Section 1374 Built-In Gains Tax: The Five-Year Window That Catches C-Corp to S-Corp Conversions

Imagine you've spent fifteen years building a successful C corporation. The business owns a warehouse purchased in 2010 for $400,000, now worth $1.2 million. Inventory is on the books at $300,000 but would sell for $500,000 at retail. Your accountant suggests converting to an S corporation to eliminate double taxation. You file Form 2553, the election kicks in, and a few months later a buyer offers $1.5 million for the warehouse.

You sell. You expect the gain to flow through to your personal return at long-term capital gain rates. Instead, you discover the corporation owes a 21% federal tax on the appreciation that built up while it was a C corp—on top of the tax you'll pay personally.

That's Section 1374, the built-in gains (BIG) tax. It's one of the most expensive surprises a business owner can encounter, and it's almost entirely avoidable with planning.

Why Section 1374 Exists

When Congress allowed S corporations in 1958, it created a single-level tax pass-through entity. Income flows through to shareholders without a corporate-level tax. C corporations, by contrast, pay tax at the entity level and again when shareholders receive dividends—the classic double tax.

That difference creates a tempting maneuver. A C corporation sitting on appreciated assets could elect S status, immediately sell the assets, and potentially pay only one layer of tax instead of two.

Section 1374 closes that loophole. It says: if you convert from a C corp to an S corp and dispose of appreciated property during a recognition period, the corporation pays a tax on the gain that built up before the conversion. The pre-conversion appreciation is treated as if the company were still a C corp.

Who Section 1374 Applies To

The BIG tax applies to S corporations that meet two conditions:

1. The corporation was previously a C corporation (or acquired assets from a C corporation in a carryover-basis transaction).
2. The corporation disposes of an asset during the recognition period that has unrealized appreciation as of the conversion date.

If you've always been an S corporation since formation, Section 1374 doesn't apply. If you started as a C corp and elected S status, it does—and the clock starts on the first day of your first S corporation tax year.

There's also a sneaky trigger: if an S corporation acquires assets from a C corporation in a transaction where the corporation takes a carryover basis (such as certain reorganizations under Section 368), those acquired assets bring their own built-in gain and recognition period along with them.

The Five-Year Recognition Period

Originally, Section 1374's recognition period was ten years. The Protecting Americans from Tax Hikes (PATH) Act made a five-year recognition period permanent for tax years beginning on or after January 1, 2015. That five-year window is still the law in 2026.

The period begins on the first day of the first taxable year for which the S election is effective. So a calendar-year corporation that elects S status starting January 1, 2026 has a recognition period running through December 31, 2030. Sell an appreciated asset on January 2, 2031, and the BIG tax doesn't apply.

That five-year window is the whole game. Plan your dispositions around it and you can sidestep most of the tax. Ignore it and you can lose six figures or more.

How the Tax Is Calculated

Section 1374 uses two key concepts that often confuse first-time readers.

Net Unrealized Built-In Gain (NUBIG)

NUBIG is a snapshot taken on the conversion date. It equals:

The fair market value of all corporate assets, minus their adjusted basis, minus liabilities and deductible items as of the conversion date.

NUBIG sets the lifetime ceiling on what can ever be taxed under Section 1374. If your NUBIG is $2 million on the conversion date, the cumulative BIG tax base over the entire recognition period cannot exceed $2 million, no matter what happens.

Determining NUBIG requires a contemporaneous appraisal of every significant asset—real estate, equipment, inventory at retail value, intangibles, customer lists, goodwill—as of day one. Skipping the appraisal is the most common mistake business owners make. Without it, the IRS gets to estimate the values, and they tend to estimate high.

Net Recognized Built-In Gain (NRBIG)

NRBIG is the annual figure that actually triggers tax. For each year in the recognition period, NRBIG equals the lesser of:

1. The recognized built-in gains for the year, minus recognized built-in losses, or
2. The corporation's overall taxable income for the year (computed under C corp rules).

In other words, you can't be hit with BIG tax in a year where you have no taxable income. Built-in gains that would have been taxed in a loss year carry forward and may be taxed in a future year (within the recognition period).

The Tax Rate

Section 1374(b)(1) imposes the tax at the highest corporate rate under Section 11(b). That rate is currently a flat 21%. So once you have NRBIG for a year, the corporate-level tax is 21% of that figure, paid by the corporation on Form 1120-S, Schedule D.

The shareholders then pay their own personal tax on the same gain when it passes through. There's a slight reduction in the pass-through gain to reflect the BIG tax paid, but the bottom line is that you pay tax twice: once at 21% corporate, once at the shareholder rate. That's the double tax the S election was supposed to eliminate—except it's not eliminated for the first five years.

A Concrete Example

Consider a calendar-year C corporation that elects S status effective January 1, 2026. On that date:

• Land: basis $200,000, FMV $700,000 (built-in gain $500,000)
• Building: basis $300,000, FMV $600,000 (built-in gain $300,000)
• Inventory: basis $100,000, FMV $250,000 (built-in gain $150,000)
• Goodwill: basis $0, FMV $400,000 (built-in gain $400,000)
• Liabilities: $200,000

NUBIG = ($700K + $600K + $250K + $400K) − ($200K + $300K + $100K + $0) − $200K = $1,150,000.

The corporation's lifetime exposure under Section 1374 is capped at $1.15 million.

