8" Type X board must be sealed with certified red fire-stop sealant. If an inspector sees an open gap around a pipe, you fail the inspection.
- Staggered Seams: The seams on the ADU side cannot line up with seams on the house side. We stagger all joints by at least 16 inches so fire cannot find a direct path through the wall.
- Double-Layer Garage Ceilings: If your ADU sits above a garage or workshop, code mandates Type X drywall on the ceiling to keep exhaust fumes and vehicle fire hazards out of the living space.
How do you soundproof an ADU party wall so tenants have complete privacy?
Under IBC Section 1206, walls separating dwelling units must achieve a minimum Sound Transmission Class (STC) of 50, which requires breaking the mechanical connection between rooms using metal resilient sound channels (RC-1), dense mineral wool insulation, and 5/8" drywall.
Here is what cheap handymen and DIY builders miss on the job:
- Back-to-Back Electrical Outlets: If an electrician puts an outlet box in the ADU bedroom directly opposite the outlet in the main house, sound pours right through the plastic boxes like an open window. Solis Interiors wraps the back of every box in heavy moldable acoustic putty pads before hanging board.
- Perimeter Caulking: We leave a 1/4-inch gap along the floor and ceiling perimeters and pack it with permanently flexible acoustic sealant. Sound travels through air gaps just like water; sealing the perimeter stops flanking noise completely.
Which drywall boards belong in ADU kitchens, bathrooms, and living areas?
| ADU Room / Location | Specified Drywall Material | Why It Is Required |
|---|---|---|
| Shared Demising Party Wall & Ceiling | 5/8" ASTM C1396 Type X | Mandatory 1-hour fire separation under IRC Section R302.3. |
| ADU Bathrooms & Kitchen Backsplashes | ASTM C1396 Moisture- & Mold-Resistant | Mold-inhibiting core protects against shower steam in small footprints. |
| Tile Shower Stalls & Wet Enclosures | ASTM C1325 Cement Backer Board | Solid cementitious base that never softens behind tile grout. |
| Living Areas & Bedrooms | 1/2" Standard Gypsum (or 5/8" for Sound) | Standard wall finish, taped and finished to GA-214 Level 4 or 5. |
2026 ADU Drywall Pricing in Lacey & Thurston County
We believe in honest, upfront pricing with zero hidden allowances. Here is what typical ADU drywall projects cost in Lacey:
| ADU Scope & Size | 2026 Lacey Cost Range | What We Include in the Job |
|---|---|---|
| Garage Conversion ADU (400 - 600 sq. ft.) | $3,400 - $5,200 | 5/8" Type X fire ceiling, mold-resistant bath board, 3-pass taping, texture match, primer |
| Detached Backyard Cottage / DADU (650 - 900 sq. ft.) | $4,800 - $7,800 | Full hang and finish (walls + vaulted ceilings), Level 4 smooth or orange peel, PVA primer |
| Acoustic Party Wall Soundproofing Package | +$0.85 - $1.65 / sq. ft. | Resilient channels (RC-1), mineral wool sound batts, perimeter acoustic caulking |
| Architectural GA-214 Level 5 Monolithic Finish | +$0.95 - $1.50 / sq. ft. | 100% full-surface skim coat for open-concept floorplans and raking light |
Why Lacey Homeowners & Builders Partner with Solis Interiors
When you are building an ADU, you cannot afford to have a drywall crew disappear for three weeks or leave sloppy seams that hold up your painter.
- Locally Based in Hawks Prairie: We are 10 minutes away from any jobsite in Lacey.
- Pre-Rock Inspection Ready: We install all fire-stopping, blocking, and Type X boards to exact city code so your inspector signs off without delays.
- Dustless HEPA Sanding: We use commercial vacuum extractors so your new ADU stays clean and ready for trim.
- Turnkey Drywall & Painting: We can take your ADU from bare framing all the way through two coats of finish paint under one clear contract.
Authoritative Building Codes & Trade References
- Washington State Legislature: HB 1337: Accessory Dwelling Unit Regulations
- City of Lacey Municipal Code (LMC): LMC Chapter 16.51: Accessory Dwelling Units
- International Residential Code (IRC): IRC Section R302: Fire-Resistant Construction
- Gypsum Association: GA-600: Fire Resistance and Sound Control Design Manual
Get a Straightforward ADU Drywall Estimate in Lacey
Building an ADU or converting a garage in Lacey or Thurston County? Text your architectural plans or cell photos to Jesus Solis, and we will give you an itemized number within 24 hours.