What level is a spell if you cast it without expending a spell slot?

Question

There are ways to cast spells without expending a spell slot such as:

Master Transmuter (PHB 119):

Restore Life. You cast the raise dead spell on a creature you touch with the transmuter’s stone, without expending a spell slot or needing to have the spell in your spellbook.

Armor of Shadows (PHB 110):

You can cast mage armor on yourself at will, without expending a spell slot or material components.

Whispers of the Grave (PHB 111):

You can cast speak with dead at will, without expending a spell slot.

See also: Channel Divinity: Read Thoughts (PHB 59-60), Mystic Arcanum (PHB 108), Beast Speech (PHB 110), Chains of Carceri (PHB 110), Eldritch Sight (PHB 110), Fiendish Vigor (PHB 111), Mask of Many Faces (PHB 111), Master of Myriad Forms (PHB 111), Misty Visions (PHB 111), Otherwordly Leap (PHB 111), Signature Spells (PHB 115), Spell Mastery (PHB 115), Shapechanger (PHB 119), Boon of Spell Mastery (DMG 232) and Boon of Spell Recall (DMG 232).

What level are these spells if you cast them without expending a spell slot?

Answer 1

The Sage Advice Compendium goes into detail about what happens when you cast a spell without expending a spell slot:

A spell is cast at its lowest possible level, which is the level that appears near the top of its description. Unless you have a special ability that says otherwise or the origin of what allows you to cast a spell without expending a spell slot also allows you specifically to expend a higher-level spell slot when you cast it; and that the only way to increase the level of a spell is to expend a higher-level spell slot when you cast it.

Sage Advice Compendium version 2.3, page 13:

What level is a spell if you cast it without a spell slot?

Such a spell is cast at its lowest possible level, which is the level that appears near the top of its description. Unless you have a special ability that says otherwise, the only way to increase the level of a spell is to expend a higher-level spell slot when you cast it. (p 13)

If you have the option to cast a spell at a higher level you have to regard the rules for Casting A Spell at a Higher Level (PHB 201):

When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting.

These origins (in the PHB) of what allows you to cast a spell without expending a spell slot also allows you specifically to expend a higher-level spell slot when you cast it:

Spell Mastery (PHB 115):

If you want to cast either spell at a higher level, you must expend a spell slot as normal.

Signature Spells (PHB 115):

If you want to cast either spell at a higher level, you must expend a spell slot as normal.

An example of special abilities that say otherwise is:

Infernal Legacy (PHB 43):

Once you reach 3rd level, you can cast the hellish rebuke spell once per day as a 2nd-level spell.

That allows you to cast the first-level spell Hellish Rebuke as a second level spell; that is at a higher level than normal, but only at that level.

The wording in the Sage Advice Compendium follows the already in the DMG established wording of:

Boon of Spell Mastery (DMG 232)

Choose one 1st-level sorcerer, warlock, or wizard spell that you can cast. You can now cast that spell at its lowest level without expending a spell slot.