My DM says that I can't multiclass but the rulebook says I can. What should I do?

This chapter defines two optional sets of rules for customizing your character: multiclassing and feats. 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.

The Player's Handbook specifies that multiclassing is an optional rule that the DM decides on if it's being used or not.

Regardless of what the Player's Handbook or any other book says, if your DM says a certain option is not allowed, or is restricted in some way, you can't just ignore that, regardless of what the results of a forum survey says. You should speak to your DM and come to a compromise, or if that's not possible, accept that your DM isn't allowing an option you want and either accept you can't use that option, or find a different game.

Davedamon is right. While the PHB and DMG have specific sections outlining the rules, the best thing to do is have an open conversation with your DM about the question. They may have a specific reason why the do or do not allow multiclassing in their game. Also, if you set out with an idea to multiclass from character creation and your DM doesn't allow it, ask them what you could do in order to try and get as close to that character as possible while staying within the boundaries set by the DM. I hope this helps!

Quote from AlexandraMelianne >>

My DM says that I can't multiclass but the rulebook says I can. What should I do?

As others have said, the rulebook says explicitly that this is optional

Furthermore, every word in the rulebook is optional, whether it says so or not. It's up to the table what to do. And up to the DM what to allow in his campaign.

Your poll is incorrect. Your choice is not whether to multiclass or not. Your DM has said no, so you CANNOT. Your choice is, keep playing in the campaign and follow the DM's rules, or quit the campaign. Arguing is not one of the choices, and multiclassing anyway definitely is not.

DM decides which rules are followed or not, when, when not, and most everything else.

A good DM is consistent and up front about things but even if they pull a switch their ruling is final.

not because they’re infallible, but because they are running the game.  Without the DM it’s a bunch of people wishing they had a game to play.  You have another DM available?  Lucky you, often there’s not one.  And if not, you sort of have to accept their rules.

Yes, open discussion is good. Coming to a mutual agreement is good.  But if at some point a decision must be made, it’s in the DM’s title that they’re the master.

I myself limited multiclassing initially in my campaign until people reached their first asi.  Mostly that was done because nearly all of them were new players and keeping things a little simpler made the game smoother.  Also avoided the pitfall of not understanding the limitations of multiclassing (like delaying feats and multiple attacks).  I am also a firm believer that you do not need to multiclass to have tons of fun. 

I have also limited multiclassing in my Roman Empire campaign. I only permit multiclassing if it is in character. Now, I actually suggested multiclassing to one of the players - he's playing a rogue who is a bounty hunter and former Roman soldier. So it would make sense that he has some fighter as well as some rogue skills, and I think he is planning on doing that. So far they just hit 2nd level, and I'm not sure what he will do for level 2.

In addition to it having to make in-character sense, it must be RPed if doing so would be warranted. Example: A fighter wants to become a fighter/wizard. OK, then you need to find a wizard character and RP learning from them. You don't just suddenly "become a wizard too." That would make no in character sense to me.

Finally, I do not allow 1-level "dips" just to get the level 1 features of a class. If you're going to multiclass, you're going to invest at least a few levels into the 2nd class. Otherwise, not allowed.

Every single rule in any version of Dungeons & Dragons (or any tabletop RPG), whether official, optional, in a core book, in a supplement, or anywhere else in the nerdverse where such things can be found are guidelines.

The only rules that matter and are in effect are those adhered to by your dungeon master. They have final say in what's allowed, not a designer, writer, or WotC employee. This is why things such as house rules exist - and there's millions of them out there. DMs take official core book rules, twist them, break them, or remove them entirely because it doesn't fit with the style of game they are running. Always remember a key point. Players should agree to playing in the game set forth by the dungeon master, under the DMs rules, not the rules in the books. 

Everything I said is facts. This has nothing to do with whether this is a good thing, bad thing, or otherwise. Should DMs change and remove official rules? Sure. Should DMs leave many things as written? Sure. Should you, as a player have the right to move on from a group or table because a DM is messin' with the rules too much? Sure.

It's all flexible. Just have fun.

