Gray Beach, down a cave
This house is one of 10 winners of 2012's spring Dream House Design contest, submitted by the player Pablityzz from Xantera.
It is the biggest house in-game. This is the only house of the ungoverned city Gray Beach. To reach it, you need access to Gray Island and special (intangible) underwater equipment (i.e. completed mission 1 of the Liquid Black Quest).
Before April 15, 2019, this house's rent was 21940 gold per month.