Do you need to let the DM know when you are multiclassing?

Question

I am planning to multi class and am curious if there is anything that says that I have to let my DM know prior to the game.

Answer 1

Yes

And to clarify, this isn't specific to multiclassing. Generally, while you can normally expect things in the PHB to be available, run your character concept by your DM before you start; Some DMs limit options, have variant rules on various races and classes, and they might run the game in a setting where some things you're planning to do just won't work (classic example would be the Ranger "Hey it's cool that you want to be a badass Underdark ranger and all that, but you should probably know that no part of this campaign will take place there. In fact, in this setting the Underdark doesn't even exist!").

For example, at my table I generally don't allow multiclassing if it's a game for new players, but I'll allow some race and class options that a new players might not be aware of, and I'll try to have a talk with them to make sure that the type of character they're trying to create will work (and have something to do) in the campaign. It's fine if you want to surprise other players with what you're trying to do, but trying to surprise the DM (without good reason) is just asking for trouble.

Answer 2

Yes.

The PHB p. 163 is explicit in that you have to have the GM's permission. Prior to the game in this context means that you have to get permission from your DM before you use it in game. If you decide during your campaign or a session of the game and ask them to multiclass before it happens in-game and they give you permission, then that works.

Even within your class and race, you have options to fine-tune what your character can do. But this chapter is for players who—with the DM’s permission—want to go a step further.

Answer 3

Yes

Both page 163 of the PHB and page 58 of the Basic Rules say this about multiclassing:

Multiclassing lets you combine classes together, and feats are special options you can choose instead of increasing your ability scores as you gain levels. Your DM decides whether these options are available in a campaign.

This means that you must tell your DM if you decide you want to multiclass. You need their permission to allow for multiclassing in the game as the DM may decide it is too unbalanced or have some other reason they do not want to allow for multiclassing. As the decision is in the DM’s hands (as are most decisions as per “Rule 0”), there is no guarantee it will be allowed in your game.

Answer 4

You should, although you don't have to

Do you need to let the DM know when you are multiclassing?

It is unclear, what you're asking. Yes, you have to tell your DM what your choice was at the very moment of leveling up. However, if you mean something like saying "I am planning to multiclass on 5th level" before the game even starts — you can do that, but you don't have to.

I am planning to multi class and am curious if there is anything that says that I have to let my DM know prior to the game.

If you have plans considering your character development, you should collaborate with the DM to make the game more interesting, but you still don't have to. There is no rule that forbids you from changing your mind.

I just was unsure of what needed to be disclosed.

There no strict rules in the book, however, there is no point in hiding information from the DM. Player should talk to their DM about character development and specifically possibly using optional rules that aren't yet defined as usable at the table (thanks, @NautArch)