Does surprise begin during the planning of an ambush?

Question

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?

Answer 1

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.

Answer 2

There are factors to combat depending on whether zero, one, or both sides are trying to be stealthy:

No one trying to be stealthy:

1. DM informs players that they stumble upon each other
2. If both sides don't see a threat or are otherwise friendly, combat doesn't occur.
3. If one side instantly becomes hostile, or after some sort of conversation hostility ensues, everyone rolls initiative and combat starts!
4. No one is surprised. Everyone rolls initiative and is allowed a reaction from the beginning.

One side trying to be stealthy:

For the sake of this example, I'll call the side actively using stealth the Stealthy side and the side not using stealth as the Passive side.

1. All Stealthy members that have made stealth rolls. These will be compared to passive perception of the Passive team.
2. If any passive perception is able to see one or more of Stealthy team, and determines them as a threat and/or starts the alarm, combat starts! Move to step 4
3. If none of the Passive teams can see a Stealthy, the Stealthy team can listen in / observe as long as they want until they choose to leave (no combat), choose to open up to a conversation (which would cause them to lose their stealth advantage, but I wanted to mention this that you aren't forced into combat) are spotted (potentially starting combat) or they choose to start the attack.
4. Once/if combat starts:
   • Everyone rolls initiative
   • None of team Stealth is surprised
   • Members of team Passive who did not see any Stealth members are all surprised.
   • Round one: Surprised Passive people take no action on their turn, but they still have a turn (in which they take no action, because they are surprised) on their initiative number. After each surprised individual's turn is over, they are able to make a reaction.
   • Round two: no one is surprised, everyone can act and react.

Both sides trying to be stealthy:

1. Everyone has a stealth roll.
2. If no stealth rolls are beaten by any passive perception (or active perception if anyone decides to actively look), no one perceives anyone and there is no combat.
3. If anyone or multiple people are able to spot someone, they have the option to begin combat / raise the alarm, or try to start a conversation.
4. If combat begins:
   • Everyone rolls initiative
   • All characters who perceived no one are surprised for the first round
   • Round one: Surprised people take no action on their turn, but they still have a turn (in which they take no action, because they are surprised) on their initiative number. After each surprised individual's turn is over, they are able to make a reaction.
   • Round two: no one is surprised, everyone can act and react.

Note on Preparation

If one side has done some pre-scouting (perhaps with an exceptionally stealthy individual or individuals), they could come back to their people and tell them to "buff up" or something, if there are such spells. However, this is outside of combat/initiative/surprise, which would be determined if after this preparation, the team with foreknowledge decided to engage the other side.

Answer 3

Planning and preparation can occur before initiative is rolled, but it could ruin surprise.

Note that sides are not surprised in the default rule set. Individuals are, based primarily upon perception rolls or passive perception, possibly mitigated by hiding or stealth. A GM can certainly roll initiative for a group of identical characters, such as a band of orcs, in the interest of speeding play.

Until the defending party is made aware of the attackers, either through perception or an initial attack, any character who starts combat unaware of the attackers rolls initiative, but lose their ability to act on their first turn. Until the moment they roll initiative, the potential for being surprised can change.

If a band of adventurers comes across a party of orcs, until the orcs become aware of them, the adventures are able to act freely. Note that the casting spells with verbal components, moving around in armor, or discussing plans will almost certainly yield more opportunities for the orcs to perceive them and the adventurers to lose the possibility of surprise.

So yes, until initiative is rolled, characters may prepare as they please. It is likely that the more they prepare, the less likely they will be able to gain surprise.