As per the “House Rules” article by Andy Collins in Dragon #305, page 16, metamagic feats can be used “on the fly” without extended casting time or preparation. The feat can be used one time daily. These spells use up the same level slot they would have if they had been cast normally, though the spellcaster is limited to using spells of a maximum level equal to the highest level spell that the caster can cast minus the spell difference given in the feat’s description. The Heighten Spell feat is treated differently— it can be used to maximize the spell to the highest level spell the spellcaster can cast.