How would this extra spellcasting rule affect game balance?

Question

I once heard of a D&D 5th edition group who played using various house rules. One such rule was as follows:

As a bonus action, a spellcaster may cast any cantrip, 1st level spell, or 2nd level spell. You may do this even if you also cast a spell using your action on the same turn.

For example, you could cast two cantrips on the same turn, or you could cast a 5th level and a 1st level spell on the same turn. The group generally played starting at higher levels.

I find it dubious, but what effect would this have on game balance in practice?

Answer 1

This is pretty dubious, especially early on

As just a simple point of example, you can use a warlock that is optimized for Eldritch Blasting, which already is one of the best strategies for them, and just double their damage output even without the unbalanced Illusionist Bracers that are a very rare item, and should not be available early on. Even on level 2 they can cast Agonizing blast for 1d10+3 (from Charisma as their prime stat) force damage -- now they can do it for 2d10+6, or on average 17 points, or about 11 including to hit rates. The expected average damage a PC could do in a turn is somewhere around 8 even for optimized builds. Spellcasters often resort to casting cantrips to conserve slots if they can, and this will just double their effectiveness in all those rounds.

Especially in tier 1, where you only have access to level 1 and 2 spells, this effectively doubles the nova ability of spellcasters. Granted, at those levels you do not have that many spell slots to throw around, but it still poses an issue for important fights and may create issues for play -- how do you now balance your encounters to account for that? A fight normally takes about 3 rounds, so a level 3 spellcaster in that fight will be able to burn both their level 2 slots in the first round, and again 2 level 1 slots in the second, and 2 level 1 slots in the third, doubling up on sleep, shatter or whatever other non-concentration spells they have. It might be more palatable at the high levels that group was playing on.

Spellcasters like Bards or some wizards are already among the strongest classes in the game. It's not that they need any help. Giving them this amount of extra power is probably not a great idea.

Answer 2

It would be very unbalancing, especially at early levels

There is a reason there's only a handful of spells with casting time of a bonus action and that if you use one, the only spells you can cast with your Action are cantrips. It's to attempt to limit to a reasonable amount the damage spellcasters can do in a single turn.

Your house rule basically for the entire tier 1 does away with the division of spells into actions and bonus actions and effectively doubles the damage output of spellcasters. AND it creates/increases (depending on how you look at this from the start) the power gap between casters and melee based classes, generally the power gap becomes more visible in the higher tiers, with your rule it's there from the very start.

To me this isn't just a house rule, it's a fundamental change to the mechanics. If you really want to keep it, I would recommend making it a feat and then limit its use to once per short rest at most. Make it clear to the players that it's something you're playtesting and still, be ready to modify it further / drop it completely if your players dislike it.

Answer 3

Effects from spellcasting are balanced around the action economy as-is, and the suggested house rule allows many spells to be cast additionally in a single turn with no restriction, significantly changing the balance from RAW. For example, the way that cantrip damage scales is inline with damage expectations from other classes without using resources, whether Fighter's extra attack, Rogue's sneak attack, Monk's damage scaling etc*. Allowing the damage output to double for these "basic" attacks, but only for spellcasters significantly improves them when comparing between classes.

In addition, most pure spellcasters have limited alternative uses of bonus actions. That means even if they only use the bonus action to cast their most effective cantrip per turn, the impact is a strict power-up.

Players could also build around the house rule, boosting cantrip damage or effectiveness (some feats, Warlock invocations and cleric domains improve some cantrips), and eschewing spells that would otherwise require using the bonus action, so they are not exposed to opportunity costs. The Cleric would never cast Spiritual Weapon, using a 2nd-level slot to get maybe +1d8+5 damage per round, limited by foes being within 20' of last attack, when they could get +3d8 damage attack per round using a bonus action Sacred Flame to hit anything in range.

If playing a high-powered game, additional house rules or homebrew items could address the balance between player characters. That might be the case in the example you found. Or they might simply prefer more powerful spellcasters.

NPC enemy spellcasters will presumably also get access to this, making them more boss-like (in that they get to perform extra actions on their turn). If the DM is setting up encounters with this in mind, it could be less of an issue, and balancing between PCs and NPC enemies is a bit looser in any case.

In general, effects that alter the action economy - whether by adding/removing actions on a turn, or changing an action type - can be very strong and can have a lot of consequences for balance. That doesn't prevent them being in the game, but you will usually see them carefully limited and/or tied to a limited resource.

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* These example classes all have options to increase damage output using bonus actions, but it comes at a cost. For the Fighter, a loss of AC, for the Rogue opportunity cost for all the other things they can do with Bonus action, for the Monk they spend Ki which is limited.