While discussing this answer I made to a recent question concerning the Bugbear's Long Limbed trait and how the reach bonus affects the character's other reach-related effects, this question naturally rose. Since both sides seem plausible for now, I decided to ask away as a separate question.
On one side, for player character attacks, the only mention of reach I could find in the books were the player character's reach, which seems to be considered as a stat owned by the player character and separate from the attacks one may take during combat.
On the other hand, a monster's attacking actions are described with the following :
Certain creatures […] have melee attacks with a greater reach than 5 feet, as noted in their descriptions.
This could imply that a creature's melee attack action could have its own reach property. While this may support the idea that an attack may have its own reach property, potentially separated from the creature's reach stat, the argument that a monster's offensive actions are different from a player character's Attack action also makes sense.
So, the question is the following: does a player character's attack, made using the Attack action, have its own reach, or is any reach used by this attack the player character's own reach?
A good answer will have citations from the books showing either definite, or in the lack thereof, tentative proof of the most plausible output to this question. Bonus points if both sides of the debate are covered.
Best Answer
Your every attack has its own reach
Reach of an attack is always based on the reach of the character that’s making the attack, it doesn’t exist independently just as the attack doesn’t, it’s always made by someone. It doesn’t have to be the same as character’s general reach though and there are features that affect the reach of individual attacks, suggesting that each attack has its own reach.
To back it up, let’s have a look at a few features that increase character’s reach and how they’re worded:
As can be seen, none of those features simply increase character’s reach, they all very specifically refer to the attack itself. This suggests quite strongly that attacks do have their own reaches.
For example, let’s take the first one, the Reach property of a weapon. It lets you add 5 feet to the attack’s reach when attacking with it, so if you attack with, say, a Halberd on your turn, then drop it on the ground and in your next turn attack with a Dagger, your first attack will have a reach of 10 feet but the second one will only have 5 feet.