Attached ADU Additions in Westlake Village

How an Attached ADU Works on a Westlake Village Home

An attached accessory dwelling unit shares at least one wall, and sometimes a roofline, with the main house. It has a separate exterior door, its own kitchen and bathroom, and no interior opening into your living space unless you want a lockable connecting door. From the street it can read as a modest addition, which helps in tracts where the HOA cares about consistent elevations.

Westlake homes from the late 1960s onward were often built as single-story ranch or split-level plans with room to extend to the side or rear. Adding the ADU along that edge keeps the backyard open and reuses the existing building pad, rather than carving a new one into the hillside.

When an Attached ADU Beats a Backyard Cottage in Westlake

The choice usually comes down to the lot. If the yard drops off steeply behind the house, as it does on many Southshore Hills and Three Springs streets, a detached unit would need its own engineered foundation and long utility runs down the slope. An addition sits on ground that is already graded and close to the existing sewer, water and electrical service.

Oaks are the other deciding factor. A big oak in the middle of a Lakeshore or First Neighborhood backyard can rule out a detached footprint once its protected root zone is mapped. Pushing the unit against the house often avoids the tree entirely.

Attached units also suit owners who want to stay close to the person living in the ADU, such as an aging parent. A short covered walk between doors is easier than crossing a yard at night.

Attached ADU Size, Height and Setback Limits

State law treats attached ADUs a little differently from detached ones. Where a city lacks a compliant ordinance, the state default allows an attached unit up to 50 percent of the existing home's floor area, but never less than 800 square feet. Height may go to 25 feet where the zone allows that much, and the unit does not have to exceed two stories. Side and rear setbacks can be as little as four feet.

Inside the City of Westlake Village, those state minimums sit alongside the standards in Municipal Code section 9.14.070. The state housing department has flagged that the City's most recent ordinance on file dates from 2022, so we check what the City applies today and design to whichever rule controls. On the Thousand Oaks side, the City of Thousand Oaks has its own ADU standards, which we apply to North Ranch and Southshore Hills parcels.

Tying an Attached ADU Into the Existing House

The hardest part of an attached unit is the connection. The new foundation has to meet the old one, the new roof has to shed water past the old roofline, and the common wall has to be built as a fire-rated, sound-insulated assembly. On older Westlake homes we also check whether the existing framing and foundation can take the new loads without reinforcement.

Utilities are simpler than for a detached unit because the runs are short. Still, a second kitchen and heat pump often mean upgrading the main electrical panel or adding a subpanel. We tap the house sewer line where the slope allows and add a separate water heater so each unit controls its own hot water. Sprinklers depend mostly on the house: if it has them, the addition usually will too, and the fire authority confirms this during plan check.

• Foundation tie-in and structural check of existing framing
• Roof transitions detailed to shed water cleanly
• Fire-rated, sound-insulated shared wall
• Panel capacity and separate water heater for the unit