Squidward lives in a well crafted chiseled stone head. Mr. Krabs has quite the house of his own in a big anchor, Sandy has a custom made dome for her specifically to live in and Spongebob is quite fond of his pineapple, as strange of a house that it is.
Considering this, it's reasonable to infer that housing access, even custom built homes, are quite easy to acquire in undersea society, yet Patrick lives underneath a rock with furniture made out of sand. Does he live like this by choice? Does he like his rock? Or is this considered substandard housing by Bikini Bottom standards, and Patrick is currently deprived of something better? Will Patrick get better housing once the UKPCBB (United Krabby Patty Cooks of Bikini Bottom) stages a proletarian revolution to overthrow the Ruling Krabbs, or will Patrick find himself still content with a rock?
Spongebob Squarepants, as a program, is vignette of American life in the 1990s.
That takes place under the sea.
Mr. Krabs, owning the Krusty Krab, represents a sneering, self-aware version the bourgeoisie from the bourgeois perspective. He is motivated soley by money, and lives in an immense house that reflects his status.
Spongebob represents an idealized version of the proletariat from the bourgeois perspective. He is tireless, and works for very little money, but still maintains a decent standard of living despite this, maintaining several hobbies, furthering his education, and even owning a home; this is to trick the audience into thinking homeownership is possible.
Patrick represents the lumpenproletariat from the bourgeois perspective. He's portrayed as foolish, and only ever engages in rest and recreational activities, two things that the bourgeois abhor for working people to do. Because of this, his housing needs are not adequately met; he literally lives under a rock.
Plankton represents the evil scientist class; it goes without saying his significance to the sociopolitical themes of the show.