Attached ADU Construction.
Share one wall with the main house, save backyard, save cost.
An attached ADU shares at least one wall with the primary residence. The shared wall cuts foundation and roof cost by 15–25% versus a detached unit and preserves more backyard, which is why attached ADUs win on small or narrow LA and Bay Area lots.
Typical range
$240K – $420K all-in (2026)
Per unit
$400 – $580 / sqft installed
Timeline
8–12 months total: 2–4 months design + permit, 3–5 months on-site, 4–6 weeks finals.
The short version.
An attached ADU adds new conditioned floor area to an existing single-family home through a shared structural wall. State law caps attached ADUs at 50% of the existing house's floor area or 1,200 sqft, whichever is less, with a 16 ft height limit. Most attached ADUs we build land between 450 and 800 sqft.
Because the unit shares a wall, the demising assembly becomes the most important detail of the entire project: a 1-hour fire-rated wall (CBC §711.2.4.3) with continuous draft-stopping at the roof line, and STC-50 acoustic separation. Get this wall right and the units feel like two separate houses; get it wrong and every footstep transmits.
Foundation, roof, siding, and one exterior wall come for free from the main house. What's added is a small slab extension, a new exterior wall envelope on three sides, the demising wall, a full MEP system, and the interior finishes of a small dwelling.
What you can actually pick.
Side addition
Pros — Easiest framing tie-in, shortest construction window, lowest cost per sqft.
Cons — Reduces side-yard, can push into setback on narrow lots.
$240K–$380K all-in50+ yearsRear addition
Pros — Preserves street-facing curb appeal, easier privacy separation for the ADU entry.
Cons — Eats backyard, often requires rebuilding the existing rear roof slope.
$260K–$420K all-in50+ years
What we deliver.
- Existing-conditions survey of the shared wall — framing, sheathing, electrical paths
- Structural review of the existing foundation and connection to the new slab
- Plan set covering both the new ADU and modifications to the existing house
- 1-hour fire-rated demising wall assembly per CBC §711.2.4.3
- STC-50 acoustic separation (resilient channel + 5/8 type-X each side + insulation)
- Slab extension keyed and doweled into the existing foundation
- New exterior walls, weather barrier, siding to match (or contrast with) the main house
- Roof tie-in — new rafters sistered to existing, continuous draft-stopping at the line
- Independent MEP — sub-panel, separate plumbing branch, dedicated HVAC
- Separate exterior entrance with its own address number per USPS rules
The code parts most owners miss.
- The demising wall must be a 1-hour fire-rated assembly with continuous draft-stopping (CBC §711.2.4.3 + §711.2.4.5).
- An attached ADU is capped at 50% of existing house floor area OR 1,200 sqft (Gov Code §65852.2).
- If the parent house has fire sprinklers, the ADU must too. If not, the ADU is exempt under CRC R313.2.
- Separate exterior entrance is required — interior connection allowed only with a 1-hour fire door.
- Title 24 envelope verification covers the new exterior walls only, not the shared wall.
Why getting this right pays off.
Attached ADUs deliver the lowest cost-per-sqft of any new-construction ADU type, because the existing house provides one full wall, often part of the roof, and the foundation tie-in point. Savings versus detached typically run $40K–$80K.
The acoustic and fire detailing of the shared wall is the single biggest determinant of whether the attached ADU rents and lives well long-term. Done right, neither side hears the other. Done wrong, the homeowner regrets the project within a year.
What goes wrong — and how to avoid it.
- Treating the shared wall as a simple interior wall — fails fire and acoustic code
- Re-using the main-house panel without a load calc — overloads the service
- Mismatching roof slope at the tie-in without a saddle flashing — chronic leak
- Forgetting the second address number — USPS rejects mail, delays first month of rent
- Not separating HVAC ductwork — cross-air movement defeats the acoustic wall
- Skipping draft-stopping at the wall-to-roof line — code violation, fire-spread risk
After we hand you the keys.
- Inspect the roof tie-in saddle and valley flashing annually
- Re-caulk the joint between new and existing siding every 5–7 years
- Confirm the demising-wall fire door (if installed) closes and latches monthly
- Service the dedicated HVAC unit annually
- Watch for differential settlement at the slab join — hairline cracks acceptable, gaps not
Attached ADU — the cost-engineered path.
An attached ADU shares one or two walls with the main house. That single connection point saves $30-80K vs detached construction — but it forces a fire-separation wall, a shared utility decision, and tighter code coordination than most owners expect.
US market size
Attached ADUs ~22% of CA ADU permits, second behind detached (~50%).
California reality
CRC R302 1-hour fire-rated assembly required at any shared wall between primary residence + ADU. CALGreen mandates separate ADU water heater, electrical sub-panel, and ventilation.
The manufacturers behind the spec sheet.
- Our default
USG / CertainTeed / National Gypsum
Chicago / Malvern PA / Charlotte NC.
Market — Three companies make 95% of US gypsum board.
Product — USG SHEETROCK Brand FIRECODE, CertainTeed FireGuard, National Gypsum Gold Bond Fire-Shield.
In California — 5/8" Type X is the CRC-mandated fire-rated drywall for attached ADU separation wall.
All three are interchangeable on UL listings; pick by lumberyard availability.
- Our default
Owens Corning / Johns Manville / CertainTeed
Toledo OH / Denver CO / Malvern PA.
Market — Top 3 US insulation makers.
Product — OC Thermafiber Mineral Wool, JM Mineral Wool, CertainTeed Sustainable Mineral Wool.
In California — Mineral wool is the spec for fire + sound-rated assemblies — R-15 to R-30 in standard cavities.
