(1) If one spouse files separately and itemizes deductions, then both must do so.
(2) An itemized deduction of a married taxpayer who files a separate federal income tax return claiming the itemized deduction may not be claimed as an itemized deduction by the taxpayer's spouse.
(3) If a married taxpayer files a separate Montana return, the following rules apply if the taxpayer filed a joint federal income tax return with the taxpayer's spouse or did not file a federal income tax return:
(a) an itemized deduction clearly attributable to one spouse may be claimed only by the spouse to whom it is attributable; and
(b) if the spouses file a separate return on the same form, an itemized deduction not clearly attributable to one spouse may be divided as provided on the form;
(c) if one or both of the spouses files a separate return on a separate form, an itemized deduction not clearly attributable to one spouse must be divided equally unless the spouses enter into a binding written agreement providing a different division;
(d) the agreement described in (3)(c) is a tax record each spouse must retain and provide to the department on request.
(4) Except as provided in 15-30-111, MCA, ARM 42.15.206 and (5), if a taxpayer files a Montana return claiming an itemized deduction that is allowed only to taxpayers claiming a specific federal filing status, the deduction is disallowed unless the taxpayer files their Montana income tax return using the same status.
(5) Married taxpayers who file separate Montana returns and are allowed a deduction for mortgage insurance premiums paid under section 163 of the Internal Revenue Code, 26 U.S.C. 163, are allowed the same deduction calculated using the federal rules for married taxpayers filing a joint return.