One shared wall, a full
second home on your Berkeley lot.

Your shared wall stops being an ordinary interior wall the moment two separate dwellings sit on either side of it. Code wants a rated separation. Usually that's a one-hour fire assembly running floor to roof — the right gypsum layers, fireblocking at every penetration, and a sound-rated cavity so a UC Berkeley grad student isn't living inside your TV. This is the part that surprises owners. Pull the finish off a 1920s brown-shingle in North Berkeley and you'll often find old knob-and-tube and board sheathing that won't carry the rating as-is. So the existing wall usually needs work too, not just the new side. We draw the assembly on the plans and build it to the listed spec.

The reason an attached ADU usually pencils cheaper than a detached one is right in the name. You tap the foundation, the PG&E service and the water line already running to your house, instead of trenching across the whole lot to a freestanding cottage. That matters more here than almost anywhere — Bay Area detached units commonly run $400K to $750K and up, and the shared wall is how you knock a real chunk off that. One honest caveat: at 750 sq ft the City can require a separate water meter, and PG&E may want a panel upgrade before a second all-electric kitchen's load. An attached unit also fits a tighter side yard than a detached one needs, which is the whole game on a narrow Elmwood or West Berkeley parcel.