What is the Built In Gains Tax (BIG Tax) and how does it apply to Corporations

The Built-In Gains (BIG) Tax is a special corporate-level tax that can apply to S corporations that were previously C corporations (or acquired assets from a C corporation in certain tax-free transactions). It’s designed to prevent companies from avoiding corporate tax by converting to S status right before selling appreciated assets.

Here’s a breakdown:

🔹 1. What is the Built-In Gains (BIG) Tax?

When a C corporation converts to an S corporation, the IRS looks at the difference between:

• The fair market value (FMV) of the company’s assets, and
  
  

• Their tax basis (book value for tax purposes),
  at the date of conversion.
  
  

That difference is called the "built-in gain."

If the S corporation sells or disposes of any of those appreciated assets within a certain period after the conversion (the recognition period), it must pay a corporate-level tax on the built-in gain.

🔹 2. The Recognition Period

Historically, the recognition period was 10 years, but under current law (as of 2025), it’s 5 years.
So, if the S corporation sells appreciated assets within 5 years of the conversion from C to S status, the BIG tax applies.

🔹 3. The Tax Rate

The BIG tax is imposed at the highest corporate tax rate — currently 21% (since the 2017 Tax Cuts and Jobs Act).
It is paid by the S corporation itself, not by shareholders.

🔹 4. Example

Let’s say:

• A C corp converts to an S corp on January 1, 2025.
  
  

• On that date, it owns a building with:
  
  

  • FMV = $1,000,000
    
    

  • Tax basis = $600,000
    
    

  • Built-in gain = $400,000
    
    

If the S corp sells the building in 2028 for $1,200,000 (within 5 years):

• The built-in gain of $400,000 is subject to the 21% BIG tax.
  
  

• The remaining gain ($200,000) passes through to shareholders without corporate tax.
  
  

If the sale occurs after 2030 (after 5 years), no BIG tax applies — the entire gain passes through to shareholders.

🔹 5. How It Affects S Corps

• The S corporation must track built-in gains and losses separately.
  
  

• It can use built-in losses to offset gains during the recognition period.
  
  

• The BIG tax reduces the amount of income that passes through to shareholders.
  
  

• After the recognition period ends, appreciated assets can be sold with no corporate-level tax — the gain flows directly to shareholders.
  
  

🔹 6. Key Planning Considerations

• Timing asset sales: waiting until after the 5-year recognition period avoids the BIG tax.
  
  

• Valuation at conversion: having a proper FMV appraisal at the time of conversion is critical.
  
  

• Tracking C-corp NOLs: certain net operating losses (NOLs) from the C corporation period can offset BIG tax.