Best Cost Segregation for Duplex, Triplex & Fourplex (2026)
Quick answer
For 2-4 unit residential properties (duplex, triplex, fourplex), all 7 reviewed providers apply the same federal framework — 27.5-year residential basis under MACRS, with rental-portion basis reclassified per Rev. Proc. 87-56. The duplex-specific wrinkle: if you live in one unit, only the rental portion's basis is depreciable, and most studies use a square-footage allocation (50/50 for two equal units). Pricing across qualified providers ranges from $495 (Cost Seg Smart, under 1 hour) to $5,000+ (KBKG, 4–8 weeks) for the same IRS-aligned methodology. R.E. Cost Seg's $950 Rapid Report is purpose-built for fourplex investors whose property fits the scope cap (≤4 units, basis <$800K, capex <$50K). For a sub-$300K duplex stake, the $495 entry tier is hard to beat on ROI.
House-hacking a duplex, triplex, or fourplex is one of the most tax-advantaged real-estate strategies on the books. You get a primary-residence mortgage (3–5% down on owner-occupied financing), rental income from the other units offsetting your housing cost, and — if you do it right — a cost segregation study that accelerates depreciation on the rental portion. The combination is hard to beat. But the cost-seg side has specific mechanics that a generalist provider may not handle cleanly.
The single biggest difference vs. a single-family rental is basis allocation. If you live in one unit of a duplex and rent the other, the IRS does not let you depreciate the whole building — only the rental portion. Most studies allocate by square footage (50/50 for two equal-sized units, or actual measured square footage if the units are unequal). Some practitioners use a room-count method. Either way, that allocation has to be documented up front, and the cost segregation MACRS reclassification then runs only on the rental basis. A provider who is used to working single-family rentals will sometimes default to "whole-property" treatment and miss this. A house-hack-aware provider will ask for unit-by-unit square footage before they start.
The other 2-4 unit consideration is what happens at the fourplex line. The IRS treats 1-4 unit residential as 27.5-year basis under the same residential class. The moment you hit 5 units, it becomes commercial-style residential (still 27.5-year, but treated differently for valuation, lender purposes, and often pricing). That puts the fourplex at an interesting ceiling: maximum-allowed unit count without crossing into small-multifamily territory. R.E. Cost Seg's Rapid Report tier specifically caps at 4 units and $800K basis — a near-exact overlay with the upper edge of the house-hack ICP.
How to read this list. For 2-4 unit residential under ~$800K, this is largely a basis-allocation and turnaround question. For fourplexes right at the Rapid Report ceiling, R.E. Cost Seg is purpose-built. For sub-$800K with the fastest turnaround and lowest entry tier, Cost Seg Smart fits. For $1M+ duplexes/fourplexes with major renovations, traditional engineering firms still have a use case — but the math gets harder to justify.
Top 7 Cost Segregation Companies for Duplex, Triplex & Fourplex
Ranked by 2-4 unit residential fit. Pricing and turnaround are estimates based on publicly available information.
| # | Provider | Duplex Score | Pricing | Turnaround |
|---|---|---|---|---|
| 1 | Cost Seg Smart * | ★★★★☆ 8.5 | From $495 | Under 1 hour |
| 2 | R.E. Cost Seg | ★★★★☆ 8.5 | $950–$5K+ | 5–10 days / 3–4 wks |
| 3 | KBKG | ★★★☆☆ 7.5 | $5K–$15K+ | 4–8 weeks |
| 4 | CSSI | ★★★☆☆ 6.5 | $5K–$15K+ | 4–8 weeks |
| 5 | Maven Cost Seg | ★★★☆☆ 6.5 | From $1,900 | 3–4 weeks |
| 6 | Engineered Tax Services | ★★★☆☆ 6.0 | $5K–$15K+ | 4–8 weeks |
| 7 | Bedford Cost Seg | ★★☆☆☆ 5.5 | $3K–$8K (Est.) | 3–5 weeks |
* Cost Seg Smart is operated by us. Full disclosure.
