You may apply metamagic to your domain spells, it'd use a higher [/i]domain[/i] spell slot. Remember, your domain spells can only be prepared in your domain slots (one slot per spell level) and vice versa! For example, a 7th level cleric with the Travel domain may prepare an Extended Fly as his 4th level domain spell, but cannot prepare an additional Dimension Door as 4th level spell.