Federal Tax Return
You have to file the extension by April 15. The extension is only for filing the tax return, not for payment of taxes due so if you owe any taxes you need to pay them by April 15 otherwise you will owe interest and possibly penalties!
One way to file an extension is via mail. Fill out Form 4868 and send it to the IRS via certified mail, mail receipt requested (to have proof). The address to which you mail the Form is on the Form (depends on the State where you live).
If you owe taxes however, you have to pay those by April 15 and you can do so by attaching a check or money order to the mailed Form 4868.
For more information on how to file an extension see this IRS Article.
An extension gives you 6 additional months, i.e., until October 15 to file your tax return with the IRS and most states.
Note: In order to know whether you owe money or not you still basically need to do your tax return, thus we highly recommend doing your taxes as far as you can get. If you are still missing information, finish your taxes without that information or an estimate of the missing information and use those numbers to fill out Form 4868.
If you do not pay any tax now but in the end will owe taxes, you will need to pay the owed taxes and interest on those when you file your return. The IRS might also charge a penalty on late payment. If you pay something now and it turns out when you file your tax return that you overpaid, you will get the excess payment back from the IRS.
State Tax Return
In most states you have to file an extension as well.
- Vermont: if you don't owe taxes, send Form 4868 to Vermont Department of Taxes, PO Box 1779, Montpelier, VT 05601-1779; if you owe taxes, send Form IN-151 with a check enclosed;
- California: gives you an automatic filing extension through October 15. You do not need to apply for one. If you owe taxes, you still need to pay those taxes by April 15. See here for more information;
- District of Columbia: send Form FR-127 to Office of Tax and Revenue, PO Box 59, Washington, D.C. 20044-0059. Any tax due must be paid in full with the request;
- Virginia: gives you an automatic filing extension through November 1. You do not need to apply for one. If you owe taxes, you still need to pay those taxes by May 1. If you owe taxes, use Form 760IP to calculate the tax and remit it;
- Maryland: If you already requested an extension from the IRS for your federal return and you do not owe additional state tax, you receive an automatic six-month extension for filing your Maryland return. If you do owe additional tax and need an extension of time to file, you must file Form 502E by the due date and enclose the amount due;
- New York: Depending on whether you are a resident or non-resident you have to file a different form to request an extension. Here is the information.
- For other States, go here.