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.

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+ years
Rear 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

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.

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