In (tabletop rpg) practices, "house rules" are modifications to the core rules; homebrews-or-whatever are more complete & complex than that, and most groups didn't think of them as house rules. (They'd agree that they are house rules. It's just that the term falls short of being specific enough.)
"House rules" were "we use the Arduin charts for critical hits" or "dice that fall off the table are always re-rolled" or "this game makes magic too easy; all spell costs are doubled."
A binder with rules for taking your D&D characters into WWII (with the premise that the Nazi occult experiments opened a rift between the worlds), or detailed writeups of settings & characters from a particular series of books, or a D&D-superheroes crossover game, are more elaborate than most gamers' (of the kind I played with) concepts of "house rules." Some of those eventually turned into complete games; a lot never got fleshed out beyond the core rules, so players who were there for the process understood the game, but there was no description that could be handed off to new people.
Re: homebrews
"House rules" were "we use the Arduin charts for critical hits" or "dice that fall off the table are always re-rolled" or "this game makes magic too easy; all spell costs are doubled."
A binder with rules for taking your D&D characters into WWII (with the premise that the Nazi occult experiments opened a rift between the worlds), or detailed writeups of settings & characters from a particular series of books, or a D&D-superheroes crossover game, are more elaborate than most gamers' (of the kind I played with) concepts of "house rules." Some of those eventually turned into complete games; a lot never got fleshed out beyond the core rules, so players who were there for the process understood the game, but there was no description that could be handed off to new people.