Hawthorne ADU Construction Attached to the House
Hawthorne ADUs built onto the back of an older house, with a rated wall between the homes, a joined roof and the original eave line left easy to read.
Could the unit be built onto the back of our house? Attaching can suit a Hawthorne lot, and it leaves more of the yard open than a detached building would. The unit shares a wall with the main house, gets its own outside door and rooms of its own, and its roof has to meet the old one cleanly. We start by opening the rear wall's lowest boards to see what the new framing will tie into.
Where new framing meets old
The new floor has to line up with the old one, so a house raised over a crawlspace gets a unit framed on a matching raised floor, with its own footing poured beside the old foundation. The two are tied with hardware an engineer specifies. Up top, the new roof is set a little below the old eave or butts into the house's wall under flashing, so the original roofline stays legible from the yard. Insetting the new walls a few inches from the old corners keeps the house's corner boards in view.
A rated wall and the city's limits
Two homes sharing a wall need a fire-resistant separation between them, typically a 1-hour assembly with every pipe and wire hole sealed, and that wall also takes insulation for sound. State law limits an attached unit to half the main house's floor area, though a city must still allow a unit of a protected minimum size. The City of Hawthorne checks both on the drawings, along with setbacks and the unit's entry. The main house gives up only the stretch of rear wall the unit covers, and its windows on other sides stay as built.
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ADU Construction in Hawthorne: common questions
Being attached shortens every run. A feeder from the house panel reaches the unit's subpanel through a short length of conduit, the water branch can tap the house supply under the floor, and the drain may join the house's waste line before it leaves for the street, once a camera shows the old pipe is sound. A separate meter stays possible if the existing service is small.
Yes, and a detached garage gives a second path if attaching does not suit the house. Slab, studs and rafters are judged against what a home has to carry, the car opening is closed in around a door and windows, and the main house stays untouched apart from the utility connections.
For an attached unit the match has to be exact where new meets old. Siding courses run straight across the joint at the same exposure, the new roof takes the house's pitch and covering, and the tops of the new windows sit level with the old ones. A slim inset or a corner board marks where the house ends, so its first outline remains visible.
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