House hacking is how a lot of investors get their first door: you buy a two-, three-, or four-unit property with an owner-occupant loan, live in one unit, and rent out the others. It’s one of the few ways to put 3.5% or 5% down on a small multifamily. But when tax season arrives, the building splits in two. The unit you live in is your personal residence, and a personal residence isn’t depreciable. Cost segregation only works on the part of the property you rent — and getting that line right is the whole game.
The good news is that the rental part is fully in play. Because you bought the building, the short-life property a study carves out of it can qualify for the 100% first-year bonus depreciation that was restored for property acquired after January 19, 2025. So the question on a house hack is never simply “does a study help?” It’s “how much of this building is business-use, and how do we accelerate that share?”
What counts as the rental portion?
You divide the property between personal use (your unit) and business use (the units you rent), and only the business-use basis is depreciable. Most owners split by square footage; if the units are similar in size, a simple unit count — three of four — is a reasonable and defensible proxy. That same business-use percentage flows through everything a study touches. The roof, the siding, the driveway, the landscaping, the shared mechanicals: each gets reclassified only to the extent it serves the rental units. Nothing tied to the unit you sleep in gets depreciated while you live there.
How the math changes on an owner-occupied fourplex
Say you buy a fourplex for $600,000 with four roughly equal units, live in one, and rent three. Strip out 20% for land and you have $480,000 of building basis. Your business-use share is three of four units, or 75%, so your depreciable rental basis is 75% of $480,000 — $360,000. The remaining $120,000 of building basis, the slice tied to your own unit, sits out; it isn’t depreciable as long as you live there.
A reasonable residential study reclassifies around a quarter of building basis into 5-, 7-, and 15-year property — appliances, flooring, and fixtures, plus land improvements like the driveway and landscaping. Applied to the $360,000 of rental basis, that’s roughly $90,000 of short-life property. Because you acquired the building by purchase after January 19, 2025, that $90,000 can be written off in full in year one under 100% bonus depreciation.
Left alone in the 27.5-year bucket, that same $90,000 would throw off only about $3,300 of depreciation in the first year. Accelerated, it produces roughly $90,000. That’s about $86,700 of extra first-year deduction, or roughly $27,700 in first-year federal tax savings at a 32% marginal rate — generated by three of the four units, with your own unit contributing nothing to the number. On a duplex where you rent only one of two units, the same logic applies at a 50% business-use share, so the accelerated dollars are real but smaller.
What happens to the unit you live in?
It waits. While it’s your residence it stays out of the depreciation math entirely. Two things can change that later. If you move out and rent it, the unit converts to business use; you place it in service as a rental at that point and can run a study on that portion then — we walk through that mechanic in our post on converting a primary residence to a rental. And when you eventually sell, the rental portion follows the rental rules, including depreciation recapture, while the portion you lived in may qualify for the primary-residence gain exclusion. The one habit that makes all of this clean is keeping a clear, consistent record of the business-use split from day one.
Can you actually use the loss?
A study on a house hack usually creates a paper loss, and for most owners those long-term rental units are passive activities. If your income is below the phase-out and you actively participate in the rental, up to $25,000 of that loss may offset other income; above it, the loss suspends and carries forward until you have passive income or sell. If one of your units runs as a short-term rental, or you qualify as a real estate professional, different rules can free the loss up sooner. We cover the trapped-loss problem in detail in cost segregation for passive investors.
Is a study worth it on a house hack?
It depends on the business-use share and the numbers behind it. On a two-unit where you occupy half, the accelerated deduction is real but modest, and the flat study fee and the effort deserve an honest look before you commit. On a three- or four-unit where 60–75% of the building is rented and bonus depreciation is on the table, a study usually pencils clearly. A defensible land-versus-building split and a clean business-use allocation keep the whole position audit-ready, which matters more on an owner-occupied property precisely because the personal-versus-rental line is one an examiner will look at. The honest way to find out is a first pass with the savings estimator on our homepage, then the free instant estimate, which shows your flat study fee before any payment. If you want the pure-rental version of these numbers, our piece on cost segregation on a duplex or small multifamily runs the math without the owner-occupied haircut.