Cost segregation on a duplex or small multifamily: how the numbers actually work.

If you own a duplex, a triplex, or a small multifamily building, you've probably heard about cost segregation in the context of single-family rentals and wondered whether the math changes once there's more than one unit under the roof. It does — and mostly in your favor. A duplex tends to reclassify a little more of its basis than a comparable single-family, the study costs the same flat fee, and the year-one number is usually big enough that it's worth running before you file. Here's how it actually pencils.

Does cost segregation work on a duplex the same way?

Yes, with one clarification. A duplex, triplex, or quad is residential rental property, which the IRS depreciates over 27.5 years — the same schedule as a single-family rental, and the same schedule whether you own one unit or four. (Buildings with five or more units are still residential rental property at 27.5 years, as long as 80% or more of the rent comes from the dwelling units.) A study doesn't change that 27.5-year clock on the building itself. What it does is pull the faster-wearing components — flooring, cabinetry, appliances, specialty electrical and plumbing, decorative finishes, and the land improvements outside — out of the 27.5-year bucket and into 5-, 7-, and 15-year buckets, where they depreciate far sooner. You're still working from one depreciation schedule for the whole building, not a separate one per unit.

Why a duplex often reclassifies more than a single-family.

Per dollar of purchase price, a small multifamily building usually carries more short-life property than a single-family house. Two units means two kitchens, two sets of appliances, twice the flooring and cabinetry and bathroom fixtures — all of it 5- or 7-year property — sitting on top of roughly the same structural shell. Then there are the land improvements that serve the whole lot: the driveway and parking pad, walkways, exterior lighting, landscaping, fencing. Those are 15-year property, and a multi-unit lot tends to have more of them. A typical single-family residential study reclassifies somewhere around 20–22% of depreciable basis. On a duplex or small multifamily, it's common to land a few points higher — call it 25%, sometimes more. Across 1-4 unit residential property we see results run anywhere from 20% to 35%, depending on the finishes.

The numbers on a $685,000 duplex.

Take a duplex bought all-in for $685,000 and placed in service in 2026. Carve out land at 20% and you're left with roughly $548,000 of depreciable basis on the 27.5-year schedule. Reclassify 25% of that — about $137,000 — into 5-, 7-, and 15-year property. Because 100% bonus depreciation is back for property acquired after January 19, 2025, that entire $137,000 is deductible in year one rather than spread across decades.

At a 32% marginal federal rate, that's roughly $43,800 in first-year federal tax savings — about $21,900 per unit — before you add whatever your state gives back. And even if you ignore bonus depreciation entirely and just take those assets over their shorter lives, the present value of pulling $137,000 of deductions forward still dwarfs the cost of the study.

What does a cost seg study on a duplex cost?

Here's the part that surprises people: a duplex study costs the same as a single-family one. Our flat fee covers 1-4 units — so a single-family, a duplex, a triplex, and a quad are all $1,750. Above four units it's the flat fee plus $125 per additional unit, up to twelve. Against a $43,800 first-year saving, a $1,750 fee is roughly a 25-to-1 return in year one. You can size your own building with the savings calculator before you ever talk to us — it has a duplex setting. And whatever the engineering work shows, every study is reviewed by an Enrolled Agent (EA) before it goes out, and built so your CPA can implement it without re-keying anything. For the full pricing picture, see what a residential study actually costs.

Can you actually use the deduction this year?

This is the honest caveat, and it matters more on a small multifamily than people expect. A long-term duplex rental is, for most owners, a passive activity. A big first-year loss from cost segregation offsets your passive income — other rentals, in most cases — but it won't automatically wipe out W-2 or active business income unless you qualify as a real estate professional, or unless the units run as short-term rentals and you materially participate. If you're a high-W-2 owner holding a long-term duplex, the deduction is still real; it just may carry forward until you have passive income or you sell. If any of the units are short-term rentals, the analysis changes — that's the short-term rental loophole, and it's worth reading before you assume. The size of the deduction and your ability to use it this year are two separate questions, and a good feasibility call answers both.

What if you bought the duplex a few years ago?

You didn't miss the window. If you've held the duplex for several years and never did a study, a look-back captures all the depreciation you should have taken — claimed in your current tax year through a §481(a) adjustment, with no amended returns. For a property you've owned for five or six years, that catch-up can be a larger one-time number than a fresh study on a new purchase.

None of this is a reason to run a study on every building you own. On a low-basis duplex where the reclassified dollars are modest, the fee may not justify it — and we'll tell you when that's the case. But for most small multifamily owners who bought after January 19, 2025, or who've been sitting on a building for years without a study, the numbers are worth fifteen minutes.