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If you live in one unit of a duplex, triplex, or fourplex and rent out the others, you are "house hacking," and the rental side of that building can usually be depreciated like any other rental property. A cost segregation study speeds that depreciation up. Instead of deducting the rental portion of the building over 27.5 years, a study identifies the components the tax code treats as 5-year, 7-year, and 15-year property, and qualifying property in those classes acquired and placed in service after January 19, 2025 is eligible for 100 percent bonus depreciation.
The catch comes first. Only the rental share of the building can be depreciated, the passive loss rules may limit how much of the deduction you can use this year, and the home sale rules treat the part you live in very differently from the part you rent. A house hacker who understands those three points can plan well. One who ignores them can end up with a big deduction they cannot use, or a surprise at sale time.
House hacking is a real estate strategy; the tax code has no name for it. You buy a small multi-unit building, move into one unit, and let the rent from the other units cover some or all of your housing cost. The tax code cares only about how each part of the building is used. The unit you live in is personal use. The units you rent are rental use. A building that is part personal and part rental is two things on your tax return, a home and a rental property, whether it is a duplex, a fourplex, a home with a legal accessory dwelling unit, or a house where you rent bedrooms to roommates.
Most house hackers focus on cash flow: does the rent cover the mortgage? Depreciation is a quieter benefit, because it reduces the tax you owe on the rental income, and in some cases on other income too. Residential rental buildings are depreciated over 27.5 years, straight-line, after the land comes out. For the rental share of a duplex, that is a modest annual number.
A building is a structure plus a collection of components, and the components have their own class lives. Appliances, removable carpet and vinyl flooring, specialty lighting, and removable fixtures can be examined as personal property with recovery periods of 5 or 7 years where the engineering supports it. Kitchen cabinets do not make that list in residential property; courts have treated them as part of the building. Site improvements like driveways, walkways, fencing, landscaping, and outdoor lighting are typically 15-year land improvements. A cost segregation study is a fully engineered and accounted analysis that identifies those components, supports their cost, and puts them on the shorter schedules.
See our guide on what cost segregation is.
Before any study is done, answer one question: what portion of this property is rental property? The allocation must be reasonable, consistent, and supportable. The common methods:
Shared spaces such as hallways, a laundry room, or the exterior structure are allocated using the same percentage, and so are expenses that benefit the whole building. Keep a written memo with a floor plan and the calculation.
The figures are illustrative. Dolores, a nurse anesthetist who files single, buys a side-by-side duplex on Bayshore Drive in Naples, Florida, for $800,000. Each unit is the same size and layout. She closes in March 2026, moves into one unit, and has a tenant in the other within weeks at $2,500 per month, or $30,000 per year.
Step 1: Separate land from building. Her county assessment and an appraisal put 20 percent of the value on land. That is $160,000 of land, which cannot be depreciated, and $640,000 of building.
Step 2: Determine the rental share. The units are identical in size, so the rental share is 50 percent. The depreciable rental basis of the building is $320,000. The other $320,000 is her personal residence and is not depreciable.
Step 3: Standard depreciation. Without a cost segregation study, the rental basis is depreciated over 27.5 years. In a full year that is $11,636. In 2026 the building takes the IRS mid-month table rate for a March placed-in-service date, 2.879 percent, so first-year depreciation is $9,213.
Step 4: Cost segregation. A study reviews the rental share of the building and identifies components that qualify as 5-year, 7-year, and 15-year property. Suppose the study reclassifies 25 percent of the rental basis, or $80,000, into those shorter-life categories. Some properties come in lower and some higher.
Step 5: Bonus depreciation. The reclassified property has a recovery period of 20 years or less and was acquired and placed in service after January 19, 2025, so 100 percent bonus applies automatically and is never prorated. The full $80,000 is deductible in 2026. The remaining $240,000 of rental basis continues on the 27.5-year schedule at 2.879 percent in the first year, $6,910.
So in 2026, instead of $9,213 of depreciation, Dolores has $86,910 on the rental side: a much larger deduction early, with less left for later years.
A larger deduction is only valuable if it can reduce your tax this year. For most house hackers, rental income is passive, and passive losses only offset passive income unless an exception applies. Finish Dolores's example. Her rental produces $30,000 of income. After the rental share of interest, property tax, insurance, and maintenance, the unit shows net income of $14,000 before depreciation. With $86,910 of first-year depreciation, the rental now shows a net tax loss of $72,910.
Can she use that loss against her wages? It depends on which rule applies:
Dolores has modified adjusted gross income of $110,000. Her $25,000 allowance is reduced by $5,000, half of the $10,000 over the threshold, leaving $20,000. So $20,000 of the $72,910 loss is usable this year against her wages. The remaining $52,910 is a suspended passive loss that carries forward against future passive income or is released when the property is sold in a fully taxable transaction. Our article on suspended passive losses at sale explains how that release works.
At her 24 percent federal bracket, using $20,000 of loss this year is worth $4,800 in federal tax, and the carryforward has future value. That is the real picture of a house hack with cost segregation: a timing benefit, well short of the full $86,910 landing against wages unless other rules are met. Under $100,000 of income, she could use the full $25,000 allowance. Over $150,000, the loss would be fully suspended unless another exception applies, which is why we ask about income before recommending a study. Our article on when not to do cost segregation explains how we think about that.
Many house hackers rent part of their property on Airbnb or a similar platform instead of on a long-term lease. Under the passive activity regulations, a rental in which the average period of customer use is seven days or less, weighted by gross rental income, is not treated as a rental activity for passive loss purposes. If you also materially participate, the income and loss are nonpassive, and depreciation from a study can offset W-2 wages. That is what people call the short-term rental loophole. We explain the details in our guide to the short-term rental loophole.
