When an owner lives in one unit of a fourplex and rents out the other three, the property is considered a mixed-use property by the IRS.
This scenario unlocks a unique tax loophole: you can combine Section 121 (the Primary Residence Tax Exclusion) with Section 1031 (the Like-Kind Exchange). Assuming all four units are of equal value, the transaction is split: 25% follows primary residence tax rules, and 75% follows investment property exchange rules.
Here are three examples of how this plays out depending on what the owner decides to buy next.
Example 1: Exchanging into a Pure Commercial Rental (The Upgrader)
The Scenario: Alex is a single filer who bought a fourplex for $400,000. He lives in Unit 1 and rents out Units 2, 3, and 4. Years later, the building is worth $1,000,000. Alex is tired of living next to his tenants and wants to move into a separate single-family home while swapping his fourplex for a hands-off commercial property.
The Math & Strategy:
Because Alex owns a fourplex and occupies one unit, the IRS views the transaction as two separate sales: a $250,000 primary residence sale (25%) and a $750,000 investment property sale (75%).
- The 25% Personal Side ($250,000): Alex’s profit on his personal unit is roughly $150,000. Under Section 121, a single homeowner can exclude up to $250,000 of capital gains from their primary residence tax-free. Alex pockets the $250,000 completely tax-free and can use it as a down payment on a new personal home.
- The 75% Investment Side ($750,000): The remaining $750,000 from the three rental units must go directly to a Qualified Intermediary (QI).
The 1031 Exchange:
Alex uses his QI to reinvest the $750,000 into a standalone, triple-net (NNN) commercial retail building worth $800,000 (adding $50,000 of his own capital or debt to cover the difference).
The Result: Alex walks away with tax-free cash from his living quarters to buy a private home, while successfully deferring 100% of the capital gains taxes on his three rental units by rolling them into a commercial asset.
Example 2: Exchanging into Another Owner-Occupied Fourplex (The Replicator)
The Scenario: Ben and Clara (married, filing jointly) bought a fixer-upper fourplex in an up-and-coming neighborhood for $300,000. They lived in one unit and managed the other three. The neighborhood boomed, and they just sold the fourplex for $800,000. They want to replicate this exact strategy in a new city—buying a nicer, more expensive fourplex for $1.2 million, living in one unit, and renting the rest.
The Math & Strategy:
- The Relinquished Fourplex ($800,000 total): * Personal portion (25%) = $200,000
- Investment portion (75%) = $600,000
- Tax Check: Their capital gains on the personal side are fully covered by the Section 121 married exemption (up to $500,000 allowed).
- The Replacement Fourplex ($1,200,000 total):
- Personal portion (25%) = $300,000
- Investment portion (75%) = $900,000
The 1031 Exchange:
To defer all investment taxes, Ben and Clara must ensure that the investment portion of the new property ($900,000) is equal to or greater than the investment portion of the old property ($600,000). It is.
The $600,000 from the old rental units flows through the QI straight into the new building. The remaining $600,000 balance for the new property (the $300,000 personal side + the remaining $300,000 for the investment side) is covered by a new bank mortgage and the cash they pocketed from their personal exemption.
The Result: They successfully upgrade to a higher-value multifamily property. The investment side is completely tax-deferred via the 1031 exchange, and their personal transition remains tax-free.
Example 3: The “Accidental Boot” (The Downsizer)
The Scenario: Diana is a single filer who has lived in her fourplex for 15 years. She sells it for $1,200,000. She wants to downsize into a smaller, quieter duplex down the road valued at $800,000. She plans to live in one unit of the duplex and rent out the other.
The Math & Strategy:
- The Old Fourplex ($1,200,000):
- Personal portion (25%) = $300,000
- Investment portion (75%) = $900,000
- Tax Check: On her personal portion, Diana has a large capital gain. Because she is single, Section 121 only shields the first $250,000 of her profit. She will owe capital gains tax on the remaining profit over that cap.
- The New Duplex ($800,000): * Because she is living in one unit and renting one unit, this new property is a 50/50 split.
- Personal portion (50%) = $400,000
- Investment portion (50%) = $400,000
The 1031 Exchange:
Here is where Diana runs into a tax trap. Her old investment value was $900,000. But her new investment value is only $400,000.
Remember the cardinal rule: to defer all taxes, you must buy an investment property of equal or greater value. Diana has downsized her investment value by $500,000 ($900,000 – $400,000).
The Result: This is a partial 1031 exchange. Diana successfully defers the taxes on $400,000 of the transaction. However, the $500,000 difference is considered “boot” by the IRS. She will be hit with a capital gains tax bill on that $500,000 gap, in addition to the taxes owed on her personal residence overage.