How would it imbalance gameplay to allow a multiclassed wizard/sorcerer to copy wizard spells in a spellbook for which one has spell slots?

Question

Background

I'm playing an eladrin wizard in a homebrew game of a friend. After hitting level 6 I'm thinking of multiclassing into sorcerer, because it fits thematically. I'm an accidental traveler from the Feywild and, through adventuring and study, I'm slowly learning more about my origins while getting more in touch with my "instinctive" side of understanding the weave.

Party composition at level 5 (currently): dwarf barbarian, half elf assassin/fighter, wood elf ranger, high elf arcane archer.

Other possibly relevant details about my wizard:

• I’m a Conjurer who focusses on utility and crowd control over blasting.
• I have +5 Intelligence and +3 Charisma.
• Thinking of multiclassing into Draconic Bloodline or Favored Soul.

Research

According to Can a multiclass Wizard copy any Wizard spell they find into their spellbook?, per the multiclassed spellcasting rules:

You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class.

[...]

If you have more than one spellcasting class, this table might give you spell slots of a level that is higher than the spells you know or can prepare. You can use those slots, but only to cast your lower level spells.

(emphasis mine)

And from the same source:

Your Spellbook. The spells copied into a spellbook must be of a spell level the wizard can prepare.

So by RAW, a wizard 12/sorcerer 8 would have one 9th-level spell slot but would not be able to learn and prepare spells of that level.

Question

How would it imbalance gameplay to allow a multiclassed wizard/sorcerer to copy wizard spells in a spellbook for which one has spell slots? Please include player and DM perspective in your answer, and support it with experience.

I'm mostly interested in how it would imbalance play at my table. For other tables, it's good to include how this example of wizard 12/sorcerer 8 compares to a wizard 20.

The house-rule would be:

A multiclassed character with primarily wizard levels is able to inscribe found wizard spells (of which one has spell slots) into one’s spellbook, and therefore be able to cast those spells.

The DM is thinking about whether to allow it or not, so I’m helping in acquiring information. It’s his first campaign as DM and my first time playing a spellcaster that’s perhaps going to make it until “endgame”, so I’m trying to plan accordingly. To be honest, all the reading about spellcasting and multiclassing is quite overwhelming, so it could be I’m misinterpreting something. Corrections in that regard are also much appreciated.

As always, please support your answer with (similar) experience(s) at your table.

Answer 1

The ability to learn and cast high level spells is the main feature you gain from high levels in a spell casting class. This is especially true of the Wizard class, which has a whole lot of "blank" levels where they don't gain any class features, only the ability to cast new spells.

If you let a Wizard cast 9th level spells despite not sticking with the class all the way, you've removed most of the opportunity cost of multiclassing. Lets look at what you give up from the Wizard progression table (PHB page 113) if you skip the last 8 levels while using your proposed house rule:

• Level 13: No class features (normally this is where you can first cast level 7 spells, but your house rule gives that ability to a multiclass wizard anyway)
• Level 14: Arcane Tradition Feature
• Level 15: No class features (normally this is where you can first cast level 8 spells)
• Level 16: ASI (you get this anyway from your other class)
• Level 17: No class features (normally this is where you can first cast level 9 spells)
• Level 18: Spell Mastery
• Level 19: ASI (you get this at character level 20 if you multiclass, since that's when you'll get level 8 in your second class)
• Level 20: Signature Spell

You also get to learn two spells for free at each level.

With your house rule, your Wizard 12/Sorcerer 8 would have all the features of levels 13, 15, 16, 17 and 19 except for the free spells learned. You do lose out on the "capstone" features that you get at levels 14, 18 and 20, but that's a reasonable trade for all the much larger number of class features you get for eight levels in Sorcerer!

Your proposed house rule could be balanced, but only to the extent that your DM doesn't let you use it because they don't give you access to any high level spells. If you never find any higher level spells, then you're just playing with the rules as written. But if they ever drop an archmage's spellbook in your lap that contains Wish and a bunch of other high-level wizard spells, then they've practically given you five Wizard levels for free. If you can reasonably expect something like that to happen, then the house rule is so overpowered that no player would ever want to play a single-class Wizard to level 20.

It might not be too unbalanced if your GM will only ever let you find a tiny number of high level spells, and that you will find them place of some other powerful magical item. But it might be simpler for your DM to just create a custom magical item that lets you cast specific high-level spells once a day instead of adding a house rule that changes how Wizard's spellcasting works (and then needing to be very careful about pitting you against powerful NPC spellcasters who might have spellbooks for the rest of the game).

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This answer is not coming from personal gameplay experience, as I've not played or GMed a game with 20th level characters. But I have theory-crafted both single- and multi-class spellcaster builds, and so I have seen the tradeoffs you need to make when choosing to multiclass. I am pretty confident that those tradeoffs are important to the balance of the game.

Answer 2

I don't recommend it

Assuming that by primarily you mean "more Wizard levels than any other class" you are removing a big multiclass penalty. For example, if the character is Wizard 11/Cleric 9, they still have all the benefits of 9 levels of Cleric, but do not lose out on casting higher level Wizard spells, greatly tipping the balance.

There are a number of trade-offs for multiclassing, such as fewer ASIs, higher level class features, and inability to get higher level spells. By removing that hurdle (and mind you, you're saying only for wizards), you are greatly swinging the balance.

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This is not from personal experience as it was never allowed. It has been clear to my groups that there are trade-offs for multiclassing and by removing them, it can greatly tip the balance.