PHB p. 168: Polearm Master
You gain the following benefits:
- When you take the Attack action and attack with only a glaive, halberd, or quarterstaff, you can use a bonus action to make a melee attack with the opposite end of the weapon. This attack uses the same ability modifier as the primary attack. The weapon's damage die for this attack is a d4, and it deals bludgeoning damage. […]
Do I use the same reach as the primary weapon as well?
Best Answer
Yes, your reach with the bonus attack is the same as your reach with the regular attack. The Reach weapon property states that:
So whenever you make an attack with a Reach weapon, your reach is 5 feet more than usual. There's no restrictions placed on how you get that attack. As long as you're using the right weapon, you get the extra reach.
In this specific case, you're attacking with the opposite end of the weapon, but it's still the same weapon. Your reach will be 10ft if it's a Reach weapon, or 5ft otherwise.