S-Corp Built-In Gains Tax: The 5-Year C-Corp Trap
If your S-corp used to be a C-corp, selling an appreciated asset within your first five years as an S-corp can trigger a separate, entity-level tax before any of that gain ever reaches your personal return. This is the S-corp built-in gains tax under Section 1374, and it catches owners who assume S-corp status automatically means pass-through treatment for gains that built up while the company was still a C-corp. It doesn’t. Here’s how the tax works, who actually owes it, and how to plan around the five-year window.
Who Owes the S-Corp Built-In Gains Tax
Section 1374 only reaches S-corps with C-corp history. A corporation that has been an S-corp for every year of its existence owes nothing under this section. The rule is written narrowly: it applies to a corporation that was a C-corp before making the S election, or that acquired assets from a C-corp in a transaction where the S-corp’s basis carries over from the C-corp’s basis — a common issue in F reorganizations and certain asset transfers. If you started life as an S-corp from day one, this exposure doesn’t exist for you at all, per IRC Section 1374(c)(1). If you’re still weighing whether converting makes sense in the first place, our S-Corp vs. LLC vs. C-Corp guide walks through the tradeoffs before you commit to a structure.
The Five-Year Recognition Period
The exposure isn’t permanent. It runs for a recognition period of five years, starting on the first day of the corporation’s first tax year as an S-corp. Sell an appreciated asset the C-corp owned before conversion during those five years, and the gain is exposed to tax at the corporate level. Sell it after the five years close, and it passes through to shareholders with no entity-level charge at all. This wasn’t always a five-year window — Congress temporarily shortened the original ten-year period several times before Section 1374(d)(7) made the five-year rule permanent for tax years beginning after December 31, 2014. It hasn’t moved since, and nothing in the 2025 tax legislation touched it, so the clock still runs from your S-election date regardless of how recently you converted.
What Counts as Built-In Gain
The tax targets the gap between fair market value and adjusted basis that existed on the conversion date — what the code calls net unrealized built-in gain. Appreciated real estate is the most common trigger, but the same exposure applies to equipment worth more than its depreciated basis, self-created goodwill, and, for businesses that used the cash method as a C-corp, accounts receivable that were earned but not yet collected at conversion. If an asset’s value at conversion already exceeded its basis, any gain recognized on its sale during the recognition period is presumed to be built-in unless the corporation can affirmatively show otherwise.
How Much It Actually Costs
The rate is steep: the highest corporate rate under Section 11(b), currently a flat 21%, applied directly at the entity level before a dollar reaches the shareholders. That’s on top of whatever tax the shareholder eventually pays when the remaining gain passes through on the K-1 — the entire reason owners convert to an S-corp is to avoid exactly this kind of double taxation, and the built-in gains tax reintroduces it specifically for pre-conversion appreciation. It isn’t unlimited, though: the amount taxed in any year can’t exceed the corporation’s total net unrealized built-in gain measured at conversion, reduced by whatever’s already been recognized and taxed in earlier years of the recognition period. Net operating loss and capital loss carryforwards from the C-corp years can be used to offset it as well.
It Doesn’t Travel Alone — Excess Net Passive Income
The built-in gains rule has a companion trap that catches the same population of converted C-corps: the excess net passive income tax under Section 1375. Any year the corporation has accumulated earnings and profits left over from its C-corp years, and passive income such as rents, royalties, dividends, interest, and annuities makes up more than 25% of gross receipts, the excess is taxed at that same 21% rate under Section 1375. Let it happen three years running and the S election terminates automatically on the first day of the following year — no vote, no warning letter, and no grace period. If your converted S-corp holds rental property or a securities portfolio left over from its C-corp days, that 25% threshold is worth checking every single year, not just once at the time of conversion.
Reporting It and Planning Around It
This liability is computed on Schedule D of Form 1120-S and flows to line 23b of the 2025 Form 1120-S. If the combined total of the built-in gains tax, the passive income tax, and any investment credit recapture reaches $500 or more for the year, the corporation has to make quarterly estimated payments the way a C-corp would — a compliance step that’s easy to miss in the first year after conversion, when everyone’s attention is on payroll setup instead. There’s a partial offset built into the code: any amount paid under Section 1374 is treated as a loss passed through to shareholders, reducing their stock basis along with it, which softens the shareholder-level result even though it doesn’t undo the entity-level hit. Practically, the planning options are timing — holding appreciated assets until the five-year window closes — installment sales that spread recognition across tax years to stay under the annual taxable-income cap (one caution: sell inside the window and the installment payments stay subject to the tax even after year five), and a qualified appraisal of the company’s assets as of the conversion date, since the tax can never exceed the net unrealized built-in gain fixed on that day and assets sold during the recognition period are presumed to carry built-in gain unless the corporation can prove otherwise.
The Bottom Line
None of this makes converting from a C-corp to an S-corp a bad move — for most profitable businesses the payroll tax savings still win out over the long run. But this is exactly the kind of rule that gets missed in a rushed conversion, and it’s expensive to discover only after the sale has already closed. If you’re planning a C-corp conversion, or you’re mid-way through your five-year recognition period and thinking about selling appreciated assets, talk to us before you sign anything.