Generally, if your name is not on the title of the property and you are not
legally responsible for the debt, you cannot deduct the mortgage
interest even if you made the payments. You must have an ownership interest in the home to deduct interest on a home loan. This means that your name has to be on the deed or you have a written agreement with the deed holder(s) that establishes you have an ownership interest.
However, Treas. Reg. § 1.163-1(b) permits a deduction for
interest paid on a mortgage when a taxpayer is the legal or equitable
owner of the property, even though the taxpayer is not directly liable
for the mortgage. To
meet the Sec 163 “equitable owner” status for the pre-loan period, the
IRS will look to facts such as whether you exclusively occupied the
residence, made all the mortgage payments directly to the lender, and
paid all expenses for repairs, maintenance, property taxes, insurance
and improvements. From your description, it sounds like you satisfy
most of these prerequisites and should be able to claim both the
mortgage and property tax payments for this period.
For more about these and other conditions for deducting home mortgage interest you can see the details in IRS Publication 936, Home Mortgage Interest Deduction
*Equitable owner: considered to have rights or obligations of an owner regardless of legal title on the ground of equity