Outperforms fiberglass batts on both fire (Class A) and STC (sound transmission class) — meaningful for sleeping-side walls.
- Spec on request
Pre-approved attached ADU plans (LADBS + AB 2221)
LADBS catalog + select architects.
Market — Limited but growing — 8+ attached plans in LADBS library.
Product — Modative, JR Design, Connect Homes attached options.
In California — Pre-approved attached plans cut permit timeline to 4-6 weeks vs 3-4 months custom.
Works when the lot + house geometry match a published plan; rare match rate (~30%).
Tier-by-tier — what you actually get.
Bump-out attached (≤300 sf)
$180K–$280K all-in
e.g. Small bedroom/bath addition + kitchen
JADU+ attached, in-law studio.
Standard attached 1BR (400-600 sf)
$270K–$420K all-in
e.g. Single-story side or rear bump
Long-term rental, multi-gen family.
Premium attached 1-2BR (700-1,200 sf)
$420K–$650K all-in
e.g. Two-story attached or garage-over
Premium rental, in-law suite with kitchen + laundry.
California distributors.
Ganahl Lumber / Dixieline (SoCal)
SoCal lumberyards.
Fire-rated drywall, mineral wool, full framing.
Truitt & White (Bay Area)
Berkeley.
Premium attached ADU framing + finish materials.
What it costs this year.
5/8" Type X drywall 4x12
+6% YTD
≈$22 / sheet
Gypsum tied to oil/transport.
Roxul SAFE'n'SOUND batts (R-15)
+4% YTD
≈$1.85 / sqft
Mineral wool premium 25% over fiberglass.
1-hour rated door + frame (Steelcraft)
+5% YTD
≈$1,400 installed
Required at shared interior door if any.
What we tell owners — off the record.
An attached ADU costs less than detached because you don't pay for foundation, roof, exterior walls, or utility runs on one or two sides. The catch is the 1-hour fire-rated wall + separate meter detail — both cost ~$15-25K and surprise owners who didn't plan for them.
The fire-rated wall assembly is what makes attached ADU code-compliant: UL U305 or U419 — typically 2x6 studs, mineral wool insulation, 5/8" Type X drywall on both sides, all penetrations firestopped with Hilti CP 25WB+ or 3M FireDam. Cut a corner and the certificate of occupancy is denied.
Stacked attached ADUs (over garages, second story over kitchen) require structural review for the existing house framing. Roughly 40% of LA single-family homes built pre-1990 cannot carry the dead+live load without joist sistering — budget $8-22K for that work.
Shared-wall sound transmission is the #1 post-completion complaint we hear. Spec STC 50+ with double drywall + sound clips (Resilmount, RSIC-1) for any wall between primary + ADU sleeping areas.
What the brand reps won't tell you.
- 'Same panel' shared electrical is allowed by code but means the renter's bill is yours. Always require separate sub-panel + sub-meter for billing — the equipment cost is $1,200-1,800 and saves years of dispute.
- Attached ADUs trigger HOA review in many newer CA subdivisions. AB 670/671 mostly preempted HOA bans, but architectural standards can still apply — confirm before design.
- Most architects + contractors size attached ADUs at the maximum allowable square footage. Often a smaller footprint that doesn't pop the wall over a setback line saves $40-90K.
Our default spec
Default: UL U305 1-hour fire-rated shared wall (2x6 + mineral wool + 5/8" Type X both sides), separate 100A sub-panel with sub-meter, dedicated heat pump HVAC, dedicated heat pump water heater, separate code-compliant exit. STC 50+ wall assembly mandatory on shared sleeping-area walls.
In short.
- Is an attached ADU cheaper than a detached ADU?
- Yes — typically $40K–$80K less for comparable square footage. The savings come from the shared wall, shared foundation tie-in, and partial shared roof. The trade-off is less design freedom and a more involved tie-in.
- How big can an attached ADU be in California?
- Up to 1,200 sqft OR 50% of the existing house's floor area, whichever is smaller. A 1,400 sqft main house caps the ADU at 700 sqft. A 2,500 sqft main house allows the full 1,200 sqft.
- Will my home feel smaller after an attached ADU is added?
- Not if the demising wall is built correctly. A code-compliant 1-hour fire-rated assembly with resilient channel and STC-50 separation means you won't hear the ADU. The main house keeps its original floor plan untouched.
- Can I connect the attached ADU to the main house internally?
- Yes, but only through a 1-hour fire-rated door, and only if both spaces are owned by the same household. If the ADU will be rented to a third party, most owners install the door permanently sealed.
- Do I need to upgrade my electrical service for an attached ADU?
- Usually yes. A 1970s-era 100A or 125A panel almost always needs an upgrade to 200A to handle the ADU's load (60–100A) plus the main house plus any EV charging. Budget $4K–$9K.
- Does the attached ADU need a separate address?
- Yes — USPS and the local fire department require a unique address (e.g. 123A and 123B). The city assigns the new number during plan check.
- How long does an attached ADU take to build?
- 8–12 months total: 2–4 months for design and permit, 3–5 months on-site, 4–6 weeks for final tie-ins and inspections. Faster than detached because there's no slab from scratch.
- Can I add a second story as an attached ADU?
- Yes — a second-story addition can be permitted as an attached ADU under state law. It requires structural review of the existing foundation and ground-floor walls. Cost is typically $340K–$520K.
Keep reading.
Planning attached adu?
Send us the address and the scope. We'll come back with a line-item budget, a permit path, and a realistic schedule — before you spend on drawings.
Start a project →Attached Adu done right — priced from the last three we built.
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- Materials sourced from our standing trade accounts
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