The mid-quarter convention rarely touches a December rental.

Here is the direct answer, because it is the one people are looking for in October: for most residential rental buyers, the mid-quarter convention does not reduce a fourth-quarter purchase at all. Two reasons. The 27.5-year building is residential rental real property, which is excluded from the 40% test outright and runs on its own mid-month convention. And the short-life property a study finds is normally deducted in full under 100% bonus depreciation, which leaves no remaining basis for any convention to prorate. The convention only starts to matter once you elect out of bonus — which is a real choice some investors make, and the rest of this note is about when it bites.

What is the mid-quarter convention, in one paragraph?

MACRS normally assumes you placed personal property in service in the middle of the year, no matter what month you actually did — the half-year convention. But if more than 40% of the total depreciable basis of the personal property you placed in service that year landed in the final three months, the half-year assumption is replaced for everything you placed in service that year with a mid-quarter convention: each asset is treated as placed in service at the midpoint of its own quarter. Property in the fourth quarter gets credit for roughly a month and a half instead of six months.

Does the 40% test include the rental building itself?

No, and this is the part most people get wrong. The test looks only at property in the MACRS personal property classes — 5-year, 7-year, 15-year, and so on. Residential rental real property and nonresidential real property are excluded from the calculation entirely, and they are never subject to the convention; the building is prorated by the mid-month convention instead.

That matters enormously for a residential investor, because the building is usually the overwhelming majority of the purchase. A $365,000 rental might carry $70,000 of short-life components and nearly $222,000 of 27.5-year building. Only the $70,000 is even looked at by the 40% test.

Does 100% bonus depreciation make the convention moot?

In practice, for most residential studies, yes. Bonus depreciation is applied before the regular MACRS tables, class by class. If you claim 100% bonus on a class of property, that class's basis goes to zero in year one and there is nothing left for the half-year or mid-quarter tables to spread out. A convention that slows down the recovery of remaining basis has no work to do when there is no remaining basis. For property acquired after January 19, 2025, the One Big Beautiful Bill restored the 100% rate, which we walk through in our note on the restored rule.

Two honest caveats. The convention question returns the moment you elect out of bonus for a class — and the election is made class by class, so electing out of 5-year property does not touch your 15-year land improvements. And some states decouple from federal bonus, so your state return may still run the full MACRS tables and care about the convention even when your federal return does not. We cover the state side in our note on state conformity.

The math on a $365,000 rental placed in service in December.

Say you close in November on a $365,000 single-family rental, get it rent-ready, and list it in early December. Carve out land at 20% and you have $292,000 of depreciable basis. Assume a study reclassifies 24% of that basis — about $70,080 — into 5-, 7-, and 15-year property, leaving $221,920 in the 27.5-year building.

Claiming 100% bonus, the full $70,080 is deductible this year. The building contributes a stub: $221,920 over 27.5 years is $8,070 a year, and a December placed-in-service date under the mid-month convention gives you half of December, about $336. First-year depreciation: roughly $70,416. At a 32% marginal federal rate, call it $22,533 in year-one federal tax savings, before any state effect. Nothing in that calculation was touched by the mid-quarter convention.

Now elect out of bonus on the 5-year class and the picture changes. Suppose $40,000 of that $70,080 is 5-year property. Under the half-year convention, 200% declining balance gives 20% in year one: $8,000. Under the fourth-quarter mid-quarter table, the first-year rate is 5%: $2,000. Same asset, same year, $6,000 of difference — and that is the scenario the warnings are actually describing.

So when should a Q4 buyer actually worry about it?

Three situations. You are electing out of bonus, usually to protect a current-year loss limitation or to keep deductions in later years. You have property that is not bonus-eligible for some reason. Or you file in a state that decoupled from federal bonus and you want the state picture modeled honestly rather than assumed.

In all three, a fourth-quarter closing is worth running both ways before you file rather than after. What is not worth doing is rushing a closing into September to dodge a convention that, on a normal residential study with bonus claimed, would never have applied.

What to check before year-end.

Confirm the placed-in-service date first, because every one of these calculations hangs off it — and it is not your closing date. We laid out the readiness test in our note on when a rental is placed in service. Then decide the bonus question deliberately rather than by default. Then run the numbers: the savings estimate takes about two minutes and shows your projected savings and your flat study fee before any payment. There are properties where a study does not pencil, and we will say so. Start with the free instant estimate, or call us for a candid yes or no.