House rules are small fan created additions and replacements to core rules in a rules set and this tag should be used when there are locally-created rules at the core of the question. In most cases, they are not officially sanctioned by the designer, in contrast to optional rules. For creation of substantial new content or rules subsystems, use the [homebrew] tag instead.
A rules set strives to be complete, but no set of rules covers every circumstance. Rules that are fan-driven and implemented unofficially are termed house rules. Contrast with optional-rules, which are officially created additional rules.
House-Rules are for new or modified rules. Whether it's creating rules to cover situations that aren't handled by RAW or customizing existing rules ("we don't use the RAW grappling rules, because they're stupid"), house rules modify the framework of the system.
House rules can completely replace certain parts of the rules set, or merely add to the existing rules to make them more complex, complete, and/or well-rounded. Many times they are created in response to a need that the designers overlooked. But just as often, they are created to take the game in a direction fitting to a certain campaign.
This tag should be used to discuss implementation, effects, and/or changes engendered by the use of house rules in the basic rules set.
GMs make table-specific rulings about how the rules work all the time; these aren't generally considered house rules unless they're codified and used consistently.
- For questions about substantial new material (e.g., whole classes, monsters, etc.) or new subsystems (e.g., adding detailed crafting or chase rules), use the homebrew tag instead.