Two cautions. Material participation is a real test: you must meet one of the seven IRS tests and keep a time log to prove it, and a property manager doing most of the work can disqualify you. And personal use can limit the deduction: if you use a rented dwelling unit for personal purposes for more than the greater of 14 days or 10 percent of the days it is rented at fair rental value, the vacation home rules apply. Living in one unit and renting a separate unit keeps the units apart; renting a spare room in the home you live in is a different fact pattern. See our guide on the 14-day personal use rule.
When you buy a property intending to rent part of it, your depreciable basis is your cost allocated to the rental portion. For the 100 percent bonus rate, the acquisition date controls, generally tied to a binding written contract: qualifying property acquired and placed in service after January 19, 2025, with no binding contract before January 20, 2025. Our guide on IRS Notice 2026-11 and acquisition dates goes into detail.
When you convert personal use property to rental use, the depreciable basis is the lesser of the property's fair market value or your adjusted basis on the date of conversion. If your home has appreciated, your adjusted basis is the number, and today's higher value stays out of it. Special rules apply to bonus depreciation on converted property, so review that point with your CPA before relying on a bonus deduction. We cover the conversion fact pattern in more depth in converting a home to a short-term rental.
Property is placed in service when it is ready and available for its intended use; closing day is irrelevant, and a vacant unit ready to rent counts.
Selling the property. When you sell a house hack, the sale is split into two pieces: the personal part and the rental part. For the part you lived in, you may be able to exclude up to $250,000 of gain ($500,000 for married couples filing jointly) under Section 121, if you owned and used that part as your main home for at least two of the five years before the sale. For the rental part, the exclusion does not apply, and the gain is taxable.
For the rental part, depreciation you claimed, or were allowed to claim, reduces your basis and is recaptured when you sell. Gain attributable to depreciation on the building (Section 1250 property) is taxed at a maximum federal rate of 25 percent as unrecaptured Section 1250 gain. Gain attributable to depreciation on reclassified personal property (Section 1245 property) is taxed as ordinary income up to the depreciation taken. Cost segregation is a timing strategy: larger deductions earlier, some of it back through recapture. Our guide on depreciation recapture shows how to plan for it.
Depreciation claimed after May 6, 1997 cannot be excluded under Section 121, even if the unit later qualifies as your main home for two of the last five years. You cannot depreciate the unit, move in, and wash the depreciation away with the home sale exclusion.
Moving out and renting the whole building. When you buy a new home and keep the old building as a full rental, the personal share becomes rental use and depreciation begins on it under the conversion rules above. If you missed depreciation on the rental share in prior years, a Form 3115 change in accounting method may be an option. Our guide on look-back studies and Form 3115 explains how that works.
A 1031 exchange can defer the gain on the rental portion, never on your residence.
An estimated study applies a percentage to the purchase price from a questionnaire, which is Approach 6 in the IRS Cost Segregation Audit Techniques Guide, the one the guide says comes with little or no documentation. A fully engineered and accounted cost segregation study works under the first two approaches the guide describes, detailed engineering from actual cost records and engineering with documented estimates reconciled to actual cost. For a house hack, that means the study:
Cost Seg America defends the study for as long as you own the building, with no time limit, no hour cap and no additional fee, through written responses and participation in examiner calls alongside your CPA. For a house hack, the most helpful document you can hand the team is the written allocation between the personal and rental space.
Cost Seg America's minimum is $180,000 of depreciable basis after land on the rental share. Below that, the fee eats the benefit. A study is also unlikely to pay when your loss would be fully suspended for many years because your income is over $150,000 and no other exception applies, or when you plan to sell within a couple of years and Section 1245 recapture takes back most of the benefit. Our guide on what a cost segregation study costs can help you weigh the numbers.
Not on the part you live in. The personal residence portion is never depreciable. You can do cost segregation on the rental portion of a multi-unit property where you live in one unit and rent the others.
The rental portion can, provided the reclassified property has a recovery period of 20 years or less and the property was acquired and placed in service after January 19, 2025. Confirm your own dates and facts with your CPA.
Possibly. If you actively participate in the rental and your modified adjusted gross income is below $150,000, part of the loss may be usable under the $25,000 allowance. If you qualify as a real estate professional and materially participate, or your rental qualifies under the short-term rental rules, the loss is nonpassive. Otherwise it is suspended and carried forward.
You can depreciate the portion of the home used for the rented room and the shared space allocated to rental use, but the vacation home rules can apply and the dollar amounts are usually small. A study is less often worthwhile here.
Depreciation taken on the rental portion reduces your basis and is recaptured on sale: unrecaptured Section 1250 gain at a maximum federal rate of 25 percent, and Section 1245 recapture on reclassified personal property as ordinary income. The Section 121 exclusion covers only the personal portion.
Yes. A look-back study claims the depreciation you missed through a Form 3115 change in accounting method, without amending prior returns, as a Section 481(a) adjustment in the year of change. Cost Seg America prepares the computation and, for a separate fee, the form; your CPA files it.
No one can promise what the IRS selects for examination. Poor documentation, rather than the study itself, is what creates risk.
If you own, or are about to buy, a duplex, triplex, fourplex, or a home with a rentable unit, send Jim Dougherty and his team the address, the purchase price, the rental share and your household income range. You will have a free proposal within 24 hours showing the projected first-year deduction on the rental share and the fee, and a plain answer on whether a study makes sense for you, including when the answer is no. Call 1-888-365-5023 or email info@costsegamerica.com, and bring your CPA into the conversation.
This article is educational and general in nature, and is not tax, legal, or accounting advice. Confirm your specific situation with your CPA before making any decision.
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