HOW COSTAL WIZARDS PLAY BX D&D: House Rules From 5e Designers

Sidney Sime

I was looking in my "Shared With Me" folder on my Google Drive and came across what appears to be a file by that Mike Mearls outlining his house rules for BX D&D- one of his favorite editions. I reproduced them below.

The context for the rules seems to be tricks picked up from 3e/4e experience and applied to BX D&D rather than house rules that arose at the time he might have played BX. I think.

Points of interest:

  • Save throws as just d20 DC checks
  • Theif skills are DC check but with a bonus based on the BX % divided by 5. So "open locks" in BX starts at 15%, by these house rules it would be a d20 DC check with a +3 bonus
  • Starting HP = Max HP + half CON score (which on average should allow 1-2 extra hits)
  • Looks like race-as-class is split to "race" and "class" but with a few bonuses. For instance, you could be a dwarf magic user which would allow you to increase your HD size one step (d4 --> d6), use "find/remove traps" as a thief, and have infravision
It is interesting that the changes are centered around "classic problems": 
  • Standardizing resolution mechanics (d20 4 eva) and removing % rolls and I'm sure X-in-6
  • Increasing survivability by increasing HD size and HP amount
  • Increasing player options by expanding access to magic, weapons, and starting character combinations (16 combinations vs. 7 BX classes) 

Mike Mearls 1981 D&D House Rules

Ability Mods: We use the 3e/4e convention (+1 or -1 per 2 above/below 10)

Saving Throws: These are ability checks, DC determined by DM

Attacks: Ability check, plus a class-based bonus

Fighter: +1 every odd level

Cleric/Thief: +1 at level 3, 6, 9, 12, 15, 18

Magic-User: No bonus

Thief Abilities: These work a bit differently.

  • Open Locks, Find or Remove Traps, Pick Pockets, Hide in Shadows are all things only thieves can do. Hide in shadows is literally that - hide in situations where other people can’t. The thief makes a check with a bonus equal to the % listed on page B8 divided by 5.
  • Move Silently, Climb Sheer Surfaces, Hear Noise are all things that anyone can try. The rogue has the advantage when he tries any of them.

Advantage: Roll an extra d20, take the highest result. If you get advantage more than once, take an extra die.

Disadvantage: Roll an extra die, but take the lowest. If you have advantage, disadvantage zaps one die per instance of disadvantage.

Hit Points: Upgrade everyone by one die type, you get maximum hit points at level 1 + half your Constitution score.

Hit Dice: You can use your hit dice to heal. You get hit dice equal to your total HD, spend them when resting, each die gets a bonus equal to your Con modifier.


  • Dwarves: Increase class’s hit die by one size. Infravision 60 feet. Can use Find or Remove Traps in underground locations.
  • Elves: Can alternate between magic-user and any other class, have infravision 60 feet.
  • Halflings: Can Hide in Shadows as a thief, get a +2 bonus to AC, but use an HD one step smaller and can’t use two-handed weapons.
  • Humans: Gain a +1 bonus to any two stats, or +2 to one stat (maximum 18).

Weapons: d4, d6, d8, or d12, class access based on die size

Fighter: All

Cleric: d6, bludgeoning d8

Thief: d6

Magic-User: d4

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