ADU Questions · 2026
What Is an Attached ADU?
An attached ADU is an accessory dwelling unit that shares a wall with, or is directly connected to, the primary residence. It’s built as an addition to the existing house rather than as a freestanding structure in the yard, which is what separates it from a detached ADU (fully standalone) and a JADU (a small unit created entirely within the existing home’s walls).
The short version: An attached ADU is physically connected to the main house, a detached ADU stands alone, and a JADU is carved out of the existing house’s interior. Attached ADUs get a taller height allowance under state law, up to 25 feet, because they’re treated more like a home addition.
Attached vs. detached vs. JADU
All three are legal ADU types under California law, but they work differently in practice.
| Attached ADU | Detached ADU | JADU | |
|---|---|---|---|
| Location | Shares a wall with the main house | Freestanding, separate structure | Within the existing home’s walls |
| Max height | Up to 25 ft | 16 to 20 ft depending on location | N/A, it’s interior space |
| Max size | Up to 1,200 sq ft (local caps may apply) | Up to 1,200 sq ft (local caps may apply) | Up to 500 sq ft |
| Kitchen | Full kitchen | Full kitchen | Efficiency kitchen allowed |
| Construction impact on main house | Requires tying into the existing structure | Independent of the existing structure | Interior renovation |
An attached ADU is legally its own dwelling unit, with its own kitchen and bathroom, it’s just physically joined to the house rather than sitting apart from it. That’s different from a JADU, which is a smaller conversion inside the home’s existing footprint, and different from a detached ADU, which is a fully separate building. For a deeper look at the JADU side of that comparison, see JADU vs. ADU.
Why attached ADUs get a taller height allowance
California’s statewide ADU law sets height minimums that cities must allow, and attached ADUs get more room to work with than detached ones. A detached ADU is guaranteed at least 16 feet everywhere, 18 feet within a half mile of a major transit stop (plus 2 more feet to match the main home’s roof pitch). An attached ADU, because it’s treated more like a standard home addition, can reach up to 25 feet.
That extra height matters if you’re thinking about a second-story addition connected to the main house, or a design that needs to match a taller roofline. It’s one of the few places where being attached is a real structural advantage over building detached.
Setbacks still generally apply to the new construction: 4 feet from side and rear property lines, with the front setback following the main home’s existing zoning. A true conversion within an already-existing structure can qualify for a 0 foot setback, but a new attached addition is usually treated as new construction for setback purposes. Your city’s specific standards, and how they apply to your lot, are exactly what a feasibility study confirms before you commit to a design.
The trade-off: construction disruption
The catch with building attached is that construction happens against, and often into, your existing house. Crews are cutting into shared walls, tying into existing roofing and utilities, and working directly adjacent to where you live. That typically means more disruption to daily life during the build, and a more complex, more site-dependent construction process than a standalone structure.
A detached ADU sidesteps that entirely. It’s built as its own structure, on its own foundation, with no need to open up or tie into the existing home. For homeowners who want a second dwelling without months of construction crews working through their walls, detached is usually the more livable path.
Why Framework First builds detached, not attached
Framework First builds detached, factory-built ADUs, not attached additions. About 97% of each home is built in our Salinas factory, then crane-delivered to your property in a single day and set on a pre-poured foundation. That approach only works with a detached structure, and it’s a deliberate trade-off in your favor: because the home is built independently in the factory while your foundation work happens on-site, your existing house stays untouched and undisturbed through the entire build.
That means no crews cutting into your walls, no months of your home life interrupted by an attached construction project, and a delivery window of 4 to 9 months depending on the model, instead of an open-ended timeline tied to how complex the tie-in work turns out to be. Every one of our 14 models, from 405 to 1,200 square feet, is a fully independent, detached home. Browse them at /models/ or see the full build process at /process/.
Frequently asked questions
Is an attached ADU the same as a home addition?
Not quite. A home addition typically becomes part of the main house, sharing the same address, ownership, and often the same systems. An attached ADU is a legally separate dwelling unit, with its own kitchen and bathroom, that happens to share a wall with the house. For a full comparison, see ADU vs. home addition.
Can an attached ADU be as large as a detached one?
Generally yes. State law allows both attached and detached ADUs up to 1,200 square feet, though your city’s local ordinance may cap the size lower in either case. A feasibility study will confirm the exact number for your specific lot and zoning.
Does an attached ADU need its own foundation?
It’s typically built off the existing foundation or tied into it, since it’s connected to the main structure. That’s part of what makes attached construction more site-dependent than a detached ADU, which sits on its own independent foundation.
Why does the height limit matter if I’m not building tall?
Even if you don’t plan to max out the height, the allowance affects roof design, ceiling height inside the unit, and how well the ADU’s roofline can match the main house. An attached ADU’s taller allowance, up to 25 feet, gives more design flexibility than the 16 to 20 foot range detached ADUs are guaranteed.
Is a detached ADU always the better choice?
Not always, it depends on your goals. An attached ADU can make sense if you specifically want the unit physically connected to the main house. But for most homeowners building a rental, a space for family, or an independent second home, a detached ADU avoids disrupting the existing house and gives you a fully separate structure, which is why it’s the model Framework First builds.
Curious what a detached, factory-built ADU could look like on your property? Start a free feasibility study or browse our models.