Inspired by a chat with Sotek, we both decided to compile our own "Do's and Don'ts" of creating homebrewing rules (and to an extent, background).
First, I shall begin with a bit of a disclaimer; these are personal opinions, but they are born from dealing with other people's creations. They are, to an extent, the rules we both work to when producing things.
Second, a question posed on your behalf - why should you listen to us? Well, I for one have been writing House Rules since before I actually understood how 40K really worked, so I've made all these mistakes and more. Since then, however, I've refined my talents and have thrown my hand at producing more rules than I care to recall. I've produced entire Codices, both alone and with others. Prime examples include helping out Dra'Tsuich-Novae with his Corsair Codex, or the work that is still ongoing with Sotek's Alliance of Tet Krom army lists.
In short, I've made rules. Moreover, I've played with those rules, and I've had some great games with them. So have the people I played, more importantly.
Now I'll give the floor to Sotek, and then I'll follow up with my own rules.

Rainman's Cardinal Sin of Homebrewing:The Stupid (n):- A. Blatantly overpowered or underpriced homebrew units/rules.
- B. Stuff that, if you look deep down and speak honestly, you included because you listened too much to "It'd Be Really Frickin' Cool" and not enough to "But Is It Justified, Really?" .
- C. Things that handwave away core game mechanics because you don't like it when your army is negatively affected by them. (Primary offender: Leadership and Morale!)
- D. Things that require excessive and painfully convoluted 'justification' for their inclusion. (e.g. "My DIY Chapter's assault companies all use Reaver Jetbikes! Everyone's totally cool with this!").
Rainman's Cardinal Rules of Homebrew:- 1. Realize that you, yes you, will inevitably end up creating The Stupid from time to time, and probably won't (want to) realize it.
- 2. Have a good buddy on hand to run your ideas by, because they can point out when you didn't notice you were making The Stupid.
- 3. Listen to your friend when they point out The Stupid. Speaking from experience, this can be the hardest step, but it is one of the most important.
- 4. Don't be That Guy.
- 5. DON'T be That Guy.
- 6. When you do a playtest game for your new homebrew, it's a good idea let your opponent be the one to use the homebrew rules. I guarantee you there is no better way to realize where The Stupid has cropped up in your homebrew than being on the ugly end of it in a match.
Rainman's Cardinal Disclaimers of Homebrew:- 1. Please keep in mind I am not being sanctimonious or mocking here, and you're not entitled to disregard me on that basis.
- 2. Why? I came up with these because at some point, I made each and every one of the mistakes I caution against. Those DIY marines using eldar jetbikes? Yeah.
- 3. Just wait 'till you see the Cardinal Rules Wargamer's had to come up with as the result of my goof-ups! But that's the point. No one is exempt.
- 4. There is no Disclaimer #4. Have fun, and happy homebrewing!
Wargamer's Guide to House Rules:- 1. Learn your background! Seriously, nothing sets my teeth on edge like a House Rule unit or character that is blatantly out of sync with how everyone and everything else in their faction works. If you're making an Imperial Guard character, he should feel like he belongs in the Guard. If you make a Tau character, he should act like Tau do. It's hard to give hard and fast rules on what is and is not 'right' for a given faction, but this is where extensive reading helps - if a Chaos character is a frothing lunatic, it is better to make them Khorne than Tzeench, for example.
- 2. Adhere to KISS - "Keep It Simple, Stupid!" In simple terms, the KISS philosophy is "don't add new things if you don't need to." If you're making an Ork Warboss, do you really need a unique statline, or will the existing one do? Does he really need unique weapon options and/or special rules? Naturally, you're probably looking to make something special, but aim to do it in the most straightforward way possible. For example, instead of inventing a 'resurrection' mechanic that can bring your character back from the dead, consider just using Eternal Warrior, Feel No Pain or It Will Not Die to represent their talent of escaping death.
- 3. Accept criticism. This includes being told you are flat-out wrong in what you are doing. On this site at least (I can't say for everywhere else) people who seem to be finding fault with your creations are doing it because there is fault, and they are bringing it up so you can change it. Even when you're good at making house rules like I am there are things you miss (I've applied bonuses and forgotten to charge for them, or put typos in statlines, or failed to include rules for wargear...), so if nothing else think of your critics as proof-readers.
- 4. Act on criticism. You don't need to totally redo your creation or bow to everyone's demands, but keep them in mind and have another look. If people are saying your rules are overpowered or under-costed, tweak them. If people are saying your idea is ridiculous and doesn't fit with the background, sit down and have a look at how to approach your idea from a new angle. Remember, there is very little you cannot pull off in rules or background - it's just a question of finding the right way to do it.
- 5. Don't use "But GW do it!" as an excuse! One of the major reasons for House Rules existing at all is because GW screw things up so badly that people can no-longer stomach playing the game with official rules. You must also remember another important fact: whilst I cannot technically stop my opponent from bringing an official unit I don't like, I can easily stop them fielding a house rule unit. When you need your opponent's permission to field the unit / army / whatever in the first place, you need to be sure that they will allow it on the table. That means making it balanced enough that they won't reject it on sight. Remember, it's better to be underpowered than overpowered.
- 6. Look at your creation from the other side of the table. You might love your new Land Raider variant, but how would you feel if you had to play against it? A prime example of bad rules writing shows up when the creator does not want to have to take on their own creation, or only does so when they can tailor their list to counter their house rule'd unit. If it's something you don't mind appearing unannounced in your opponent's list, then odds are you're on the right track.
No doubt we'll have more 'rules' of homebrew popping up, but this should be a good set to start with. If anyone else has their own rules do share with us.
