Rental Property Tax Question

[Matt Whitt]

I'm new here and joined specifically to ask this question.
I have a mortgage on my home. Included in my property is an apartment above my detached garage which I rent out. The water bill is included together between the two structures. My question is this: on my taxes, am I able to write off a percentage of my mortgage insurance for the year as an expense for the rental property just like you can the whole mortgage insurance for a normal rental property?
I called a local H and R Block and they told me that I should do it by square footage. I calculated that the apartment is 18% of the square footage between the two. So according to him, mortgage interest, insurance, water bills, etc. can all have 18% written off as rental expenses.
Thanks for any help!

Sorry for double post, I put it in the wrong part of the forum from the start.