Short answer: usually no, and the reason is the prior-year safe harbor. If what you have already paid in through withholding and your first three estimates adds up to 100% of last year's total tax — 110% if your prior-year adjusted gross income was over $150,000 — you have satisfied the estimated tax rules for the year, and the tax on your sale is simply due when you file. No Q4 payment required, no underpayment penalty, even if the sale created a five-figure bill.
If you have not hit that number, the Q4 payment for a 2026 sale is due January 15, 2027, and paying the shortfall by then stops the penalty clock.
That is the whole answer. The rest is how to tell which side of it you are on, because a sale after a cost segregation study tends to produce a bigger number than the year's estimates assumed.
Why a sale after a cost segregation study surprises people
A study moves part of your building basis into 5-, 7-, and 15-year property, which is what lets you take the deduction early. When you sell, the depreciation taken on those shorter-life components comes back as Section 1245 recapture — ordinary income at your marginal rate, not the 25% cap that applies to the building portion. That is the trade the study makes, and it weights the tax on the sale toward ordinary income, where your bracket is highest.
The quarterly estimates you set back in April were built on rental income and your other earnings. They did not know a sale was coming. So the year closes with a liability materially larger than your payment schedule assumed, and the question is whether that gap costs you a penalty.
What the safe harbor actually requires
The underpayment penalty is avoided if your payments for the year reach the lower of two targets: 90% of your current-year total tax, or 100% of your prior-year total tax (110% where prior-year AGI exceeded $150,000). The prior-year figure is the useful one in a sale year, because it is a known, fixed number. Your current-year tax is not knowable until the dust settles on the closing.
Two things people get wrong here. First, the test is on total payments for the year, not any single quarter — withholding from a W-2 job counts, and counts as paid evenly across the year no matter when it was withheld. Second, the penalty is interest on the underpayment, not a punitive add-on. Being short does not void anything; it costs a modest, calculable amount.
A worked example: a $540,000 rental sold in November
Take a single-family rental bought for $540,000 and placed in service in 2021, with $90,000 allocated to land and $450,000 depreciable. A study reclassified $99,000 of that basis into 5- and 15-year property. With 100% bonus depreciation available that year, the full $99,000 came off in year one, against roughly $3,600 if it had stayed on the 27.5-year schedule — an extra $95,400 of first-year deduction. At a 32% marginal rate, that is about $30,500 of federal tax the owner did not pay in 2021, and had the use of for five years.
The property sells in November 2026, producing about $47,000 of additional federal tax once recapture, unrecaptured Section 1250 gain, and capital gain are added up.
The owner's 2025 total tax was $38,000, and 2025 AGI was over $150,000, so the safe harbor target is 110% of $38,000, or $41,800. Run the two cases:
If withholding plus the Q1 through Q3 estimates already total $42,000, the safe harbor is met. The owner writes no check in January and settles the full $47,000 on April 15, 2027 — and keeps that money working for another three months at no cost.
If those payments total only $30,000, the owner is $11,800 short of the harbor. Sending $11,800 by January 15, 2027 caps the exposure; waiting until April instead adds roughly three more months of interest on the shortfall.
Does it matter that the sale closed in the fourth quarter?
It can help, and this is the piece most often missed. The penalty is computed quarter by quarter, and the default assumes your income arrived evenly across the year — exactly wrong when a sale lands in November, because it retroactively makes your Q1, Q2, and Q3 payments look short.
Annualizing instead, on Form 2210 Schedule AI, matches each required payment to when the income actually showed up. For a Q4 sale that often erases the underpayment attributed to the first three quarters and leaves only the fourth. It does not reduce the tax, only the interest — but on a large gain that is worth your preparer's hour.
What to settle before year-end
Three things, and there is still time for all of them. Pull your prior-year total tax off last year's return and compute the safe harbor figure — one line, and it decides everything else. Total what you have actually paid in for 2026, withholding included. Then ask your CPA for a rough recapture number off the closing statement, so you know whether January 15 is a date you need to act on or one you can let pass.
If you are still choosing a closing date, which year the sale lands in is its own question; we ran that one in December or January closing after a cost segregation study. For how the recapture figure itself is built, see depreciation recapture when you sell.
None of this changes whether the study was the right call. A deduction taken five years early and partly returned at sale is still five years of use of your own money, which was the point. The estimated payment question is narrower: not paying interest on top of a bill you were always going to owe. Run the savings estimate if you are weighing a study, and settle the harbor math with your CPA if you have already sold.