Somewhat tricky - here's a simplifed answer:
There is a 50% limit on all charitable donations - so you can never deduct more than 50% of your income no matter how large your gift.
The limit drops to 30% for "capital gain property"
You get a choice of how to value the house. If you use the current market value, you are subjet to the 30% limit. Or you can use what you paid for the house + cost of improvements and be subject to the 50% limit.