I plan to get an extension on my taxes. I am an LLC filing as an S-Corp, with everything flowing through personally. I am hoping to be able to fund my SEP later in the year, prior to paying my final taxes, which I will do around October 15th. How late can I fund a SEP for the prior tax year (2016)?
Since it is already past March 15, 2017, it is too late for a calendar-year S corp to now request a filing extension for a 2016 tax return. Unless your S corp did so by March 15, 2017, the opportunity for your S corp to make a SEP contribution for 2016 has been lost. If your S corp did request a filing extension by March 15, your S corp has until September 15, 2017 to make your SEP contribution.
According to the IRS Pub. 560 - Retirement Plans for Small Business
Deadline for setting up a SEP. You can set up a SEP for any year as late as the due date (including extensions) of your income tax return for that year.If your S-Corp uses a calendar year for its accounting, please keep in mind that the original filing date for an S-Corp return is March 15. A six-month extension would bring the filing deadline to September 15 (October 15 only for your individual return).