I always like to go to the horse's mouth. I didn't see the statement that any portion of the common area is treated as a personal portion, but it is written somewhere. An interpretation of this might be that if only one room is rental and the balance of the house is common area, then any expenses directly related to that room are deductible and since the common areas are personal use, then how is your tenant responsible for 50% of HOA, and taxes?
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Renting Part of Property
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, 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 are normally 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 (Form 1040).
You don’t 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 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 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.
Example.
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 × 0.10) is a rental expense. The balance, $540, is a personal expense that you can’t deduct.
