crooked-v parent
Also, other options that used to be more efficient to build multiple homes at once, like row houses or duplexes/triplexes, are generally directly or indirectly illegal to build in most US cities now. Row houses and brownstones were the cheap-and-fast housing option once upon a time; they're now rare and expensive because in most places nobody's allowed to put up a new one.
I've seen sixplexes around here, but unless they're apartment buildings (which have better designs anyway) they're relatively undesirable.
A duplex saves you some coin, but each additional house you stick on the side saves you less, and the desirability goes down. People don't like duplex/triplex/sixplex living.