Buying a Rental in Q4? How the Mid-Quarter Convention Affects Your Cost Segregation Deductions

It's November, and many investors are racing to close on a rental before December 31. Their question: what will my first-year depreciation be? The answer depends on a rule most buyers have never heard of: the mid-quarter convention.

It can sharply cut regular first-year depreciation on short-life property when you place a lot of it in service in the last three months of the year. A cost segregation study is exactly the thing that creates that short-life property. This guide explains how the convention works, when it applies, why 100% bonus depreciation usually makes it a non-issue, and what to check before year-end.

The Half-Year Convention: The Default

Conventions decide how much depreciation you get in year one. For personal property, the default is the half-year convention. The IRS treats everything placed in service during the year as if it went into service at the midpoint of the year, no matter whether you bought it in February or November.

That applies to the property classes a cost segregation study creates:

• 5-year property, such as appliances, carpeting, and certain fixtures
• 7-year property, such as some furniture and equipment
• 15-year property, such as land improvements (paving, fencing, landscaping) and qualified improvement property

Under the half-year convention, the first-year MACRS rates are roughly 20% for 5-year property, 14.29% for 7-year property, and 5% for 15-year property. Those figures come from the IRS tables in Publication 946, which your CPA will use.

The Mid-Quarter Convention and the 40% Test

The half-year convention has a catch. If more than 40% of the year's depreciable basis in qualifying property is placed in service during the last three months of the tax year, the mid-quarter convention applies to all of that property for the year. For a calendar-year taxpayer, the last quarter is October 1 through December 31.

Under the mid-quarter convention, each asset is treated as placed in service at the midpoint of the quarter it actually entered service. A Q4 asset gets only a sliver of a year:

• It's an annual, aggregate test. It counts everything you placed in service that year, including business equipment and furniture, not just the rental.
• It uses depreciable basis, not bonus. The test basis is reduced by any Section 179 expense, but generally not by bonus depreciation. That's why the test can trip even when you plan to take 100% bonus.
• One bad quarter taints the year. If the test fails, mid-quarter applies to your Q1 through Q3 personal property too.

Real Property Isn't Part of the Test

The 40% test covers 5-, 7-, and 15-year property. It excludes 27.5-year residential rental and 39-year nonresidential real property. Those assets use the mid-month convention regardless: placed in service on any day of a month, they're treated as placed in service at the middle of that month. A building placed in service in December gets only 0.152% of its 27.5-year basis in year one, and that's true with or without a study.

Buy a rental without a study and there's usually nothing to trip, unless you also placed other personal property in service that year.

How a Cost Segregation Study Can Trip the Test

A study carves short-life components out of the building's basis. Before the study, you have a single 27.5-year asset outside the test. After the study, you may have 20% to 35% of the depreciable basis in 5-, 7-, and 15-year buckets, and all of it entered service in Q4 because you closed in Q4.

That can tip the 40% test in two ways:

The rental is your only personal property for the year. Then 100% of your short-life basis is in Q4, so you fail the test automatically.

You have other personal property, and the Q4 amount pushes you over 40%. For example, $200,000 of equipment placed in service in Q1 plus $160,000 from a December study puts 44% of the basis in Q4 ($160,000 / $360,000), which fails. If the earlier purchases had been $300,000, the Q4 share would be about 35% and you would pass.

Why 100% Bonus Largely Neutralizes the Penalty

The One Big Beautiful Bill Act (OBBBA) restored 100% bonus depreciation for qualifying property acquired after January 19, 2025. Our guide to 100% bonus depreciation in 2026 covers the details.

Bonus depreciation is taken before regular MACRS, on the full basis of the asset. The convention only governs the regular MACRS deduction on whatever basis remains after bonus. If bonus takes 100%, there's no remaining basis for the convention to act on.

So for bonus-eligible 5-, 7-, and 15-year property, tripping the mid-quarter test usually costs you nothing. The study's reclassified components are deducted in full in year one, whether they went into service in January or December. That's why late-year closings with a study are so attractive, as our year-end tax planning checklist shows.

Bonus eligibility generally requires that you acquired the property after January 19, 2025 with no earlier binding written contract, and used property must meet the acquisition rules. The 27.5- and 39-year building shell doesn't qualify for bonus, so the study is what unlocks the deduction.

