Owners of 2-to-4-unit residential rentals (duplex, triplex, fourplex) need a provider that treats a four-unit building as what it is: four units of the same residential asset class, not a scaled-down commercial job. Virtual Cost Segregation runs an engineering-based cost segregation services study for $2,200 flat per property, delivered in 3 to 5 business days with no site visit required, built to the criteria in the IRS Cost Segregation Audit Technique Guide. The steps below cover how to confirm your property qualifies, lock in the right bonus depreciation rate for 2026, and hand a completed study to your CPA without rework.
A duplex, triplex, or fourplex sits in an odd spot for tax planning. It is residential real estate, but IRS depreciation rules split it further depending on how you rent it out and how long guests or tenants stay. This guide walks through the sequence a 1-to-4 unit owner should follow before paying for a study, what documents to have ready, and how the report moves from your desk to your CPA’s return in 2026.
What You’ll Need
- Your closing statement or settlement sheet, showing the purchase price and closing date
- The placed-in-service date for the property (when it was first available to rent, not when you closed)
- A record of whether the property runs as a short-term rental (average stay of 7 days or less) or a long-term rental
- Receipts or invoices for any renovation, furnishing, or amenity costs added after purchase
- Your CPA’s contact information, since the study is not filed on its own
- A cost segregation provider that works with 1-to-4 unit residential property, such as cost segregation services for duplex-to-fourplex owners
Step 1: Confirm Your Property Fits the 1-to-4 Unit Residential Class
Start by confirming your building is genuinely a duplex, triplex, or fourplex, not a 5+ unit property that falls into commercial multifamily rules. This distinction matters because providers and depreciation treatment both shift once a building crosses into 5 or more units.
Count units exactly as recorded on your deed or local zoning record, not as marketed. A property billed as a “4-plex” that actually has a detached accessory unit may have a different unit count on paper. Confirming this upfront avoids ordering the wrong type of study and having to redo intake paperwork.
Common mistake: assuming a mixed-use building with ground-floor commercial space still counts as 1-to-4 unit residential. If any unit is non-residential, flag that for your CPA before ordering anything.
Step 2: Pin Down Your Rental Type and Its Depreciation Life
How you rent each unit changes the building’s depreciation class, even though the property itself is residential. A long-term rental building depreciates over 27.5 years as residential property. A short-term rental where the average guest stay is 7 days or less, the threshold tied to the STR loophole, depreciates the building over 39 years as nonresidential property.
This matters because the building structure itself never qualifies for bonus depreciation regardless of which life applies. What does qualify for faster write-offs are the components a study pulls out separately: flooring, certain electrical and plumbing tied to specific assets, and furnishings, typically assigned 5- or 7-year lives. Land improvements such as fencing, paving, and pools are 15-year property.
If you run a duplex where one unit is long-term and one is short-term, document that split clearly. Mixed-use buildings need the rental history broken out per unit before a study can classify the building correctly.
Step 3: Lock In Acquisition and Placed-in-Service Dates for Bonus Depreciation
Under the One Big Beautiful Bill Act (OBBBA), signed July 4, 2025, bonus depreciation on qualifying components is 100% only for property both acquired and placed in service after January 19, 2025. Both dates have to clear that line. A fourplex under contract before January 20, 2025 but placed in service later does not get the 100% rate; it falls under the prior phase-down instead, 40% if placed in service in 2025, 20% in 2026, and 0% from 2027 onward.
Pull your purchase contract date and your placed-in-service date before you order a study. If either date predates January 20, 2025, read through bonus depreciation eligibility for residential rental property so you know which phase-down rate applies and can confirm the exact figure with your CPA. A converted primary residence does not reset its acquisition date just because it starts renting in 2026; a home bought before January 20, 2025 and later converted to a rental does not qualify for the 100% rate, and the applicable rate depends on the original purchase date.
Step 4: Gather Your Closing and Rehab Documents
A study needs source documents, not estimates. Before ordering, collect your settlement statement, any renovation invoices broken out by trade, and a list of furnishings or amenities purchased separately from the building (hot tubs, decks, appliances, smart locks).
For a fourplex with four separate unit renovations, keep invoices sorted by unit so the engineering review can allocate costs correctly instead of averaging across the building. Owners who skip this step usually end up sending follow-up documents mid-study, which adds days to turnaround.
Common mistake: lumping all rehab costs into one “renovation” line item on a spreadsheet instead of keeping original invoices. The more specific the paper trail, the more defensible the component allocation ends up being.
Step 5: Order an Engineering-Based Study Built to the IRS ATG
This is the step that actually produces the depreciation schedule, and it is where provider choice matters most for a 2-to-4 unit property. Virtual Cost Segregation runs this type of study specifically for owners of 1-to-4 unit residential rentals, whether rented short-term through Airbnb or VRBO or rented long-term, for a flat $2,200 per property with no site visit required and delivery in 3 to 5 business days.
For a duplex or fourplex, the engineering-based approach matters because a rule-of-thumb percentage study applies one blanket reclassification rate to the whole building, which tends to miss unit-by-unit differences in finish level and renovation history. An engineering-based study built to the IRS Audit Technique Guide’s criteria itemizes each component with its own cost source, which gives your CPA a defensible basis for every asset life on the return.
