Metamagic FeatsOn orfinlir there are two traditions of casting metamagic spells, the first is the tradition explained in the SRD, the second follows the rules from Unearthed Arcana:
To use metamagic feats, a caster must expend extra spell slots in order to apply the effects of a the feat. These spell slots must be of a level equal to or higher than that of the spell being modified by the metamagic feat. In essence, the character pays for the metamagic effect by "using up" other spells of the same or higher level. The caster need not prepare the metamagic version of the spell ahead of time, and the spell’s actual level and casting time are unchanged from normal.
The number of extra spell slots required is equal to the spell level adjustment of the feat. For example, a wizard who wants to cast a quickened magic missile would expend the magic missile spell, plus four additional spells of 1st level or higher.
If the caster has more than one spellcasting class, all extra spell slots expended must come from the same spellcasting class as the spell to be modified. A 7th-level cleric/4th-level wizard who wants to cast a maximized cure moderate wounds can’t spend any of her wizard spell slots—they all must be from her cleric spellcasting ability.A character may only expend bonus spells from school specialization on spells of that school (so an evoker could spend her bonus 3rd-level evocation spell to pay for a metamagic feat applied to another evocation spell, but not to a spell of any other school). She can’t expend domain spells to pay for a metamagic feat’s added spell slot cost, even if the spell is a domain spell.
In the case of the Heighten Spell feat, a caster expends one additional spell slot for each effective level she wishes to add to the spell. To heighten a fireball to the equivalent of a 5th-level spell, a caster must expend two extra spell slots of 3rd level or higher.
A caster can apply more than one metamagic feat to a spell, or even the same metamagic effect more than once (if allowed by the feat’s description). She must simply pay the additional cost in spell slots. For instance, a wizard casting a stilled and silent invisibility spell would spend two spell slots in addition to the invisibility spell: one for Still Spell and a second for Silent Spell.
If a caster doesn’t have enough remaining spell slots to cast the metamagic spell, she can’t apply that metamagic effect to the spell.