Sam and Riley's situation
Sam Park is a UX designer making $135K. Riley Park is a project manager making $105K. They bought a 2-flat in Chicago's Logan Square neighborhood in late 2024 for $620,000. They live in the upper unit (1,200 sqft) and rent the lower unit (1,100 sqft) on a 12-month lease to a long-term tenant. The tenant pays $2,400/month.
Their CPA mentioned that "cost seg might be worth looking at" but didn't give a clear answer. Sam asked on a real estate forum and got responses ranging from "definitely yes, $40K easy" to "no, it doesn't apply to your primary residence." Both are wrong, and both are right — depending on how you allocate the basis.
The rule, plain
Cost segregation only depreciates property used in a trade or business or held for the production of income (IRC §167). Your primary residence isn't either. Cost seg cannot touch the basis allocated to the unit you live in. But the rental portion IS held for income production. Cost seg applies to it normally.
The mechanics are simple in concept and exacting in execution:
- Allocate purchase price between personal-use and rental-use portions. Square footage is the most common allocation method. Sam and Riley's 1,200 sqft personal + 1,100 sqft rental = 2,300 total, with the rental portion at 47.8%.
- Carve out land separately. Land doesn't depreciate. In Logan Square, county assessor records suggest ~18% land allocation. So $620K × 18% = $111,600 of land, $508,400 of depreciable improvements.
- Apply the rental percentage to the depreciable improvements. $508,400 × 47.8% = $243,015 of rental-side depreciable basis. This is what cost seg works on.
- Run cost seg on the rental basis. Apply LTR reclassification rates (~18% for residential rental) to the carved-out portion.
The math, worked
| Line | Amount | Source |
|---|---|---|
| Purchase price | $620,000 | closing docs |
| Land allocation (18%) | −$111,600 | assessor |
| Depreciable improvements | $508,400 | computed |
| Rental allocation (47.8% by sqft) | $243,015 | computed |
| 5/15-year reclassification (≈18%) | $43,743 | benchmark |
| OBBBA bonus dep (100%) | $43,743 | §168(k) |
| Year-1 federal tax savings @ 24% | $10,498 | — |
Study fee: $1,295 at Cost Seg Smart for properties in the $300-700K range. ROI: 8×. Net benefit: roughly $9,200 of federal savings on the Park's 2024 joint return.
Smaller absolute number than the W-2 Doctor's case because: (a) only 47.8% of the basis qualifies, (b) LTR reclassification rates are about 60% of STR rates (no FF&E uplift), and (c) the Parks are in the 24% bracket, not 35%. But still a clean 8× return on the study fee.
The §469 trap that catches house-hackers
Sam and Riley have W-2 income totaling $240K and a long-term rental. They're not REPS-qualified (full-time W-2 jobs). Average customer stay on the lower unit is 365 days, so the STR exception doesn't apply. Their rental losses are passive under §469.
The $10,498 of cost-seg-driven loss CAN'T offset their W-2 income. It offsets the rental's income (about $28,800/year) and any excess gets carried forward. In their case, the cost-seg deduction more than covers their first-year rental income, so the entire $10,498 is suspended on Form 8582 to be used in future years against passive income or freed when the property is sold.
This isn't a reason not to do cost seg. It's a reason to understand timing. The deduction lives on the return as a suspended loss. When the Parks eventually sell or generate passive income (a second rental, a syndication interest, a passive partnership), the suspended loss applies. Or — more relevant for house-hackers — when one spouse becomes REPS-qualified, the suspended losses unlock.
The $25,000 special allowance under §469(i) doesn't help here either. The allowance phases out completely above $150K AGI. The Parks are at $240K. They're past the phase-out.
The basis-allocation gotchas
Square footage isn't the only method
Square footage is the simplest and most defensible. But if the units have meaningfully different finishes (e.g., the rental unit has builder-grade everything while the owner unit was renovated), an allocation by relative fair market value or by appraisal can produce a different — and usually more favorable — split. The IRS accepts any "reasonable method" of allocation; defensibility comes from documentation.
Common areas count partially
Roof, foundation, exterior walls, basement utilities, shared driveway — components used by both units allocate to both. The cost seg engineer separates these explicitly in the report. House-hackers shouldn't see common-area components carved entirely into one side.
Owner-occupied conversion timing
If Sam and Riley later move out and rent the upper unit too, the personal-use basis converts to rental basis at fair market value on the date of conversion (not original cost). A second cost seg study on the converted unit, dated from the conversion date, captures the upper unit's basis at that point. Don't try to amend the original study to include the upper unit retroactively.
The decision tree
When this fails
- Tiny rental allocation. If you're renting one bedroom in a 4-bedroom house, the rental allocation might be 15% by sqft. Study fees plus complexity make the math marginal.
- Below-FMV rental to family. §280A treats below-market rentals as personal use. The "rental" portion may not qualify as held for income production.
- Plan to convert primary residence within 2 years. Recapture on sale plus the basis-conversion mechanics get complicated.
- Property under $300K total purchase. Rental basis after allocation often falls below $100K. Study fee economics don't justify it.
What Sam and Riley should actually do
Get the cost seg study now (year one of ownership = maximum benefit under OBBBA bonus). Hand it to their CPA along with the explicit basis allocation methodology. The deduction creates a suspended passive loss that sits on Form 8582 until they (a) acquire a second rental that generates passive income, (b) one of them transitions to REPS qualification, or (c) they sell. Any of those events frees the suspended loss.
For a worked second-rental scenario where the Parks acquire another property in year 2 and unlock the suspended loss, see the Real Estate Pro persona. For the timing on conversion if they decide to move out and rent the upper unit, see the Form 3115 lookback section of the pillar guide.
Order a study on your house hack
Cost Seg Smart handles owner-occupied multi-unit allocations as a standard offering. The study models the rental basis separately and produces a clean component breakdown for your CPA.
Disclosure. This page is operated by Cost Seg Smart LLC. The "order a study" CTA routes to costsegsmart.com, the same operator. Numbers in the worked example are modeled from Cost Seg Smart's 2026 benchmarks dataset (n=260 studies). Your actual study will differ. Nothing on this page is tax, legal, or financial advice — consult a qualified CPA.