Attached ADU Additions in Calabasas

When an Attached ADU Is the Better Fit in Calabasas

Many Calabasas homes sit on pads graded flat for the house and little else. Behind the patio the ground falls toward a canyon or climbs toward a slope bank. A detached cottage on that land needs retaining walls and a new foundation. An addition built off the existing pad avoids most of that, because it extends a level surface the original builder already created.

Attached units also suit yards that are already spoken for. If the rear yard holds a pool, a sport court or mature oaks you want to keep, the side yard or the area beside the garage may be the only buildable space. We look at which wall of the house has room, where the existing plumbing runs and how a separate entry can reach the unit without crossing your private outdoor space.

Attached ADU Height, Size and Setbacks Under State Law

California treats attached and detached units differently on height. An attached ADU may reach 25 feet or the height limit of the zone, whichever is lower, and does not need to exceed two stories. That gives more room than the one-story, 16-foot rule the City has applied to detached units, which is why a second-level ADU in Calabasas often makes more sense when it connects to the house.

Size limits follow the state floor: any local maximum must allow at least 850 square feet, or 1,000 for a unit with more than one bedroom. The 4-foot side and rear setbacks protected for units up to 800 square feet apply here too. Front setbacks are a local matter, and on corner lots they can decide which side of the house is usable. We map all of these on the site plan before drawing the unit.

HOA Design Review for Attached ADUs in Calabasas Communities

An addition changes how the house looks from the street, so associations in places like The Oaks and Calabasas Park Estates pay close attention. State law limits what they can do. Under Civil Code sections 714.3 and 4751, CC&Rs that ban or unreasonably restrict an ADU on a single-family lot are void, but an HOA may still enforce reasonable, objective standards on materials, colors and rooflines.

The City's own rule asking ADUs to conform to the design of the main home was one of the standards HCD flagged as subjective in its May 2026 letter. For an attached unit, matching the house is usually the natural choice anyway: same roof pitch, same stucco or siding, same window style. We prepare drawings that satisfy the association's checklist and the City's plan check together, so neither review sends the design back for basic changes.

Building an Attached ADU Next to an Occupied Calabasas Home

Two dwellings sharing a wall need a fire-rated separation, sound insulation and, usually, separate heating and cooling. We detail the shared wall or floor as a rated assembly, seal penetrations, and route ducts so the units do not share air. On a parcel mapped Very High for fire hazard, the new exterior walls, vents and roof must meet wildland-urban interface standards even though the old part of the house may predate them.

Most owners stay in the house while the addition goes up. We sequence the work so the tie-in to the existing structure, which means opening the wall, connecting the roof and extending plumbing, happens late and quickly. Before that, the new foundation, framing and roof go up behind a temporary barrier, and your kitchen and bathrooms stay in service.

Utilities and Systems for an Attached ADU

Short distances help. An attached unit can often draw from the house's sewer lateral and water line a few feet away, rather than trenching across the yard. Las Virgenes Municipal Water District serves Calabasas, and its connection requirements apply if a new meter or larger service is needed. On the electrical side, a load calculation shows whether the panel can feed a second kitchen and HVAC system, and whether Southern California Edison must upgrade the service.