One shared wall,
a second legal home on your San Francisco lot.

The second two separate dwellings sit on either side of a wall, it stops being ordinary framing. Code wants a rated separation — usually a one-hour fire assembly running floor to roof, the right gypsum layering, fireblocking at every penetration, plus a sound-rated cavity so a tenant in a Sunset flat isn't living inside your TV. This is the part that surprises people. SF housing is old. On a 1920s Richmond Edwardian we've opened a shared wall expecting clean framing and found knob-and-tube wiring, shiplap sheathing and balloon framing that won't carry a one-hour rating as-is and runs fire straight up to the attic. So the existing wall usually needs work too. We draw the assembly on the DBI set and build it to the listed spec.

The reason an attached ADU pencils cheaper than a detached one is built into the name. You tap the foundation, the PG&E service and the water line already running to your house instead of trenching across a lot that, in this city, you probably don't even have. One honest caveat: at 750 sq ft the City can require a separate water meter, and PG&E may want a service upgrade before a second all-electric kitchen's load — so we size the unit knowing where those lines sit. An attached unit also fits a far tighter side yard than a detached one, which is the whole reason it works on a 25-foot SF lot. Bay Area numbers are a different planet — detached units routinely run $400K to $750K-plus here — but the high rents are exactly why the math still works.