Owner-Occupied Duplex and Fourplex Sellers: Splitting Section 121 and Section 1031

The regulation draws the line at the dwelling unit, and your depreciation schedule has already drawn it for you

Reg. section 1.121-1(e)(1) says section 121 "will not apply to the gain allocable to any portion (separate from the dwelling unit) of property sold or exchanged with respect to which a taxpayer does not satisfy the use requirement," and that no allocation is needed when the residential and non-residential uses are inside the same dwelling unit. A rented unit with its own kitchen and entrance is a separate dwelling unit; a rented bedroom in your own unit is not.

Paragraph (e)(3) then fixes the method: basis and amount realized are allocated "using the same method of allocation that the taxpayer used to determine depreciation adjustments." If you have depreciated 75% of a fourplex's cost for years, the sale is 75% rental for section 121 and for section 1031. Changing the method at the sale invites the question why the depreciation was wrong.

The example in the regulation is a townhouse with a separate basement apartment, and the owner must allocate because "the basement apartment and the upper floors of the townhouse are separate dwelling units." A duplex, triplex or fourplex is the same case with more units.

The home share is the easy part, unless you moved between units or once rented the one you live in

Section 121(a) excludes gain on property you owned and used as your principal residence for periods aggregating two years in the five years before the sale, up to $250,000 or $500,000 on a qualifying joint return, and section 121(b)(3) allows the exclusion only once in any two-year period. Depreciation claimed after May 6, 1997 is never excluded under section 121(d)(6), which matters if the unit you now live in was once a rental.

Section 121(b)(5) removes from the exclusion the share of gain allocable to nonqualified use, meaning periods after 2008 when the unit was not your residence, but section 121(b)(5)(C)(ii) carves out time after the last date you lived there. So moving from unit A to unit B and renting A afterwards does not create nonqualified use on A; moving into a unit that had been a rental since 2010 does. The 1031 versus section 121 guide sets out the direction-of-travel rules.

If the unit you live in was rented first and the exclusion is cut down, Rev. Proc. 2005-14 lets a single sale use section 121 first and section 1031 on the rest, including depreciation, provided the property is investment property on the sale date; using both on one property explains the ordering.

Worked model: a $1,200,000 fourplex, one quarter home, and the decision on the other three quarters

These figures are hypothetical and rounded. A couple bought a fourplex with four equal units for $400,000, have lived in one unit for twelve years and rented the other three, taking $150,000 of depreciation on the rental share. They sell for $1,200,000 net of costs.

• Home quarter: $300,000 of price against $100,000 of basis, a $200,000 gain, fully excluded on a joint return.
• Rental three-quarters: $900,000 of price against $300,000 of cost less $150,000 of depreciation, a $750,000 gain, of which $150,000 is unrecaptured section 1250 gain at up to 25% under Topic 409 and $600,000 is long-term gain at 15% or 20% depending on where taxable income lands against the 2026 bands in Rev. Proc. 2025-32.
• Exchange the rental share: reinvest $900,000 and replace the rental share of any loan, and the full $750,000 is deferred. The $300,000 from the home unit comes to the couple tax-free and never touches the intermediary.
• Have your CPA run the rental line against your real depreciation schedule, because that schedule sets the split.

When paying the tax beats the exchange: small rental shares, low-income years and a replacement you do not want

An exchange is worth its cost when the deferred tax is large and the replacement is something you would own anyway. Three cases cut the other way: a duplex where the rental half carries little gain, a retiree couple whose 2026 taxable income leaves room under the $98,900 joint ceiling for the 0% long-term rate in Rev. Proc. 2025-32, and a seller whose only reason to reinvest is the exchange itself.

The 0% band applies to the long-term gain, not to the unrecaptured section 1250 gain, which is taxed at up to 25% regardless. Run the rental share through both lines before deciding; a half-and-half duplex with $80,000 of depreciation and $100,000 of appreciation may cost less to sell outright than a seller expects.

Where the rental share is large, the exchange defers everything, and the home share's cash is free to use. That combination, exclusion on one unit and deferral on the others, is the reason to keep the two halves on separate lines of the closing statement.

Exchanging the rental share into another building you will partly live in brings its own rules

Many sellers want the next building to be a house hack too. Only the units you will rent are replacement property; the unit you will live in is personal-use property bought with exchange money, which is taxable to the extent of gain. Exchanging into a duplex or fourplex you will partly occupy covers that purchase, including the occupancy rules an owner-occupied loan adds.

If the plan is to rent the replacement first and move in later, section 121(d)(10) denies the exclusion on a sale within five years of acquiring property in a section 1031 exchange, and Rev. Proc. 2008-16 gives a safe harbor for the rental period: own the dwelling 24 months after the exchange, rent it 14 days or more at fair rental in each 12-month period, and keep personal use under the greater of 14 days or 10% of rental days.

For a rental share that is not large enough to buy a building, an all-cash DST is the usual replacement

Three quarters of a fourplex, or half of a duplex, often leaves equity that does not buy a whole rental in the same market without new debt. A traditional DST takes the rental share at its size: Rev. Rul. 2004-86 treats the beneficial interest as an interest in real property, an all-cash trust needs no loan if the rental share carried none, and the home share's cash stays outside. DST interests cannot be sold on demand, the sponsor controls the sale, and the property can lose value, so principal is at risk.

A seller who wants to keep title and the decision to sell can look at a direct title security instead, and a seller buying another small multifamily building outright will find the underwriting on the duplex and fourplex property page.