#1. Cost Seg Smart — Fastest, Lowest Entry Tier for Sub-$800K
★★★★☆ 8.5/10 • From $495 • Under 1 hour
Disclosure: Cost Seg Smart is our company. We built it, we run it, and we are obviously biased. We include it because it would be dishonest to rank duplex providers and exclude ourselves. Also note: we were founded in 2025, so we do not have the multi-decade audit-defense history of legacy engineering firms. Take this review with that context.
Cost Seg Smart is fully automated and works well for the typical owner-occupied duplex or pure-investment fourplex stake. You enter the basis, the property details, the rental-vs-personal split (if any), and pay. A completed study lands in your inbox in under an hour. The engine handles 27.5-year residential basis correctly, applies the rental-portion allocation, and itemizes 5-year (appliances, carpeting, window treatments), 15-year (driveway, landscaping, exterior lighting), and 27.5-year (structural) components against the rental basis only.
The price tiers map well to house-hack reality: $495 covers a rental-portion stake under $300K (typical for a 50/50 split on a $600K duplex), $795 covers $300K–$700K (most fourplex rental basis), and $895 covers up to $1M. For a study that would cost $3,000-$5,000 elsewhere and arrive 4-6 weeks later, the ROI math is straightforward. The trade-off: no human engineer reviewing your specific property. For a standard 2-4 unit residential, that trade works. For a $2M+ property with unusual conditions, you probably want a human.
#2. R.E. Cost Seg — Purpose-Built for the Fourplex Cap
★★★★☆ 8.5/10 • $950–$5K+ • 5–10 days (Rapid) / 3–4 weeks (full)
Yonah Weiss is, by reputation, the most visible name in residential cost segregation. He shows up on house-hack podcasts, BiggerPockets threads, and Twitter/X investor circles constantly. The team's Rapid Report tier is the cleanest scope match for a fourplex investor we have seen in the market: ≤4 units, basis under $800K, capex under $50K. That is a near-exact overlay with the upper end of the house-hack / small-portfolio ICP.
For the rental side of a duplex or a pure-investment fourplex inside that cap, the Rapid Report at $950 delivers a defensible study in 5-10 business days. They itemize appliances and FF&E reasonably well, document basis allocation, and produce CPA-ready depreciation schedules. The brand recognition also helps if a skeptical CPA wants a name they have heard of.
The limitation kicks in above the cap. A fully-furnished fourplex listed on Airbnb with $80K in renovations crosses the capex line and pushes you into their Fully Engineered Residential tier ($2,800+, 3-4 weeks). At that point the price-and-speed advantage narrows, though they are still cheaper than KBKG and faster than CSSI. For investors right at the cap — basis $700K-$800K, modest capex, classic 2-4 unit residential — R.E. Cost Seg is purpose-built.
#3. KBKG — Best for $1M+ Fourplex with Major Renovations
★★★☆☆ 7.5/10 • $5K–$15K+ • 4–8 weeks
KBKG is the gold standard in cost segregation generally — 30,000+ studies, IRS ATG contributors, engineers who have testified in Tax Court. For a 2-4 unit residential property, that firepower is usually overkill. At $5K–$15K, the math on a typical duplex stops working: a $500K rental basis with 25% reclassification produces ~$125K in accelerated depreciation, worth roughly $40K at a 32% marginal rate. A $5K KBKG study is an 8:1 ROI. A $495 Cost Seg Smart study on the same property is an 80:1 ROI.
Where KBKG earns the #3 duplex spot: high-basis fourplexes ($1M+), properties with major renovation history that require document-heavy basis support, or situations where a sophisticated CPA insists on a name-brand provider for audit defense. If you are buying a fully-renovated $1.5M fourplex in a high-cost MSA, KBKG's site-visit methodology and engineering rigor are worth paying for. For a $400K house-hack duplex, they are not.
