Adding detached accessory dwelling units (ADUs) in California is no longer just for single-family lots. State law now lets multifamily owners build up to two detached ADUs, provided each structure meets basic height, setback, and fire‐safety rules. This case study walks through a high-end duplex ADU in Glendale—two mirror-image, 1,000 square-foot homes rising behind an existing apartment building. Each residence delivers three bedrooms and three full baths across two stories, with nine-foot ceilings, upscale finishes, and a ground-floor guest suite.
Glendale sits in the heart of the Los Angeles rental market, where vacancy rates are chronically low and demand for family-sized units is high. The property owner already held a four-plex on a deep, 7,500 sq ft lot zoned R-2250 (medium‐density residential). By tucking a detached, two-unit ADU at the rear—outside the shared yard and within the city’s four-foot side- and rear-setback rule—the owner could add 2,000 sq ft of rentable space without triggering a discretionary review. Because each unit stayed below the 1,200 sq ft ADU size cap in state law, the project qualified for Glendale’s by-right ministerial approval process.
State standards allow multifamily ADUs up to 18 feet tall (plus a modest roof pitch) when they sit at least four feet from side and rear property lines. The design team chose a 22 × 48 foot footprint—long enough to fit two units split by a rated demising wall yet narrow enough to clear mature jacaranda trees near the fence. A low-slope gable roof hides a compact attic for mechanical ducts and keeps the overall ridge below the 18-foot threshold. Fiber-cement clapboard, charcoal standing-seam metal accents, and black-framed windows match newer infill projects a few blocks away on Brand Boulevard, helping the ADU blend into the neighborhood fabric.
A private entry for each home faces a new concrete walkway that wraps the main building, keeping residents out of the shared driveway. Inside, the first floor offers an open great room anchored by an L-shaped kitchen. A guest bedroom in the rear corner has direct access to a full bath—perfect for aging parents or a roommate who prefers no stairs. The second level holds two more bedrooms, each with an ensuite bath. The larger of the two claims the title of primary suite thanks to a five-piece bathroom, walk-in closet, and a nine-foot tray ceiling that makes the 1,000 sq ft footprint feel generous.
Placing plumbing stacks back-to-back at the midline keeps rough-in costs predictable, and stacking bathrooms upstairs helps with fire separation at the common wall. Each home meets the California Residential Code’s egress-window rule by using 4' × 4' casement units in every bedroom—an approach that balances daylight, ventilation, and emergency escape in one move.
Glendale’s renter pool skews toward young professionals and extended families seeking “move-in ready” homes with a touch of luxury. To meet that demand, the owner selected the following finish palette:
California Title 24 sets minimum performance thresholds for envelope insulation, windows, and equipment. The duplex complies with an R-15 blown-in fiberglass wall cavity, an R-38 high-density roof deck, and U-0.28 low-E windows. A dedicated heat‐pump water heater per unit, placed in a first-floor utility closet, captures indoor heat in summer, reducing cooling loads, while a 3.2 kW rooftop solar array offsets common-area electric use under Glendale Water & Power’s net-metering program.
Because the lot already hosted a multifamily building, the application followed California’s “two detached ADUs” path. Key steps included:
Plan-review times vary, but Glendale typically returns first corrections on ADUs in six to eight weeks. A complete resubmittal often clears within the following month, making a four-to-five-month total entitlement window realistic when drawings arrive “permit ready.”
Site work began with trenching for a 4-inch sewer lateral and a joint utility trench for electric, water, and telecom. A slab-on-grade foundation with deepened perimeter footings provided a cost-effective base. Framing used kiln-dried Douglas fir, and fire-rated Type X sheathing lined the party wall to meet one-hour separation. Because the demising wall runs from footing to roof sheathing, noise and fire protection meet code for town-house separation, even though the structure is still classed as a single ADU building under California law.
Rough trades closed in under eight weeks, followed by insulation, drywall, and finish carpentry. Setting cabinets and tile in both homes concurrently kept subcontractors on site and reduced mobilization costs. Total build time finished at just over seven months, including rainy-season delays.
High-finish ADUs in Los Angeles County often land in the $300–$350 per-square-foot range by the time design, permits, and trenching are folded in. At 2,000 finished square feet, the duplex budget came in near the midpoint of that band. Because rental demand in Glendale for new three-bedroom units sits north of $4,000 per month, the duplex is projected to gross roughly $96,000 a year. Even after maintenance, insurance, and property-tax carry, the net operating income is strong enough to support favorable refinancing terms, unlocking equity for future projects.
Building a detached duplex ADU behind an existing apartment can feel complex, but three practices minimize surprises:
By following state ADU laws and Glendale’s streamlined permit path, this project converted underused rear‐lot space into two full luxury homes. The result is extra housing in a supply-constrained city, a stronger balance sheet for the owner, and a case study other multifamily landlords can mirror—right down to the last square foot.
Explore your ADU possibilities with Gather. We provide ADU planning, design, permitting, and construction management services in California, Georgia, Washington, Oregon and beyond!
