No matter how well they are written or how many times they are edited, no rules for a tabletop role playing game (RPG) will ever be entirely perfect. Given that the purpose of the rules for an RPG is to simulate an alternate reality, it makes sense that no set of rules are ever perfect. The rules that govern reality are something that has not fully been understood, despite millennia of research.
Despite the lack of perfect rules, millions of people play role playing games every year. And, the solution to the imperfect rules is designed into every single game published. A person, known as the game master (GM), is the final arbiter for every given situation. Thus, a single person exerts essentially God-like power over the events that occur while playing an RPG.
Ironically, despite the fact that every RPG explicitly places the final arbitration in the hands of a GM, the rules for most role playing games are written to avoid the necessity of the GM making an arbitrary decision. Complex rules dictate the procedure for evaluating the results of actions ranging from combat to social interactions to auto repair. As much as possible, these rules are intended to be inclusive, though results vary greatly on how well various game rules succeed at that goal.
Unfortunately, because the rules generally attempt to be inclusive and because rules tend to be complex, there are often unforeseen interactions in the rules that create unexpected and unintended results. For example, in 3rd edition Dungeons & Dragons there were items that weighed nothing, which meant characters could carry infinite numbers of them. Creative players were able to abuse this rule to perform unintended and unrealistic actions, within the game world.
As a GM, when you come across a rule interaction like this, you have two choices. You can either permit it to happen and accept the unintended consequences or you can create a house rule. A house rule is a rule created by a GM that either modifies a published rule of the game or exists besides the published rules of the game. House rules, once instituted by a GM are followed by every player of the game though only apply to the individual game being run by the GM.
House rules are often a point of contention among players and a GM. Many players feel that since a RPG is a game, it should be played only using the published rules, without modification. In fact, some players will absolutely refuse to play in a game that uses any house rules at all. In contrast, some GMs will heavily house rule the game they are running, sometimes to the point that the game rules are barely recognizable.
For the most part, house rules that simply modify an unexpected interaction of multiple rules, referred to as a corner case situation, are common and rarely create friction between players and the GM. But, house rules can also be used to simply modify the game rules for any reason the GM feels like. For example, in a Vampire: The Masquerade game a GM may decide they feel that Potence is too powerful a discipline and house rule that it costs twice as much experience points to purchase as other disciplines. This kind of house rule exists to support the world view of the GM and may also exist to create a specific unique feel to the game. But, it is also the kind of house rule that can annoy players.
If you wish to add house rules to a game that you are a GM for, you should always publicize all of your house rules in advance to starting your game. Players much prefer to be forewarned about house rules than to discover them after they have started to play. It helps to create trust between you and your players, something which is absolutely necessary to successfully run a game.
In addition, you should be careful about creating a house rule specifically for the purpose of making a particular player’s character more or less useful than normal. If you make a specific character more powerful, it will look like favoritism to the other players. If you make a specific character less powerful, the player of that character is likely to feel slighted. In a situation where you feel it is necessary, either to preserve game balance or make the game more enjoyable, you should be very open with everyone involved about why you are making the house rule and you should be willing to negotiate with the player directly affected.
Finally, when you make house rules, be prepared to defend your reasons for making them. It is your game and you are the final arbiter, but your players deserve to know the reasons why you made a specific house rule, even if they don’t agree with those reasons. For example, in a Shadowrun game you might create a house rule that armor piercing ammo costs 10 times the listed price. You should then explain to the players that your reason is you want to portray a world where armor is highly common and effective and this rule helps that portrayal. The players may not agree with your approach, but at least the explanation will take some of the sting out of the decision.
Nearly every single GM makes a few house rules. These often range from slight changes to character creation to entirely changing or removing some classes from a game. There is nothing inherently wrong with using house rules and in fact, most games encourage them. The only true obstacle to using house rules is recalcitrant players. A little discussion will usually solve any problems. Finally, just remember that you always have the option to modify or discard your house rules, especially if they don’t work as well as you hoped.