Hey, I was doodling up a couple homebrew spell ideas and I could use some clarification. I was just wondering when creating a spell how should one determine what classes/subclasses should or shouldn't be able to use said spell? Thanks.

Yes, I have almost the same question, because I can confuse so easily the magic school of the spell should be related to.

When you create the spell there is a box at the bottom right "available for classes" - start typing and it brings up the classes letting you add the spell to that class spell list. 

Oh no I don't mean technically. I was more wondering on a creative standpoint when you say: "I've made this spell, it can be cast by Warlocks and Wizards, but not Sorcerers!" how do you go about deciding Sorcerers can't do it?

Suspiciously Sorcerers are so OP, by the wild magic birth.

If the spell is a cantrip or is higher than 5th level, I begin by ruling out paladins, rangers, and artificers (only for spells higher than 5th level). Then, I think about archetypes. Magic users typically fall into three categories, arcane, divine and nature. While the PHB defines divine and nature magic as being one in the same, the flavor in spells on the druid spell list and the cleric spell list are very different. Once you decide on the form of your spell, you can more easily assign it to classes. But of course, not all arcane spellcasters get every spell the wizard gets.

A good case example is magic circle. As a spell that wards an area from extra-planar creatures, it has many uses. It can be used to trap and contain a monster, or it could be used as part of a seance or summoning. The idea of circles of power and containment is an old idea and both holy men and people summoning demons would use similar circles. This is why clerics, paladins, warlocks, and wizards get it. There is also something artificial about a magic circle. It sounds like you might find it in some ancient text, unread for centuries. It does not sound like something that you one day wake up and know how to make. That is why sorcerers don't get magic circle.

In short, it depends on the spell. If you share a spell and mention that you're thinking about the classes that get the spell, you will get feedback around that topic. Hope this helped!

Quote from DoodTheMan >>

Oh no I don't mean technically. I was more wondering on a creative standpoint when you say: "I've made this spell, it can be cast by Warlocks and Wizards, but not Sorcerers!" how do you go about deciding Sorcerers can't do it?

 
Only Wizards create spells. Their spells are a result of practice, study, learning, and understanding. It is a mental process and can be achieved by anyone who practices magic in the same way.

Sorcerer spells are not created, they are inherent, they are powers. Some spells overlap - the Sorcerer may just "do" what the Wizard had to spend years learning how. Where a wizard provides Somatic components by tracing strange arcane symbols in the air or performing complicated gestures, and povide Verbal components as words of old languages, achieving certain pitches and tones to get the right resonance -- a Sorcerer, for the same effect, might provide a somatic component of just raising a hand and the Verbal component for them might just be "Burn!" or a scream of anger.

Warlocks do not create spells either. Their patrons, however, do and confer those spells either as gifts of knowledge or manifestations of borrowed powers.

Clerics get given their spells by the god they serve.

Druid magic is part created but mostly developed instinctually by a bond with nature.

And so on.

Basically Wizards are the only ones who get to say "I created a spell that does this" and they don't decide who gets to cast it. It might be they invented a spell never before achieved. Or maybe they invented something some Sorcerer somewhere already could do. The Wizard might invent the spell, but somebody else, not having the intelligence for wizardry may just make a deal with a Devil or Archfey to be able to cast it and just be "given" the knowledge and power instantly - for a price, of course.

First I write a cool spell. Then I think about who that spell best serves. If it’s a great spell for a squishy character to get out of melee it goes to Wizards, Sorcerers, Warlocks, and maybe Bards, Druids, and Artificers. If it’s a combat spell I start thinking Clerics, Paladins, Rangers, Druids and maybe Artificers. If it’s a utility spell I start with Bards and Artificers. If it’s a battlefield control spell I start with Wizards, artificers and Bards. High Damage spells usually go to Sorcerers first and foremost. Stuff like that.

Then I consider the theme of the spell. Nature stuff goes to Druids and Rangers, holy/unholy stuff goes to Clerics, Paladins, and sometimes Warlocks.

Between what the spell does, how it does it, and who it serves I usually narrow it down pretty easy. Just remember that Paladins and Rangers don’t get Cantrips, or spells above 5th level. Sometimes I forget that and it’s annoying.

In example, if I make a spell that creates a Holy Bell which it dolls for at least 1 hour, with Concentration required, then in what Magic school should I put in ???

Should I focus on Cleric, Light Domain only, or in the Conjuration school ???

Classes and spell schools are not the same thing. All spells have a school of magic, no matter which class gets it. For your holy bell, I would recommend making it a conjuration spell and giving it to clerics and paladins.

In the PHB, there is a description of each spell school.

For Holy Bell it depends on what effects it has. If it's just a mechanism to make noise (like a church bell) then aside from the name there'd be no point in ruling out Wizards as well and just chaning the name to Mystic Bell. If there's other effects like deterring particular creature types, etc then you can start to see which spells have similar effects and base class limitations around those.