#4. CSSI
★★★☆☆ 6.5/10 • $5K–$15K+ • 4–8 weeks
CSSI (Cost Segregation Services, Inc.) is one of the longest-running engineering-led firms in the space. Their methodology is solid, their staff is credentialed, and their reports are accepted by CPAs nationwide. For 2-4 unit residential the price-vs-benefit gap is similar to KBKG's: their pricing is built around commercial-scale studies, and a typical duplex does not justify the spend. They become more compelling if you have a portfolio of 5+ rental properties and want a single provider relationship across SFR, duplex, fourplex, and commercial.
#5. Maven Cost Seg
★★★☆☆ 6.5/10 • From $1,900 • 3–4 weeks
Maven sits in the middle tier on pricing and turnaround. $1,900 entry is more expensive than the automated tier but cheaper than the legacy engineering firms; 3-4 weeks is slower than Rapid Report but faster than KBKG. For duplex investors who want a human engineer involved without paying KBKG-level prices, Maven is a reasonable middle option. They are not specifically house-hack-specialized, so expect to walk them through the rental-portion allocation if that applies to your property.
#6. Engineered Tax Services
★★★☆☆ 6.0/10 • $5K–$15K+ • 4–8 weeks
ETS is a multi-disciplinary tax engineering firm that does cost segregation alongside R&D credits, 179D energy deductions, and other specialty services. For a duplex or fourplex specifically, they are not the obvious choice — their workflow is built for larger commercial engagements, and pricing reflects that. Investors with a complex tax situation that needs both cost seg and 179D might find value in the cross-service bundle. Standalone duplex studies are not their sweet spot.
#7. Bedford Cost Seg
★★☆☆☆ 5.5/10 • $3K–$8K (Est.) • 3–5 weeks
Bedford is engineering-led with residential experience, but their work-flow and pricing are oriented toward larger residential portfolios and commercial properties. For a single duplex or fourplex, the price-and-turnaround math doesn't typically favor them vs. the top three on this list. They are included for completeness — if you have a personal CPA relationship that recommends Bedford, the study itself will be defensible. Independently, they are not the first call for a 2-4 unit residential study.
Duplex, Triplex & Fourplex Tax Considerations
The MACRS framework for 2-4 unit residential is the same as for an SFR — 27.5-year residential basis under Rev. Proc. 87-56, with reclassification into 5-year, 7-year, and 15-year property where supported. The differences are in how basis is allocated, how conversions are handled, and how unit count interacts with classification.
Basis allocation between owner-occupied and rental portions
This is the single most important duplex/fourplex-specific consideration. If you live in one unit and rent the others, only the rental portion's basis is depreciable. Most studies use a square-footage method: total the rental units' square footage, divide by the building's total square footage, and apply that fraction to the depreciable basis. For two equal-sized units this is 50/50. For unequal units (e.g., a 1,200 sqft owner unit + a 900 sqft rental unit), it's measured directly. Some practitioners use a room-count or unit-count method when square footage is genuinely ambiguous, but square-footage is the IRS-preferred default. The cost segregation MACRS reclassification then runs only on the rental basis. The owner-occupied basis is not depreciable at all.
Conversion-date basis for properties converted from primary residence
If you bought a duplex as your primary residence, lived in both units (or rented out one), and later converted the second unit (or the whole property) to rental, the depreciable basis is the lesser of your adjusted basis or fair market value at the conversion date. This is a different number than your original purchase price. Document the conversion date carefully — your CPA needs it for Form 3115, and your cost segregation study should run against the conversion-date basis, not the original purchase basis.
Form 3115 lookback for properties bought years ago
If you bought a duplex five years ago and never did a cost segregation study, you can still capture all missed accelerated depreciation. Your CPA files Form 3115 (change of accounting method) and takes the entire missed deduction in the current tax year — no amended returns required. This is one of the largest single-year deductions available to small landlords. The lookback covers the period from the conversion-to-rental date (or original placed-in-service date) through the current year.
