Iowa Admin. Code r. 701-101.15
The return must be filed with the director on or before March 31 following the tax year. There is no authority for the director to grant an extension of time to file a return. Therefore, any return which is not filed on or before March 31 following the tax year is untimely.
When the due date falls on a Saturday, Sunday, or holiday, the return will be due the following day that is not a Saturday, Sunday, or holiday. If a return is placed in the mail, properly addressed and postage paid in ample time to reach the director or the department on or before the due date for filing, no penalty will attach should the return not be received until after that date. The functional meaning of this requirement is that if the return is placed in the mail, properly addressed and postage paid, on or before the due date for filing, no penalty will attach. Mailed returns should be addressed to Department of Revenue, Attention: Local Government Services, Hoover State Office Building, Des Moines, Iowa 50319.
Iowa Admin. Code r. 701-101.15