calculating percentage of house rented out to a roommate

[pirate_wench]

I rented a room in my house for 4 months in 2015.  Turbo tax and the IRS website claims for calculating a percentage of my home for rental purposes, to measure the room and divide by total square footage.  That doesn't make sense to me though, because the roommate had full access to the rest of the house she shared with us!  What do you all recommend?

Detailed info:
two-bedroom house
one roommate with her own room, two home owners who share a room (me and my better half)
renter had access to entire house except the other bedroom and half the basement.
square footage of house 1700 sf including unfinished basement. Upstairs liveable space is 940sf, though the washer and dryer are in basement.
rent charged was $1,000/month, lease four months, included all utilities, internet, etc... (haven't calculated these expenses yet)
we paid for a lawn service as well, about $160/month

If it matters, we weren't even here, as we were thru-hiking the PCT...We made it a "roommate" agreement though in case we were injured and had to come home, but effectively she had the whole house to herself. We plan on dividing the utilities by three as if we had lived there.

What say you mustachian tax experts?

[bacchi]

Everything I've read indicates that you take a percentage of shared living space, too. If you have an IRS link that says take only the bedroom, then use that.

Remember, though, that the % you use determines how much of the mortgage, repairs, and taxes you can deduct.

[Retire-Canada]

I would charge 50% of the house to her if she had 1 of the 2 bedrooms and an estimate of the % utilities based on use....if you weren't there that would be 100%.

For services like lawn care I would charge 50% to her....if you can only enter a single % for all home costs go with 50%.

[Goldielocks]

If your rented your whole home out for 4 months, with you not there,  I would take the whole home  x 4 mos / 12 mos. 

Just like if you had a vacation home that you used 8 months a year and rented out 4 months a year.

I would charge 100% of the lawn service during those months, too, and any other costs your were required to maintain as part of your agreement.

[El Marinero]

Remember, though, that the % you use determines how much of the mortgage, repairs, and taxes you can deduct.

Even without renting out the room, you should be able to deduct all mortgage interest and property taxes if you itemize. 

The % allocation to the roommate is important because determines how much depreciation and perhaps utilities can be considered a  deductible expense.  Interest expense and property tax deductions really just move from one form to another.

[bacchi]

Remember, though, that the % you use determines how much of the mortgage, repairs, and taxes you can deduct.

Even without renting out the room, you should be able to deduct all mortgage interest and property taxes if you itemize. 

The % allocation to the roommate is important because determines how much depreciation and perhaps utilities can be considered a  deductible expense.  Interest expense and property tax deductions really just move from one form to another.

Yeah, good point.

Being able to deduct repairs could be helpful though -- delaying expenses until a room is rented is a good strategy.