Does Filing a Tax Extension Cost Money?
No, not by itself.
For a federal individual income tax return, requesting an automatic extension of time to file does not itself require a separate IRS filing fee. You can request the extension by filing Form 4868, and the IRS also allows taxpayers to request an extension electronically through IRS Free File at no cost. However, if you owe tax, filing an extension does not extend the time to pay.
Any tax you pay with the extension is a tax payment, not a fee for the extension. If you do not pay enough by the original due date, interest and possible penalties may apply. To avoid confusion, note that while the IRS does not charge a filing fee, some third-party providers or paid preparers may charge their own service fees for preparing or transmitting an extension; those are separate from federal law and IRS charges.
State Law Note
State extension rules are administered at the state level and can differ from the federal rules above. A state may have its own extension procedures, payment rules, and related fees or penalties. Some states accept a valid federal extension, while others require a separate state extension request and may impose state-level fees or minimum penalties, so taxpayers should consult the applicable state revenue department or tax agency.
Sources
IRS — Get an extension to file your tax return
IRS — Topic no. 304, Extensions of time to file your tax return