I would like to add to all of this... to the OP, as some friendly advice. It is bad form to come onto this forum, and try to get the DMs on D&D Beyond to side with you and "overrule" your DM. As a player, you're supposed to abide by the rulings of the DM. If they upset you enough, leave the campaign. But don't come on here and appeal to us. As you can see from the many responses above, no one here is going to overrule your DM.

The answer is the same as should I get married.

As BioWizard points out, the only thing going to your DM with "DND Beyond users say I can Multiclass!" (though currently, your poll is overwhelmingly "NO, you can't") is going to do is create a lot of hurt feelings and possibly get you booted from the game.

The DM decides which of the optional rules are going to be used in his game. That's why they're distinctly marked as optional.

Now, yes. I know that the DM can change any rules that he wants, but the optional ones are just way clearly marked.

The better thing to do, to my mind, is present your case to your DM in a calm, reasonable manner. If they still say no, then it's no. Your choice then becomes: do I continue playing in this group?

Myself? If I was having fun and not multiclassing would still be fun (if not AS fun), then I'd just go, "Ok! When's the next session?!"

Either way, I hope you get this amicably resolved and continue throwing those bones!

Aside from multiclassing being an optional rule - if your DM says no then the answer is no. You can talk to them and see if a compromise can be reached but the final say is theirs - nobody else's.

Quote from BioWizard >>

I would like to add to all of this... to the OP, as some friendly advice. It is bad form to come onto this forum, and try to get the DMs on D&D Beyond to side with you and "overrule" your DM. As a player, you're supposed to abide by the rulings of the DM. If they upset you enough, leave the campaign. But don't come on here and appeal to us. As you can see from the many responses above, no one here is going to overrule your DM.

 I would like to add: I can see that you are both new to DnDBeyond, and D&D in general, AlexandraMeliannne, based on your join date and posts. I will also say that I understand the poll. For a lot of people new to D&D, there is a bit of a learning curve on the whole "rule based game" vs "anything and everything is possible game (and not possible)" aspect. You will see a lot of things in the books, both optional and not optional, and might have questions about them. I have played in a lot of games, from very RAW (Rules As Written) to RAI (Rule As Intended) to ROF (Rule Of Fun). All different, because of the DM. What I'm saying is: based on one of your previous posts that you have never been in a campaign before, I'm assuming that you're just looking for guidance on the whole "Why is there a rulebook when the DM just does whatever they want" situation. My advice is: use the other resources you have available to you, which is to ask questions about it online. Trust me, I wish I had a forum on DnDBeyond to ask questions when I started playing D&D. (In truth, I wish I just had internet to ask questions about the game or how my DM was running it)

In the old days, you could write letters to Gary Gygax and the people at TSR (original makers of D&D). And once, I even did.

I was in 8th grade. We were playing a level 4 or 5 adventure. It was just me, as player, and my best friend, as DM. I had a Wand of Lightning Bolts. I had read up on Lightning Bolts and knew how the spell worked. We were in a corridor, and approaching us were 2 wraiths. Well, the Lightning Bolt in those days was, I think, 10' across and something like 60' long. According to the rules, the bolt would reflect back off of any solid surface, such as a wall. The Wraiths were about 20' away, and the far wall was about 60'. So, I figured it was safe to fire the bolt, and I shot it down the hall at them. The wall was at the limit of the bolt's length so it could not reflect back at me.

My friend, the DM, ruled that the lightning bolt reflected off the wraiths (who in those days were semi-substantial like ghosts and not fully solid) and came back up the hallway, bathing the entire party. I argued with him. I pulled out the book and showed him what it said. He argued that the wraiths were solid enough to be that reflective surface. I insisted that was not what today we call "RAI" (rules as intended). No it does not say that a person or a monster is NOT a reflective surface, but surely if they had intended that, given the commonality of monsters in D&D, they'd have said so. The example they gave was a stone wall, not a wraith! And so we argued, for like 15 minutes, until he finally said, look, this is what happens. I grumbled about it and we continued (with my party almost dying as a result).

