Content from the IRS
If you rent part of your property, you must divide certain expenses between the part of the property used for rental purposes and the part of the property used for personal purposes, as though you actually had two separate pieces of property.
You can deduct the expenses related to the part of the property used for rental purposes, such as home mortgage interest, qualified mortgage insurance premiums, and real estate taxes, as rental expenses on Schedule E (Form 1040). You can also deduct as rental expenses a portion of other expenses that normally are nondeductible personal expenses, such as expenses for electricity, or painting the outside of the house.
There is no change in the types of expenses deductible for the personal-use part of your property. Generally, these expenses may be deducted only if you itemize your deductions on Schedule A.
You cannot deduct any part of the cost of the first phone line even if your tenants have unlimited use of it.
You do not have to divide the expenses that belong only to the rental part of your property. For example, if you paint a room that you rent, or if you pay premiums for liability insurance in connection with renting a room in your home, your entire cost is a rental expense. If you install a second phone line strictly for your tenant's use, all of the cost of the second line is deductible as a rental expense. You can deduct depreciation on the part of the house used for rental purposes as well as on the furniture and equipment you use for rental purposes.
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.
You rent a room in your house. The room is 12 × 15 feet, or 180 square feet. Your entire house has 1,800 square feet of floor space. You can deduct as a rental expense 10% of any expense that must be divided between rental use and personal use. If your heating bill for the year for the entire house was $600, $60 ($600 × .10) is a rental expense. The balance, $540, is a personal expense that you cannot deduct.
A common situation is the duplex where you live in one unit and rent out the other. Certain expenses apply to the entire property, such as mortgage interest and real estate taxes, and must be split to determine rental and personal expenses.
You own a duplex and live in one half, renting the other half. Both units are approximately the same size. Last year, you paid a total of $10,000 mortgage interest and $2,000 real estate taxes for the entire property. You can deduct $5,000 mortgage interest and $1,000 real estate taxes on Schedule E, and if you itemize your deductions, you can deduct the other $5,000 mortgage interest and $1,000 real estate taxes on Schedule A.