27.5-year residential basis (same as SFR, not commercial)
A 1-4 unit residential property uses 27.5-year residential basis, the same as a single-family rental. The MACRS classes (5/7/15/27.5) and the bonus depreciation treatment (100% under OBBBA) are identical. This is in contrast to a 5+ unit multifamily property, which is still residential under the IRC but is often handled with different valuation assumptions and pricing tiers. The 4-unit cap matters: it's the upper limit of residential as most providers define it.
FF&E nuances for furnished sub-units
If you offer one of the rental units as a furnished short-term rental (e.g., the basement unit of a duplex listed on Airbnb), that unit's FF&E — furniture, decor, linens, kitchenware, electronics — becomes 5-year personal property and is bonus-eligible. The other (unfurnished long-term) unit doesn't get this treatment. A good provider will itemize FF&E on the furnished unit only, rather than averaging across the property.
House-hack-specific: STR loophole on a furnished sub-unit
The short-term rental loophole — which converts rental losses from passive to non-passive, allowing offset against W-2 income — can apply to a furnished sub-unit in an owner-occupied building if (a) the average stay is ≤7 days and (b) you materially participate. House-hackers who furnish their basement or garage unit and list it on Airbnb sometimes qualify, depending on their personal involvement in cleaning, guest communication, and turnover. The cost segregation study amplifies this by accelerating the depreciation that becomes the deductible loss. A CPA decision, not a cost seg provider decision — but the cost seg study is what makes the math work.
Editor's Pick by Segment
Duplex, triplex, and fourplex cost segregation has 3 sub-segments. The right provider depends on whether you're house-hacking (owner-occupied + rental) or pure-investment:
$495 entry tier covers a sub-$300K rental-portion stake. Under 1 hour delivery means you can order in January, file in February. Built for sub-$1M residential default.
Order at Cost Seg Smart →$950 Rapid Report is a near-exact scope match. Yonah Weiss's team is THE house-hack name in the BiggerPockets / STR investor world. 5–10 day turnaround.
Read review →At this basis size with renovation complexity, an engineering-led site-visit study is worth the $5K+ spend. KBKG has the strongest audit defense in the market.
Read review →Once you cross the 4-unit line into 5+ units, classification mechanics, valuation, and provider tiers shift. We have a dedicated ranking.
See multifamily ranking →What to Look for in a Duplex / Fourplex Cost Segregation Provider
Not every cost segregation study handles 2-4 unit residential well. Here are the criteria that actually matter for duplex, triplex, and fourplex investors.
1. Basis allocation between personal and rental use
If you live in one unit and rent the others, the study has to allocate basis correctly. The owner-occupied portion is not depreciable. A house-hack-aware provider will ask for unit-by-unit square footage up front, document the allocation method (square footage is the default), and run the MACRS reclassification only on the rental basis. A generalist who treats the property as a whole building is making an error that could surface in audit.
2. Form 3115 readiness for lookback
Most duplex and fourplex investors buy the property years before they look into cost segregation. The study has to be structured so your CPA can file Form 3115 cleanly and capture all missed accelerated depreciation in the current year. That means clear placed-in-service dates, conversion-date basis documentation (if applicable), and a depreciation schedule your CPA can plug directly into the 3115 calculation.
3. Fourplex scope fit
A fourplex right at the 4-unit ceiling is in a particular spot in the market. Some providers (R.E. Cost Seg's Rapid Report specifically) have scope caps that exactly match it. Others price it as "small multifamily" and quote $5K+. Ask up front: is a 4-unit residential property in your standard residential tier, or do you treat it as small multifamily? The answer determines whether you pay $950 or $5,000.
