So, you're a Canadian, or other non-US citizen, who owns and rents out US property, hasn't yet obtained an ITIN and you don't expect to make your June 15, 2018 1040NR filing due date. No worry, go ahead and file an extension request on Form 4868 by writing “ITIN to be requested” on SSN line of form. Note - if US tax is due and paid after June 15, then the late payment penalty (usually ½ of 1% of any tax) and related interest does arise. But your extension is assisting you regarding the late filing penalty (usually 5% of the amount due for each month or part of a month your return is late; the maximum penalty is 25%) that is usually charged if your return is filed after the due date - as such due date includes extensions.