this is from IRS pub 936 - home mortgage interest
Divided use of your home. The only part of your home that is considered a qualified home is the part you use for residential living. If you use part of your home for other than residential living, such as a home office, you must allocate the use of your home. You must then divide both the cost and fair market value of your home between the part that is a qualified home and the part that isn't.
