Attunement slots are an important tool for maintaining game balance - especially in high magic settings where magic items are common.

If I've homebrewed a new magic item for my players, how can I decide if balance-wise this item should require attunement or not?

It's fairly trivial to say that the most powerful items should probably require attunement, whereas the least powerful probably shouldn't. But, in between these two extremes, where should the line be drawn?

• Are there any guidelines in offical materials for when homebrewed items should require attunement?


• Or, can any guidelines be sensibly inferred through careful analysis of existing official magic items (contrasting those that do require attunement with those that do not)?

There is some official Guidance

On page 285 of the Dungeon Master's Guide there is a short section on giving attunement to magic items:

Attunement

Decide whether the item requires a character to be
attuned to it to use its properties. Use these rules of
thumb to help you decide:

• If having all the characters in a party pass an item
  around to gain its lasting benefits would be disruptive,
  the item should require attunement.


• If the item grants a bonus that other items also
  grant, it's a good idea to require attunement so that
  characters don't try to collect too many of those items.

To that last point we can collect a few common types of bonuses which induce a need for attunement (non-exhaustive list):

• Non-armour bonuses to AC (e.g. Ring of Protection)


• Resistances or immunities (e.g. Ring of Resistance)


• Setting an ability score (e.g. Helm of Brilliance, Belts of Giant Strength)

This isn't particularly much to go on, but from looking at the items in the DMG we can find a couple other things that tend to be coupled with attunement:

• Interaction with a class feature. Most commonly this is spellcasting, where it would say: "requires attunement by a spellcaster" or specify a specific class (or set of classes).




• Charges and command words. A lot of these require attunement, which has the nice upside that command words are automatically learned through attunement and having them require attunement limits the number of different resources each character can have.




• Consumables don't require attunement. Despite allowing powerful effects, items like potions, necklace of fireballs, and up to Ring of Three Wishes these don't require attunement. (Related to that last one, Luck Blade presumably requires attunement because of the reroll ability and not the wish one.

I don't stand a chance to find all the "trends" myself, so I welcome suggestions from the comments.