- The GM is very familiar with the rules, and there are a few places he wants to improve them. This is common in popular games, and you can find some common house rules for popular games online. For instance, a common change to 3rd edition d20 games is to make character death occur at [Level+10] negative hit points. Another example: Critical fumble house rules have been cropping up in just about every game.
- The game has supplemental and optional rules, and the GM wants to make clear which ones are being used. This is a house rule even though it's part of the original game. A storyteller running a LARP using Laws of the Night may say "we are not using any rules out of Dark Epics," for instance.
- The game has inconsistencies or unclear spots, and the GM wants to clarify the problem. This is more common in games that didn't get a lot of playtesting (that would be most games).
- The GM wants to emphasize elements of the setting specific to the plot he wants to run. This commonly means character creation house rules. A Cyberpunk 2020 GM might give extra points to starting characters to represent that they're experienced characters. Or he might say everyone has to play a Cop because they represent a team of detectives.
Here are some tips for designing house rules.
I. Make sure they work.
Duh. This sounds simple, but you'd be surprised how often me and other GMs have produced dysfunctional house rules! Because of the size and format of LARP, this is more common in LARP -- especially delicately balanced elysium-style LARPs. Remember, your house rules are a comment on the system. You're saying "these are better than the original, for my game." If your house rules don't work, the players will be pretty confused. Your rules might
...work, but be onerous
...work, but unbalance the player characters
...work fine at low levels, but break down later
...work fine in tabletop play but break down in LARP
...work fine, but be hard to keep track of
II. Keep It Simple (KISS)
Your house rules should always be as simple as possible. Adding a house rule complicates the game, so keep the rule itself simple to avoid overcomplicating things.
Rule of thumb: If your rule makes the game easier to understand, keep it. If not, weigh its benefits against the added complexity. Some of my favorite house rules (below) are more complex than the original, but they're all still pretty simple. Notice how I was able to write them without pages of explanation or tables.
III. If you wind up with lots of house rules, just rewrite the whole thing.
A good rule of thumb is this: Take just the rules part of your houserules (sometimes GMs package setting house rules along with setting fluff information). If your house rules can all fit on one page, you're OK. If you have to shift the margins and print on the back of the page, you should consider rewriting the whole game. If you can't manage to fit them on one piece of paper without sacrificing readability, you really should write your own game rules. It's not that hard to write the system for a game based on an existing game, especially compared to writing three pages of house rules and making sure they all work.
IV. Borrow with impunity
If game A has a bad rule for something and game B has a good rule for the same thing, it's usually better to import the good rule than to make your own new rule. Why? First, it may have been playtested, and it was almost certainly edited for clarity. That saves you a lot of work!. Second, you already know the rule and its practical effects from the other game. Third, your players might know it from the other game, as well..
If another GM has used a house rule for the system, you should seriously consider borrowing that as well, if you feel the need to change the rule at all. The other GM's game serves as a playtest, you can get feedback from his players, and he will be able to explain the rule in a clear way.
You can't get rid of house rules once you implement them. It's not fair. The exception is if the rule is broken -- then you have to get rid of it. It's always hard to change the rules mid-game. When you do, you have to give players a chance to change their characters to account for the rule changes. ("No more critical fumble rules? I built my character to force enemies to fumble!") This is another reason why you need to make damn sure your rule works!
V. Keep it until it breaks
To wrap up, here are my top five house rules!
- d20v3.x: Critical Skills: A roll of 1 on a skill check is -10, a roll of 20 is 30. This avoids characters always succeeding on a 1, except for those who hyperspecialize; and it lets anyone trained have a chance of hitting over-30 DCs. I don't generally play with Epic skill rules unless a magic spell is involved, though. I would not use this rule for a game with a tighter skill system (e.g. 4e D&D) where guaranteed success (succeed even on a 1) has been accounted for (can a Paragon fighter with a running start jump over a 10' wide chasm every time? Yes!).
- Mind's Eye Theater (original): No Trait Loss: This is really simple: Characters don't lose physical, mental or social traits when they lose challenges. Any time a character would have to "risk an extra trait" it's equal to -1 on ties (this makes negative traits worse at first, but less harmful in the long run; and that's OK because they add to roleplay). Any time a character would lose a trait other than losing a contested challenge (say, due to magic), they take a -1 penalty (per "trait lost") to that category for the rest of the night, and they can clear all such penalties in a trait category once per night by spending 1 Willpower. Easy peasy. This also lets you use scrip for all consumable traits (tickets, tokens, etc.) without breaking the bank.
- D&D 4e: Stuns that don't suck: I haven't actually tried this, but I intend to! It's the Online DM's houserule for Stun (shout out!). Change the Stunned condition to "A character who is stunned is dazed and cannot attack.." For monsters, it's basically the same as being stunned, but with moving (so with a good defender, it's exactly the same as being Stunned). For a player, being stun-locked for 3 rounds is about 20 minutes of doing absolutely nothing in a dramatic scene. It sucks. The difference between doing absolutely nothing and doing almost nothing is literally infinite.
- Cyberpunk 2020: Multiclassing: Characters can take up to 15 total points in career skills, from up to 3 careers. Characters may still only have 1 career skill above 7. (I also tended to limit starting characters to 6 or lower in career skills, and I rewrote the character advancement system entirely, but those aren't in my top 5.)
- d20v3.x: Less Random Death: Player Characters die at negative [level+10] hit points. Variability in damage increases dramatically at higher levels, leading to one-hit kills of PCs, even PCs with over half their HP left, far too often without this rule. It's almost ridiculous that this rule was not the official rule from the start. I've been running 3rd ed D&D for so long that it seems obvious, but I can see how it was not noticed in playtest, if most playtesting happened at lower levels (3.5 edition D&D is beautiful below level 9, especially below level 7).
What are your favorite house rules?