In your enforcement of the rules of D&D 5th edition as supported by the text in the PHB and the Sage Advice document, do you require the assassin to pre-declare their action before initiative is rolled.
Initiative isnt rolled unless someone declares a hostile act. Hostile
acts are the domain of the combat sequence, are performed on your turn and in initiative order.
Seeing as the Assasin in this example is hiding in a bush, and the Orc is standing 30' away twiddling his thumbs and unaware of the Assassin, its doubtful that the Orc is going to be the one to get hostilities under way.
It goes:
Bob: OK, so this is the place my target is supposed to be, I peer in the warehouse doorway. What do I see?
DM: Yeah see a the warehouse is full of boxes and crates scattered about ad-hoc. Really cluttered. The roof is about 20' above you. Its quite dark in here.
Bob: It's OK, im a half orc and have darkvision. Can I hear anything? I'll stand quietly outside the doorway for a minute listening for any noise.
DM: Roll perception.
Bob: 13. Anything?
DM: Yes. You can just make out a muffled cough. It seems to be coming from the middle of the warehouse.
Bob. Ill silently enter the warehouse, moving behind crates as I do towards the centre of the room.
DM: Roll stealth.
Bob: I got a 16.
DM: (Compares steath check with guards passive perception. Notes Bob succeeded). OK Bob, your assasin sneaks to the middle of the warehouse. Peering over a crate, you notice a lone human. He wears chainmail and has his shield slung on his back. He's currently having a smoke and looks more intersted in the porn magazine he reads than his job. In front of him you see a trapdoor. Its currently open.
Bob: Is this my target?
DM: No. This looks like some kind of hireling.
BOB: Can I sneak past him and down the trapdoor?
DM: He's right in front of it at the moment, but distracted with his magazine, so you can with disdvantage on your stealth check.
Bob: Hmmmm.... damn. Screw that - OK, I'll draw my handcrossbow and shoot him.
[AT THIS STAGE THE GAME SWITCHES FROM NARRATIVE MODE TO COMBAT MODE]
DM Roll initiative. He's surprised.
Bob: Damn... a 9.
DM: 10! He wins!
Your crossbow makes a distinctive creak as you crank it back and his head snaps up from his magazine; the cigarette falling from his mouth. He quickly fumbles for his sword with no idea where the sound was coming from. It's now your turn Bob, resolve your attack. You have advantage as you are attacking from hiding.
Bob: Rolls a 7 and a 21. [The 21 hits, deals hand crossbow damage, sneak attack and Dex]. Is he still up?
DM: Yes, but your bolt is solidly embedded in his shoulder [unbeknown to the player, the guard is on 1 HP]
Bob: Damn! Is he looking at me?
DM: No, he was distracted by his porn magazine. Now he's staring in shock at the crossbow bolt in his shoulder with a fair amount of blood running down his chest already.
Bob: Ok, I' ll move to behind another crate and use cunning action to hide as a bonus action.
DM: OK, roll stealth..
etc
Honest question here: Where in the PHB does it say this? I'm not being a smart-aleck here. I can see that initiative is rolled at the start of combat. That combat is a clash between two sides. That surprise is determined by the DM. That characters get actions on their turn (unless surprised) and that movement can be taken before and/or after an action. That there are a number of actions characters can take. Nowhere do I see it say "The ONLY way combat can be initiatied is when someone declares a hostile action."
And underneath those rules you can see the rules for surprise. They are lacking in 'when surprise ends' granted.
Combat rounds and the attack action doesnt start after an attack,
as an attack can only happen during combat. It starts when hostilities are declared (by either the player or the DM). Then all hell breaks loose.
Well this Sage Advice's "clarification" (smells like a stealth errata to me) changes those rules significantly.
No, it doesnt. This is how combat in DnD has been since. well.. ever. Declaring a hostile act only commences the combat sequence. It doesnt grant you a free round of combat.
Otherwise players are just gonna be interrupting you halfway though your monster descriptions with:
DM: OK Bob, in front of you is a 30 x 30 room with an exit on the east wall. In front of you, you see an Hobgoli..
BOB: I ATTACK!
DM: Roll to hit. (should be roll initiative)
Or:
DM: The Lizard folk shaman thanks you for your assistance and asks you if you kept your promise not to loot the kings body.
Bob: I ATTACK!
DM: Roll to hit. (should be roll initiative)
Or:
DM: The cute little puppi...
Bob: I ATTACK!
DM: Roll to hit.
Etc.