I rent my home. We split the rent and all bills 3 ways (per bedroom). One of these 3 rooms is my home office. Can I claim 1/3 of all costs as a home office deduction?

[ColeenD3]

I'm not sure I understand the question. Can you clarify exactly who lives/works where? Which three people split the 3 rooms?

In one scenario, you own the house and rent out two bedrooms and your home office is the other bedroom. What about common areas? 

Common areas are not rental space. They belong to the landlord. In this case, this is personal space that can't be included as rental. If you are self-employed and meet the qualifications for a home office, then you can prorate the square footage of your office contrasted to the square footage of the entire house, just as you would do as if you occupied the entire residence.

[best-jaimies-gma]

Thank you for taking the time to reply. Let me clarify a bit. My husband, myself, and a roommate rent the home from a landlord. We split the cost of rent by bedroom. My husband and I pay for 2 bedrooms, one is our bedroom, and one is my office. Therefore, the office space costs me $400 a month. My husband and I pay all utilities. I calculated the utilities of the office space by deducting my roommates bedroom and bathroom from the square footage of the home (the parts I do not use at all). I listed the cost of the office as $400, not a percentage of the space it occupies in the home.  I would not be paying the extra $400 for the office, if I did not need it, so that is what it cost to rent that space. I am basically asking if this is an accurate way to deduct, since it actually mirrors the cost, or if I have to use the percentage, even though that does not represent the actual cost. This is just for the physical room. I have calculated all other home office costs, such as utilities, by percentage of square footage. 

[AmyC]

Since you have a verifiable method and can easily prove your office usage and rent, it is a valid expense at $400/month rent for your office. If you are audited, you can get a notarized statement from the other renter and the landlord. A picture showing the room exclusively used for your office is also good to add to your tax file.

The other expenses pro-rated is perfect.