How to divide expenses. If an expense is for both rental use and personal use, such as mortgage interest or heat for the entire house, you must divide the expense between rental use and personal use. You can use any reasonable method for dividing the expense. It may be reasonable to divide the cost of some items (for example, water) based on the number of people using them. The two most common methods for dividing an expense are (1)the number of rooms in your home, and (2) the square footage of your home.
Renting Part of Property
http://www.irs.gov/publications/p527/ch04.html#en_US_2015_publink1000219159
