[RPG] Does surprise begin during the planning of an ambush


From the rules in 5e, I understand that a combat encounter starts with the DM determining surprise, then setting starting positions, and rolling initiative. Then round 1 starts. Surprised participants can't perform any actions or bonus actions on their first turn. They would, of course, have the ability to take reactions after the end of their individual turns in the first round.

When one side has foreknowledge of the enemy through divination, good spying, scouting, or other means, and wants to use that to set up an ambush, has the combat already started? Should initiative be rolled now? Are they forced to act at this point if they want to exploit the surprise condition (for example, to use the Assassinate action)?

For me it makes sense that the combat starts then, and also to roll initiative, since many participants start doing combat actions like spell buffing. But I see no reason why the surprise condition should end then. The way I see, it the party could prepare for an unlimited number of rounds before the other side becomes aware of a threat. Is the surprise condition maintained until then?

Best Answer

This is what the PHB says about scenarios that demonstrate one group surprising another:

A band of adventurers sneaks up on a bandit camp, springing from the trees to attack them. A gelatinous cube glides down a dungeon passage, unnoticed by the adventurers until the cube engulfs one of them. In these situations, one side of the battle gains surprise over the other.

So it's being shown to us that the encounter wherein creatures can be considered surprised begins when one group takes hostile action against the other group that has not noticed the first. And note that surprise can only happen when you are in the first round of the encounter, which means that, if you want to take advantage of surprise, Initiative cannot be rolled before the start of the ambush.

It also doesn't make sense to give someone the surprised status when nothing is surprising them (ie, the players are planning, not ambushing). When you are surprised, you cannot move. Does the act of the players planning somehow paralyze the other group? No, it shouldn't.