Attached ADU Construction in Alhambra
Attached Alhambra ADUs with their own heat pump, water heater and subpanel, a fire-rated wall shared with the house and a foundation joined to the old one.
Will an attached unit run off the house's heating and water heater? For an Alhambra ADU built onto the back of the home, we plan separate equipment: a heat pump, a water heater and an electrical subpanel of its own. The mechanical code does not let one dwelling draw return air from another, and a duct crossing the rated wall between them would need fire protection. What the two homes do share is that wall.
Building the shared wall
Two dwellings joined under the residential code need a fire-rated separation between them, carried from the foundation to the underside of the roof deck. The wall is built to a tested assembly, usually layers of Type X gypsum board on each face, with insulation in the cavity that also cuts sound. Pipes and wires that cross it pass through sleeves sealed with listed firestop material, and electrical boxes on opposite faces are offset or wrapped with putty pads. Where the existing rear wall becomes the separation, its finishes come off on the new side so the rated layers can be added and inspected.
Equipment, power and review
A ductless heat pump with heads in the main rooms keeps all the ductwork on the unit's side. A heat pump water heater sits in a closet with enough air around it, or a compact electric model serves a small layout. The subpanel takes its feeder from the main panel after a load calculation, and a separate meter is possible if the utility agrees. The new footing meets the old one with dowels or a separation joint, as the engineer directs, and if the yard falls behind the house, raised floor framing brings the new floor level with the old. Alhambra plan check reviews the separation, the tie-in and the equipment together.
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Built onto the house, the unit has to meet the existing floor line, which is simplest where the ground holds level along the rear wall. If an Alhambra yard drops away from the house, raised framing can carry the difference. Set apart as a detached unit, it still needs room inside the setbacks and a sewer branch with fall.
A qualifying single-story design is within its reach, up to a floor area of 500 square feet. On an Alhambra unit joined to the house, the wall sections detail the rated separation and its penetrations, and the city reviewer reads them before anything else. Structural engineering for the tie-in, and the construction, are quoted once we have opened the old footing.
The rated separation wall with its firestopping, and a second set of heating and hot water equipment for the unit. Exposing and doweling into the existing footing takes careful digging next to the house. Where the ground drops behind the rear wall, raised floor framing adds lumber and a few steps outside the unit door.
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