As an illustrative example only, if a study reclassifies 25% of a $500,000 fourplex’s depreciable basis into shorter-life property, and the owner is in the 37% bracket with components placed in service after January 19, 2025 and acquired after that date, the 100% bonus depreciation rate applies to that reclassified portion in the year it’s placed in service. This is a simplified example, not a projection of your actual results; actual savings depend on your specific property and should be confirmed with your CPA.
Audit support is included with the study at no added cost, meaning the report and its documentation are available if your return is reviewed. The study itself is not a CPA service and is not filed with the IRS; your CPA applies it when preparing your return, and the firm does not represent anyone before the IRS.
If you’re still comparing what a study should include before committing, review what an engineering-based cost segregation study includes and the questions to ask before hiring a cost segregation firm.
Step 6: Hand the Report to Your CPA for Form 4562 (and Form 3115 if Needed)
Once the study is delivered, it goes to your CPA, not the IRS. Your CPA uses the component schedule to complete Form 4562 for the current tax year, applying the correct recovery periods and bonus depreciation rate to each reclassified asset.
If the property has been in service for a prior year and depreciation was never segregated, your CPA may file Form 3115 to catch up the missed depreciation through a Section 481(a) adjustment on the current return, rather than amending prior-year returns. For a walkthrough of how that handoff works in practice, see how CPAs implement a cost segregation study on Form 4562.
Expected outcome: your CPA has a line-by-line schedule to enter into their tax software, whether that’s Lacerte, Drake, or UltraTax, without needing to estimate asset lives themselves.
Troubleshooting Common Mistakes
You ordered a study before confirming your placed-in-service date. Pull your rental listing history or occupancy records to pin down the exact date before the study starts; it changes which bonus depreciation rate applies.
You assumed a DIY percentage calculator was enough. A manual estimate gives you a ballpark, not a component-level schedule a CPA can file from. Treat calculator output as a starting point, not a final number.
You mixed short-term and long-term rental history for a multi-unit property. Document each unit’s rental pattern separately; a duplex with one STR unit and one LTR unit needs both treated correctly for building classification.
You forgot the acquisition-date qualifier on bonus depreciation. Placed-in-service date alone does not determine the 100% rate; the property also has to have been acquired after January 19, 2025.
You sent the report straight to your CPA without reviewing it first. Check the component breakdown yourself for units or costs that look misallocated before your CPA builds the return around it; see how to review a residential cost segregation report before filing.
Tools and Resources
- Virtual Cost Segregation (cost segregation services): engineering-based studies for 1-to-4 unit residential rentals, $2,200 flat per property, 3 to 5 business day turnaround, no site visit
- Cost segregation for duplex and triplex owners: background on how multi-unit residential properties are classified
- Cost segregation for fourplex investment properties: specifics for four-unit buildings
- How much does a cost segregation study cost: pricing context across providers
- Cost segregation study audit documentation for rental owners: what to keep on file after the study is delivered
FAQ
What does a cost segregation study cost for a duplex or fourplex in 2026?
Pricing varies by provider and model (flat fee versus percentage of savings). Virtual Cost Segregation charges $2,200 flat per property for 1-to-4 unit residential rentals, regardless of unit count within that range.
Does bonus depreciation still apply if I bought my fourplex in 2024?
It depends on your acquisition date, not just when you placed it in service. Property acquired before January 20, 2025 follows the prior phase-down: 40% if placed in service in 2025, 20% in 2026, 0% from 2027. Confirm your exact contract date with your CPA before assuming a rate.
Is a cost segregation study for a fourplex different from one for a single-family rental?
The process is similar, but classification depends on how the units are rented, short-term versus long-term, not strictly on unit count. A fourplex with mixed rental types needs each unit’s history documented separately.
Do I need to be home for a site visit on a 2-to-4 unit property?
No. Virtual Cost Segregation does not require a site visit for its studies, which keeps turnaround to 3 to 5 business days.
Who actually files the cost segregation study with the IRS?
No one files the study itself. It’s a supplementary report your CPA applies when preparing Form 4562 (and Form 3115 if catching up missed depreciation); the study provider does not represent you before the IRS.
Does my state’s tax rules affect the bonus depreciation benefit?
Bonus depreciation is a federal rule. Some states decouple from federal bonus depreciation for state income tax purposes, so check your state’s conformity with your CPA separately from the federal calculation.
Conclusion
For a duplex, triplex, or fourplex, the sequence that works in 2026 is: confirm the unit count and rental type, lock in your acquisition and placed-in-service dates, gather source documents, then order a study from a provider built around 1-to-4 unit residential property rather than a generic commercial process. Virtual Cost Segregation’s flat $2,200 fee and 3-to-5 business day turnaround fit that sequence without adding a site visit to the timeline. Whichever provider you use, the study is only as useful as the documentation behind it and the CPA who applies it, so treat the cost segregation services you choose as step five of six, not the whole plan.