4. FF&E itemization on furnished sub-units
If one of your rental units is offered furnished (long-term furnished rental, or short-term Airbnb sub-unit), the FF&E component matters. A good provider itemizes furniture, appliances, electronics, and decor on that unit only, capturing the 5-year personal property that an unfurnished unit doesn't have. Lumping the property into a flat reclassification percentage misses this.
5. Pricing relative to rental-portion basis
The ROI math runs against the rental-portion basis, not the full purchase price. A $600K duplex with 50/50 allocation has a $300K rental basis. A 25% reclassification produces ~$75K in accelerated depreciation — worth roughly $24K at a 32% marginal rate. A $795 study has a 30:1 ROI. A $5K study has a 5:1 ROI. Both are "worth it" on absolute terms, but the price-tier match matters for sub-$800K rental-portion basis. Use our ROI calculator to model your specific property.
Frequently Asked Questions
Do I claim cost segregation on the whole duplex or just the rental side?
Only the rental portion's basis is depreciable. If you live in one unit of a duplex and rent the other, the IRS allows you to allocate basis between personal and rental use — most commonly using a square-footage method (e.g., 50/50 for two equal units) or in some cases a room-count method. The owner-occupied portion is not depreciable at all, and cost segregation only applies to the rental basis. A good study will document the allocation method up front and run the MACRS reclassification only on the rental side.
What is the difference between cost seg for a duplex and an SFR?
The federal framework is the same — 27.5-year residential basis, MACRS reclassification, 100% bonus depreciation under OBBBA. The differences are practical: a duplex study has to handle basis allocation if the owner lives on-site, often involves two separate sets of appliances and finishes (which can increase the 5-year personal property allocation), and may benefit from sub-unit FF&E capture if one side is offered as a furnished short-term rental. Reclassification percentages are usually similar to an SFR (20–28%), unless a unit is furnished and STR-listed, in which case the rental side can hit 30–40%.
Can I do a lookback study on a duplex I bought as my primary residence years ago, then later moved out?
Yes, but the math gets specific. When you convert a unit (or the whole duplex) from personal use to rental use, the depreciable basis is the lesser of your adjusted basis or the fair market value at the conversion date. Your CPA files Form 3115 to capture any missed accelerated depreciation since the conversion date — not since the purchase date. So if you bought the duplex in 2018, moved out and converted the second unit to a rental in 2022, the cost segregation lookback covers 2022 through current year. Pre-conversion personal-use years do not generate any deduction.
Does fourplex cost segregation hit different MACRS classes than a duplex?
The MACRS classes themselves do not change — a 2-4 unit property is still 27.5-year residential under Rev. Proc. 87-56, the same as a duplex or SFR. What changes is scale: a fourplex has more appliances, more flooring, more cabinetry, and often more site-work (parking, exterior lighting, fencing). That can push the 5-year and 15-year buckets up in absolute dollar terms, even if the percentage allocation looks similar to a duplex. The IRS treats anything 5 units and above as multifamily commercial-style residential, so the 2-4 unit ceiling matters for classification.
Are appliances in the rental unit included in cost seg, or just the building?
Appliances in the rental unit are absolutely included — they are 5-year personal property under MACRS, fully bonus-eligible at 100% under OBBBA. That includes refrigerator, range/oven, dishwasher, microwave, washer/dryer, and any landlord-provided items like window AC units. If the rental side is offered as a furnished short-term rental, the FF&E line expands to cover furniture, decor, linens, kitchenware, and electronics — all 5-year. A good provider will itemize these as separate components rather than burying them in a generic personal-property bucket.
Related
For the full breakdown of what these providers charge, see our pricing comparison across 27 firms. If you want to model the tax savings for your duplex or fourplex, use the ROI calculator (set the property type to residential and use your rental-portion basis, not the full purchase price). If you are stepping up to a 5+ unit property, see our multifamily ranking. And if one of your sub-units is — or could be — offered as a furnished short-term rental, our STR provider ranking covers that segment in detail.