This, from IRS Publication 526:
Partial Interest in Property
Generally, you can't deduct a charitable contribution of less than your entire interest in property.
Right to use property. A contribution of the right to use property is a contribution of less than your entire interest in that property and isn't deductible.
Example 1.
You own a 10-story office building and donate rent-free use of the top floor to a charitable organization. Because you still own the building, you have contributed a partial interest in the property and can't take a deduction for the contribution.
Example 2.
Mandy White owns a vacation home at the beach that she sometimes rents to others. For a fund-raising auction at her church, she donated the right to use the vacation home for 1 week. At the auction, the church received and accepted a bid from Lauren Green equal to the fair rental value of the home for 1 week. Mandy can't claim a deduction because of the partial interest rule. Lauren can't claim a deduction either, because she received a benefit equal to the amount of her payment. See Contributions From Which You Benefit , earlier.
Since you would still own the entire property of which the carriage house is just a part, I think the above would apply.
Here's the entire publication. I was looking under Partial Interest in Property:
