Does renting out a room in my house mean I lose part of the capital gains exclusion?

No. Renting a bedroom to someone who shares your kitchen and bathroom keeps everything inside one dwelling unit, and the last sentence of Treas. Reg. §1.121-1(e)(1) says no allocation is required in that case, so §121 reaches the whole gain. The one piece it cannot reach is depreciation. Under §121(d)(6), gain up to the depreciation allowed or allowable after May 6, 1997 stays taxable as unrecaptured §1250 gain, at a rate capped at 25% by §1(h)(1)(E).

How do I allocate the gain between the residence and the rental unit?

Treas. Reg. §1.121-1(e)(3) has you allocate the basis and the amount realized between the two portions. Square footage holds up best because it is objective and reproducible, and it is what I use unless the facts argue otherwise. Relative fair market value is also acceptable and occasionally gives a better answer, for instance when the owner's unit has the view and the rental unit is a windowless basement. Write the method down and keep the measurements, because the same allocation drives the exclusion, the depreciation schedule, and anything you have to defend in an examination.

Is splitting one sale between §121 and §1031 actually legitimate?

Yes, and the IRS wrote the procedure for it. Rev. Proc. 2005-14 addresses exactly the case where both provisions could apply to a single sale, and it sets the ordering: §121 first, then §1031 on the remaining gain. The mechanics are what is strict, not the concept. The rental portion has to run through a qualified intermediary, you get 45 days to identify replacement property and 180 days to close, and property used solely as a personal residence does not qualify for §1031 at all, which is why the allocation has to be defensible before you close.

I rented the basement for years and never claimed depreciation. Does that help me?

No, it costs you twice. §1250(b)(3) defines the depreciation adjustments as the amounts allowed or allowable, so the gain is figured as though you had taken the deductions whether or not you did. You lose the deductions and still owe the tax on them. If you still own the property, Form 3115 with a §481(a) adjustment catches up every missed year on a single return, and it has to be filed before the sale to do you any good.

Does adding a kitchen to my basement so I can rent it out change my taxes right away?

Not much in the year you build it. The construction is a capital improvement you add to basis and depreciate over 27.5 years for a long-term rental, and the rent and expenses go on Schedule E. What changes is the exit. Under Treas. Reg. §1.121-1(e)(4), Example 3, installing the kitchen and bath is the act that turns the basement into a second dwelling unit, so that portion's gain comes out of the §121 exclusion when you sell. Example 4 is the reverse: remove the kitchen and reconnect the space and it is one dwelling unit again, though you then need two years of residential use of that portion inside the five-year window before it rides along with the exclusion.

The Journal / Real Estate Tax

Selling a House With a Rental Unit: A Kitchen Downstairs Costs $38,604.

By Ewan Morkel, EANovember 2, 20267 min read

Treas. Reg. §1.121-1(e) excludes the whole gain on a house with rented bedrooms and only the owner's share on one with a basement apartment. The hinge is whether the rented space has its own kitchen, and on an $850,000 sale it is worth $38,604.

Contemporary residential home behind a green yard and trees

A couple in Utah County buys a house in 2016 for $420,000, lives on the main floor, and rents the basement for ten years. In 2026 they sell for $850,000 and want to know whether the home sale exclusion covers the whole thing. The answer turns on something nobody mentions at the closing table: whether the basement has a stove. Selling a house with a rental unit is the one transaction where a $38,604 tax difference rides on a kitchen.

One question

Selling a house with a rental unit turns on whether the rented space is its own dwelling unit.

The allocation rule is two sentences long. §1.121-1(e)(1) says §121 does not apply to gain allocable to any portion of the property, separate from the dwelling unit, that fails the use requirement, and then adds the sentence that decides most of these sales: no allocation is required if both the residential and non-residential portions are within the same dwelling unit. The whole analysis collapses into a definition.

§1.121-1(e)(2) borrows that definition from §280A(f)(1), with one carve-out: appurtenant structures do not count as part of the unit. §280A(f)(1) lists a house, apartment, condominium, mobile home, or boat, and Prop. Reg. §1.280A-1(c)(1) supplies the test practitioners actually use, which is basic living accommodations, meaning sleeping space, a toilet, and cooking facilities. That proposed regulation dates to 1980 and was never finalized, so it does not bind anyone. It is also the only definitional guidance on the point and the IRS applies it, so I plan around it rather than bet against it.

The regulation's own example is this exact fact pattern. In Treas. Reg. §1.121-1(e)(4), Example 3, a taxpayer buys a three-story townhouse in 2002, converts the basement into a separate apartment by installing a kitchen and a bathroom and removing the interior stairway, rents it from 2003 to 2007, and claims $2,000 of depreciation. The conclusion the regulation reaches is that after the conversion the property is two dwelling units, so the gain has to be allocated. Example 4 runs it backwards: he takes the kitchen out, reconnects the basement to the house, and it is one unit again. The kitchen does the work in both examples.

This is the simultaneous version of a question the site has already answered the other way. Selling a rental I used to live in covers the sequential case, where the whole house was your residence first and a rental second, and the governing rule there is §121(b)(5) on periods of nonqualified use. That rule is about time. This one is about space, and the two do not overlap.

Three configurations cover nearly every house I see in this spot.

Bedrooms rented with a shared kitchen and bathroom. One dwelling unit, no allocation, and §121 reaches the entire gain except the depreciation.
A basement apartment, a mother-in-law suite, or an interior accessory dwelling unit with its own kitchen, bath, and entrance. Two dwelling units under Example 3. Allocate.
A duplex, or a detached garage apartment or guest house. Two units either way. The detached structure is appurtenant under §280A, but §1.121-1(e)(2) deliberately leaves appurtenant structures out of the dwelling unit for §121 purposes, so it gets allocated even though it shares your water meter.

The same house, twice

What a stove downstairs costs on an $850,000 sale.

Hold every other fact still. The basement is half the square footage, the depreciation schedule shows $61,091 of accumulated depreciation against it, and the only variable is whether the tenants cooked downstairs or came upstairs to do it.

Case A: the basement has its own kitchen and bath, so it is a separate dwelling unit

Sale price: $850,000
Commission and closing costs: $55,000
Amount realized, allocated 50/50: $397,500 each
Residence half: gain of $167,500, excluded under §121: $0
Rental half: adjusted basis after depreciation: $168,909
Rental half: total gain: $228,591
Unrecaptured §1250 gain of $61,091 at 22%: $13,440
Long-term capital gain of $167,500 at 15%: $25,125
Net investment income tax at 3.8%: $6,026
Utah at 4.45%: $10,172
Total tax: $54,763

Tax year 2026. Married filing jointly with $180,000 of wages and the $32,200 standard deduction under Rev. Proc. 2025-32. Bought for $420,000 in 2016 with $40,000 of improvements since, allocated 50/50 by square footage. The $61,091 of accumulated depreciation is ten years of straight line at $6,109 a year on the $168,000 of building basis assigned to the rented half, which is half the purchase price less a 20% land allocation; the improvements were made in 2025 and add to basis without changing the depreciation materially. Ordinary taxable income of $147,800 plus the $61,091 of unrecaptured §1250 gain tops out at $208,891, inside the 22% bracket that runs to $211,400 for joint filers, so the 25% ceiling in §1(h)(1)(E) never binds. The long-term gain stacks to $376,391, under the 15% bracket ceiling of $613,700. NIIT under §1411 applies to the lesser of net investment income or modified AGI over $250,000, which is $158,591 here. Utah's flat rate is 4.45% for 2026 under S.B. 60.

Case B: the tenants rent bedrooms and share the upstairs kitchen, so it is one dwelling unit

Amount realized, not allocated: $795,000
Adjusted basis after $61,091 of depreciation: $398,909
Total gain: $396,091
Excluded under §121: $335,000
Unrecaptured §1250 gain of $61,091 at 22%: $13,440
Net investment income tax at 3.8%: $0
Utah at 4.45% on the taxable $61,091: $2,719
Total tax: $16,159
What the kitchen downstairs costs: $38,604

Tax year 2026, same house, same basis, same depreciation. Under §121(d)(6) the exclusion cannot reach gain up to the depreciation allowed or allowable after May 6, 1997, so $61,091 stays taxable and the other $335,000 is excluded, comfortably under the $500,000 joint cap. Modified AGI is $241,091, below the §1411 threshold, so no net investment income tax applies. Utah's return starts from federal adjusted gross income, so excluded gain never reaches the state at all.

Two things open that gap. The first is obvious: $167,500 of appreciation on the rented half loses the exclusion and gets taxed at 15%, which is $25,125. The second is quieter and catches people. In Case B the only income added to the return is $61,091, so modified AGI lands at $241,091 and the 3.8% net investment income tax never starts. Put $228,591 on the same return and $158,591 of it sits above the threshold, which is another $6,026. The $250,000 figure in §1411 is statutory and has never been indexed, so it catches more of these sales every year. The $500,000 joint exclusion in capital gains tax when selling a house has not moved since 1997 either.

The lever

The rental half can go into a 1031 exchange on the same closing.

If you land in Case A, you are not stuck with the $54,763. Rev. Proc. 2005-14 exists specifically because neither §121 nor §1031 said what to do when both could apply to one sale, and it sets the order: §121 first, §1031 on what is left. The residence half takes the exclusion, the rental half goes through a qualified intermediary into replacement real estate, and the whole $228,591 comes off the 2026 return, including the $61,091 of depreciation §121 could never touch. Where the exclusion and the exchange overlap on the same square footage, the procedure increases the replacement property's basis by the §121-excluded gain attributable to the business portion. How to avoid depreciation recapture walks the rest of that trade.

Whether the exchange is worth doing is a different question, and my answer is usually no if you wanted the cash. A §1031 exchange buys a deferral and costs you a 45-day identification window, a 180-day closing, and a replacement property you have to genuinely want. $54,763 of tax against $397,500 of proceeds from the rental half is a 13.8% toll, and plenty of sellers should just pay it and move on. What I would not do is pay it by accident. Two items ride along either way: every suspended passive loss from the rental years releases under §469(g)(1) when you dispose of the entire interest in a fully taxable sale to an unrelated buyer, which is often worth thousands against ordinary income in the year of sale, and depreciation you never claimed still comes out of basis, because §1250(b)(3) measures the adjustments as allowed or allowable. Suspended passive losses on a rental sale covers the release.

On the return the two halves go to different places. The rental half starts on Form 4797, Part III. The residence half shows up on Form 8949 with code H, and only if you received a Form 1099-S or the gain ran past the exclusion. The title company reports gross proceeds of $850,000 on one 1099-S for one address, which is why these sales draw CP2000 notices: the return has to account for a number that was never split in the first place. IRS Publication 523 states the same test the regulation does, in plainer words, under Business or Rental Use of Home. Both directions of this error show up in second-opinion reviews, and allocating when you did not have to is the expensive one, because nobody ever gets a notice for overpaying.