When the Mid-Quarter Penalty Still Matters

Bonus doesn't make the convention irrelevant in every case. It can still matter if:

• You elect out of bonus. The election is generally made by property class (5-year, 7-year, 15-year) for the whole tax year, and it's hard to reverse. Once you elect out for a class, regular MACRS applies to that class with the mid-quarter convention if the test fails.
• Your property isn't bonus-eligible. Examples include property subject to a pre-January 20, 2025 binding contract, or property that doesn't meet the used-property rules.
• Your state doesn't conform to bonus. Some states require you to add back federal bonus and compute depreciation under regular MACRS. If the mid-quarter convention applies federally, the state calculation usually uses it too. Model the state return separately.

Why Elect Out of Bonus?

Usually because the deduction wouldn't help this year: passive limits would suspend the loss, you expect a higher bracket later, or your state doesn't conform. Electing out in a mid-quarter year is expensive, since year-one deductions get very small. See our comparison of Section 179 vs. bonus depreciation, and note that Section 179 generally isn't available for residential rental property because of the lodging exclusion. Ask your CPA before electing.

Placed-in-Service Date vs. Closing Date

The convention is driven by the placed-in-service quarter, not the closing date. That cuts both ways:

• A closing on September 28 with the property rent-ready on October 5 puts the property in service in Q4.
• A December 28 closing on a property needing a two-month renovation likely places it in service next year, outside this year's test entirely.

Depreciation starts when the property is ready and available for its intended use, which for a rental generally means it's rent-ready and offered to tenants. Our guide on the placed-in-service date covers what to document. Your CPA makes the final call.

A Simple Worked Example

The numbers below are illustrative only, not a quote or a prediction.

An investor closes on a turnkey rental on December 10, 2026, lists it for rent the next day, and has no other personal property placed in service this year. The purchase price is $800,000.

• Land: $160,000 (not depreciable)
• Depreciable building basis: $640,000
• Hypothetical study result: $120,000 of 5-year property and $40,000 of 15-year property (25% reclassified)
• Remaining 27.5-year building: $480,000

All of the short-life basis is in Q4, so it exceeds the 40% threshold, and mid-quarter applies.

Had the half-year convention applied, regular MACRS would have given $24,000 on the 5-year property and $2,000 on the 15-year property, about $26,730 with the building. Mid-quarter cuts that to about $7,230, which is the penalty if you elect out. With 100% bonus, the convention has no basis left to act on, so the result is the same about $160,730 either way.

Without a study, first-year depreciation would be roughly $970 (about 0.152% of the $640,000 building basis). At a 32% federal rate, the extra ~$160,000 deduction could be worth around $51,000 in 2026 federal tax, if you can use it this year. Passive loss rules and state conformity may change that.

Year-End Checklist for Q4 Buyers

Confirm the placed-in-service date. Gather the closing statement, listing date, and any renovation schedule. The quarter matters for the convention.

Inventory all personal property placed in service this year. Include equipment, vehicles, and furniture, not just the rental. Ask your CPA to run the 40% test with and without the study results.

Confirm bonus eligibility for each asset. Check the acquisition date, any pre-January 20, 2025 binding contract, and the used-property rules.

Decide whether to elect out, class by class. Do this only after your CPA models passive limits, state conformity, and future income. The default is to take bonus.

Check state conformity. If your state decouples from bonus, assume regular MACRS with the mid-quarter convention on the state return.

Start the study now. The property must be in service by December 31, but the report needs to be ready before your preparer files. See our year-end tax planning guide for timing.

Plan the exit. Accelerated deductions can come back as depreciation recapture when you sell, unless you defer with a 1031 exchange.

The Bottom Line

The mid-quarter convention is a real rule, and a Q4 closing plus a cost segregation study can trigger it. But for most investors buying after January 19, 2025, 100% bonus depreciation takes the reclassified basis off the table before the convention applies. The cases where it still bites are elections out of bonus, ineligible property, and non-conforming states. Those are the cases to raise with your CPA before December 31, not in March.

This article is general education, not tax advice. The 40% test, convention rates, bonus eligibility, and elections depend on your specific facts and on how the IRS rules are applied to them. Confirm everything with a qualified CPA before you act.

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ROOM42 prepares engineering-based cost segregation studies for residential and commercial investors, including late-year acquisitions. Get your free estimate to see what your first-year depreciation could look like, with no credit card required.