The S-Corp Built-In Gains Tax: The Five-Year Trap in a C-Corp Conversion

A general contractor I will call Dave built his business as a C corporation back in 2010, when his accountant told him the corporate rate looked fine on paper. Fifteen years later the company is worth far more than its book value, most of that in goodwill and a few pieces of paid-off equipment, and Dave wants to sell within the next two or three years. His new CPA suggests electing S-Corp status now to escape the double tax that hits a C corporation sale. That instinct is right in general and wrong on the timing, because of one provision that catches almost every C-corp-to-S-corp conversion. Dave is theoretical, but the trap is real, and it is one of the more expensive mistakes I see.

What the built-in gains tax actually is

When a C corporation converts to an S corporation, the assets it holds on the conversion date usually carry appreciation that was never taxed. Congress did not want owners to dodge the corporate-level tax on that appreciation by flipping to pass-through status the day before a sale. So IRC §1374 imposes a corporate-level tax on the net recognized built-in gain when a former C corporation sells or disposes of appreciated assets during a defined window after it elects S-Corp status. The total gain exposed to the tax is capped at the company's net unrealized built-in gain, the spread between the fair market value and the tax basis of all its assets on the first day of the S election, defined in §1374(d)(1).

The five-year clock

The window is the recognition period, and it is the 5-year period beginning on the first day of the first tax year the corporation is an S corporation, under §1374(d)(7). That length has moved around over the years. It started at 10 years, dropped temporarily during the recession, and the Protecting Americans from Tax Hikes Act of 2015 made the 5-year period permanent. Sell an appreciated asset in year two, and the built-in gain on it is taxed at the corporate level. Sell the same asset in year six, one day past the recognition period, and §1374 does not apply to it at all.

The 21% rate and the two ceilings

The tax rate is the highest rate under IRC §11(b), which since the 2017 Tax Cuts and Jobs Act has been a flat 21%. The IRS computes it in Part III of Schedule D of Form 1120-S, and the instructions spell out the 21% multiplier directly. Two limits can shrink the bill. First, the taxable income limitation in §1374(d)(2) caps the taxed amount at the income the company would have reported as a C corporation that year, with any excess carried forward inside the recognition period. Second, the total built-in gains taxed across all five years cannot exceed the net unrealized built-in gain measured at conversion, under §1374(c)(2). Neither limit helps a profitable company selling a large appreciated asset, which is exactly the situation that makes this tax bite.

A theoretical sale, inside the window versus outside

Say Dave's company elects S status effective January 1, 2026, and an appraisal pegs its net unrealized built-in gain at $600,000, almost all goodwill and depreciated-out equipment. If a buyer closes an asset sale in 2027, inside the recognition period, the full $600,000 is net recognized built-in gain. Here is the difference the calendar makes. The figures are illustrative and rounded.

The double tax, and the small offset

There is a second bite. After the entity pays the §1374 tax, the same gain still flows through to Dave on his personal return, because an S corporation is a pass-through. The one piece of relief is IRC §1366(f)(2), which treats the built-in gains tax the corporation paid as a loss passed through to the shareholders, reducing the gain they report. In the example, the $126,000 of entity tax passes through as a $126,000 loss, so Dave reports $474,000 of gain personally instead of the full $600,000. He still paid the corporate tax, then paid again at his individual rate on what was left. Selling one day past the five-year mark erases the entire $126,000 entity-level layer.

Why most LLC owners never see this

If you formed an LLC and it has only ever been taxed as a sole proprietorship or a partnership, electing S-Corp status does not create a built-in gains problem, because §1374 reaches only corporations that were C corporations or S corporations holding assets with a basis carried over from a C corporation. A partnership-to-S or a straight LLC-to-S election does not carry any C-corporation basis history, so there is nothing for §1374 to tax. This is one more reason the "S-corp or LLC" question so often resolves toward the LLC that later elects S. It never carries this baggage. The built-in gains tax is almost entirely a former-C-corporation problem.

How to plan around it

The cleanest fix is patience. If a sale is not imminent, converting well ahead of time starts the five-year clock, and a sale after the recognition period pays no §1374 tax. If a sale is close, run the math both ways before electing, because in some cases staying a C corporation through the sale, or timing the deal past the window, beats a rushed election. An appraisal at the conversion date matters too, since a defensible net unrealized built-in gain figure sets the ceiling and lets you support the value assigned to assets you do not plan to sell inside the window. This is a spot to bring in a CPA before you file Form 2553, not after.

For the broader entity picture, see how S-Corp and C-Corp taxation compare and how a disregarded entity, S-corp, partnership, and C-corp differ once you sell. This article is educational and is not legal or tax advice. The numbers are illustrative, and Dave is theoretical. Talk to a qualified CPA about your own conversion before you elect.