Now suppose in March 2027 (year two of the recognition period), the corporation sells the land for $750,000. The recognized built-in gain is $500,000 (the appreciation locked in on January 1, 2026; any appreciation between then and the sale is post-conversion gain and not subject to Section 1374). Assuming positive taxable income that year, NRBIG is $500,000.

The corporate-level BIG tax is $500,000 × 21% = $105,000.

The shareholders then report the same gain on their personal returns (reduced by the $105,000 already paid), and pay their own capital gain tax on top.

If the same sale had happened in March 2031 (after the recognition period closed), the entire gain would have flowed through to shareholders at long-term capital gain rates with no corporate-level tax. Same economics, same business, very different outcome.

Seven Strategies to Minimize or Eliminate the BIG Tax

1. Defer Dispositions Past the Recognition Period

The simplest planning move is the most powerful. If you can hold appreciated assets for sixty months after the S election, the BIG tax disappears entirely. Always check the disposition timeline before signing a purchase agreement.

2. Use Section 1031 Like-Kind Exchanges

A like-kind exchange of real property under Section 1031 is not a recognized disposition for BIG tax purposes (except to the extent of boot). The replacement property inherits the recognition period clock, but the gain doesn't accelerate. After the Tax Cuts and Jobs Act, Section 1031 only applies to real property—personal property exchanges no longer qualify.

3. Use Up C Corporation Tax Attributes

Net operating losses, capital loss carryforwards, minimum tax credits, and general business credits generated during C corp years can be used to offset BIG tax. These attributes don't otherwise survive an S election, so applying them against BIG tax is essentially free. Coordinate with your tax preparer to ensure they're claimed correctly on Schedule D of Form 1120-S.

4. Sell on the Installment Method

Section 453 installment sales let you recognize gain over multiple tax years. For BIG tax purposes, the installment-method gain remains subject to Section 1374 in the year it would otherwise have been recognized if not deferred—but spreading over multiple years can match payments to years of low taxable income, reducing or eliminating NRBIG in some years.

5. Lease or License Instead of Sell

A lease or license arrangement—if it's a bona fide lease and not a disguised sale—doesn't trigger gain recognition. Equipment, real estate, and intellectual property are all candidates. Be careful: if the lease is structured to transfer benefits and burdens of ownership, the IRS may recharacterize it as a sale.

6. Donate Appreciated Property to Charity

A charitable contribution of appreciated property is not a recognition event under Section 1374. The corporation gets a charitable deduction (subject to the usual limits) and avoids the BIG tax on the contributed asset. This works particularly well for assets the owner doesn't strictly need to sell, like artwork on the company books or appreciated stock holdings.

7. Sell Stock Instead of Assets

If the entire business is being sold during the recognition period, structure the deal as a stock sale rather than an asset sale. The selling shareholders recognize capital gain on their stock, but no asset-level gain is realized inside the corporation, so Section 1374 doesn't apply. Buyers usually prefer asset sales for the basis step-up, so the negotiation often turns on a Section 338(h)(10) election or other structuring tools that balance both sides.

Common Pitfalls That Cost Real Money

No appraisal at conversion. Without contemporaneous fair market value documentation as of day one, you can't credibly establish how much of any later gain is built-in versus post-conversion appreciation. Get a qualified appraiser before you file the S election.

Forgetting about goodwill. Self-created goodwill often has zero basis and substantial value, especially in service businesses. It rarely shows up on the balance sheet but can dwarf every other built-in gain on the appraisal.

Inventory blow-up. Inventory carried at LIFO can have massive built-in gain hidden in old layers. Section 1374 captures it as recognized built-in gain when sold during the recognition period.

Ignoring carryover-basis acquisitions. If your S corp acquires another business in a tax-free reorganization, the acquired assets come with their own built-in gain and a fresh recognition period.

Cash-basis accounts receivable. A C corp on the cash method that converts to an S corp brings unbilled receivables to the table. Those receivables are built-in gain when collected during the recognition period.

State-level BIG tax. Many states piggyback on Section 1374 with their own corporate-level tax. California's franchise tax and Iowa's BIG tax are two examples. Federal planning isn't enough; check your state.

The Bookkeeping Connection

Section 1374 turns clean record-keeping into real money. To compute NUBIG correctly, you need a precise asset-by-asset tally of basis, fair market value, and acquisition date as of the conversion date—then a way to track those figures forward through every disposition during the recognition period. Five years of precise tracking, applied to assets that may have been on the books for decades.

Spreadsheet-based tracking breaks down fast under that kind of pressure. So does opaque accounting software where the audit trail lives somewhere only your software vendor can see. Plain-text accounting solves both problems: every transaction, every asset, every basis adjustment lives in human-readable text files you control. When the auditor asks how you arrived at a NUBIG figure five years ago, you can show them the exact entries—and the exact commits that produced them.

Keep Your S-Corp Election Defensible from Day One

Whether you're approaching an S election, navigating the recognition period, or planning a sale, accurate financial records are the difference between a clean conversion and a six-figure surprise. Beancount.io provides plain-text accounting that's transparent, version-controlled, and AI-ready—so every basis adjustment, asset disposition, and tax election is tracked in a format your CPA, your auditor, and your future self can all read. Get started for free and see why developers and finance professionals are switching to plain-text accounting.