Can I modify the mortgage on my property in chapter 13 bankruptcy, if the property is not my principal residence on the filing date?

Yes.

In a recent Houston bankruptcy court case, a lady lived in one house when she filed chapter 13, but decided to sell it. She moved into her second house, that she owed over $100,000 on, but was worth only $60,000.

The Court allowed her to propose a plan to pay only the $60,000 in full during her 5 year chapter 13 case, plus some interest. The other $45,000 would be paid only a small amount as an unsecured claim and discharged or canceled when she completed her case. She would have the property paid for in only 5 years. If the second house had been her principal residence on the date she filed bankruptcy, she would not have been allowed to do this.

The court found that the "the date on which a debtor’s principal residence determined for purposes of 11 U.S.C. § 1322(b)(2) is the petition date." In re Collins, Case No. 14-34816, US Bankruptcy Court, Southern District of Texas.