Single-member LLCs are initially classified as a disregarded entity by the IRS, and for federal tax purposes, they are taxed as a sole proprietorship.
A disregarded entity needs to file Form 4868 (Personal tax extension) to extend the filing deadline of tax Form 1040 by 6 months.
Single-member LLCs can elect to be taxed as,
C Corporation by filing Form 8832 with the IRS. To extend the tax Form 1120, use the extension Form 7004
S Corporation by filing Form 2553 with the IRS. To extend the tax Form 1120-S, use the extension Form 7004