But I was still sure he was wrong, and I was kind of ticked about it. So like you, OP, I decided to appeal to a higher authority. No forum in those days, but I had the address of TSR. So I sat down with my blue Eraser Mate pen (yes, I still remember it) and hand-wrote out a page-long letter describing what I have above. I mailed it away to TSR, and then, I admit, forgot about it. 

About a month later I got a letter in the mail. At that age I hardly ever got letters addressed to me unless it was my birthday (i.e., cards), so I was surprised. I opened it up and found my original letter had been sent back with hand-written commentary, in red ink (like a teacher would do). On the front (first page) of the letter, the commenter had marked "--YES" next to my statement about lightning bolts bouncing off walls, and "--NO" off the comment about it bouncing off wraiths. Aah! I thought. I'm vindicated! Can't wait to show the DM!

Then I turned the paper over and on the back, below my signature, in red ink, were several very pointed sentences taking me to task. Even though the person marking up the letter agreed with my interpretation of the rules, he wrote back that I should not have argued with the DM. The DM's word is law during a session, and the most important thing is to keep the session moving and continue the play. Bogging down a session to argue with the DM is not acceptable. And although he didn't say it, the responding TSR employee heavily implied that I was being a poor sport for even sending this question to them -- because he knew I was doing exactly what this OP is doing: trying to get some "higher authority" to overrule the DM. He explained that in D&D, there is no higher authority than the DM -- period. 

In those days, even Gary Gygax, the writer of the rules, would have said this (as a kid, I used to imagine GG himself had written that response but as an adult, I highly doubt it). He wrote the rules but, he would have said, "Whatever your DM says happens is what happens, not what I say."  The rules are there to help you have a good session but the rule-writers are not at your table; the DM is. The DM is the one who has to make sure things keep moving and everyone has a good time, and there is a lot that goes into "having a good time." It's not just about "am I having fun now?" but what decisions the DM makes now that could make the game more, or less, fun in the future. 

So if we get back to multiclassing... that is an option rule because, depending on what the DM allows, it has the very real potential to unbalance the game with characters who are overpowered for their level. That's why, as I said above, I do not allow the "1 level dip." It's know to experienced D&D players that this is highly effective because a single level will not limit your main class much (few if any classes have "must have" abilities at level 20, so missing out on level 20 is usually not a big deal), and will often make a character far more versatile than a single-class character would be. Your DM, having noted this potential for unbalancing things, might have forbidden multiclassing period (instead of just 1 or 2 level "dip" the way I did). His or her goal is not to harm your character but to ensure the greatest possible fun for everyone, and you need balanced, non-overpowered characters, for that to happen on a consistent basis.

It is entirely fair to ask your DM, if there is a reason why multiclassing is not allowed. But whatever the answer is, you need to (like me with the lightning bolt) accept it and move on, not argue about it.

Give the feats a shot if the DM allows it. It's a different kind of advancement than multiclassing.

If not, still give the game a shot. Not being able to multiclass doesn't necessarily destroy a game. It will require you to re-strategize from this point forward, which is not a bad thing.

Yeah it's not like the game is miserable if you only play one class. Only playing one class is the default assumption.

Out of sheer curiosity, what are you trying to multiclass into?

I was going to respond to this yesterday but had some internet trouble and missed out. 

As everyone else has already pointed out, everything is optional and the DM can allow or disallow anything and everything. 

In the two Avernus campaigns I run, I don't allow multi-classing, because the party is on the first level of Hell trying to get out. They don't have time to study another class and nowhere to train for it if they did.

I've played in and ran campaigns that were no multiclassing, only humans, only humans aasimar and tieflings, no magic at all, no arcane casters, no divine casters, only kobolds, no money, etc. 

Your DM said no, so it's a no.

It looks like the general flow of things is that I should talk to my DM and try to make a compromise. Thank you for your insight. I'm not a very expierienced player so thank you for your opinions. Thank you!

In a certain sense, multi classing is binary - either you can or you cannot.  So compromise may not be available.  But perhaps ask him why he does not allow it and see what sense that makes and explain to him why you wish to and maybe he can see what that makes sense.  So the compromise may be in understanding each other's views.  Sometimes you just have to settle for that.