If you know you cannot file your return by the due date you do not need to file for an extension. You will automatically be granted an extension until October 15. If you anticipate that you will owe additional tax on your return you should submit your payment with a payment voucher (Form 40V) by April 15th.
Except in cases where taxpayers are abroad, no extension will be granted for more than 6 months.
An extension means only that you will not be assessed a penalty for failure to timely file a return. Interest is due on any additional tax not paid by the original due date. Interest on the additional tax due from the due date of the return and any penalties will be assessed if applicable to your return.