I got this after finishing the main storyline and playing only a few of the bonus levels. There is no in-game progress counter, but if it hasn't unlocked for you by the end of the game, it should shortly.
If it hasn't unlocked for you and you just want to grind it out, replay level 1. Build cabins on the empty lots and restart.
Houses, as well as Markets, Sawmills, Banks, and Dovecotes do count toward this achievement. (Confirmed by Hotdogmcgee.)