Houserule: Inspiration Die

An inspiration die provides a player with the means to alter d20 rolls in dramatic situations, reflecting the luck that can change crushing failure into heroic success. Your DM can choose to give you an inspiration die for a variety of reasons. Typically, DMs award it when you play out your personality traits or otherwise portray your character in a compelling way.
You can never have more than one inspiration die at a time. When you first earn inspiration it starts as a d4. If the DM grants you inspiration again, you don’t gain another die; instead, the size of the inspiration die increases for that check, from 1d4 to 1d6, or 1d6 to 1d8. If you have a 1d8 inspiration die, you cannot earn more inspiration until you use the die.
You can spend an inspiration die to improve the result of an attack roll, a skill check, an ability check, a level check, or a saving throw. When you spend an inspiration die, you add the result to your d20 roll to help you meet or exceed the target number for the roll. You can declare that you are spending an inspiration die after you have already rolled the d20, but you must do so before the Dungeon Master reveals the result of your roll (whether the attack roll or check or saving throw succeeded or failed).
You can’t use an inspiration die on a skill check or ability check when you are taking 10 or taking 20.