What Happens to a $900,000 401(k) When the Owner Dies and the Beneficiary Falls Under the New 10 Year Rule

A 65-year-old who just inherited a $900,000 401(k) from a parent who died at age 75 faces a tax bill most beneficiaries never see coming. The stretch IRA, the estate planning move that let prior generations spread inherited account withdrawals over their own life expectancy, is gone for non-spouse heirs. In its place sits the SECURE Act 10-year rule, effective for deaths after 2019, and final IRS regulations published in July 2024 made the math considerably worse for most heirs.

When the original account owner died after their required beginning date for RMDs, the heir cannot simply wait until year 10 and withdraw everything at once. The final regulations confirm that annual RMDs are mandatory during years one through nine, with the full remaining balance due by December 31 of year 10. The IRS waived enforcement of those annual distributions from 2021 through 2024 while the rules were being finalized. Starting with the 2025 tax year, annual RMDs from inherited accounts became fully enforced, and the 25% penalty for missed withdrawals (reducible to 10% if corrected within two years) now applies in full.

The Tax Bomb in Plain Dollars

Assume the inherited 401(k) earns a 6% blended return. By year 10, cumulative distributions plus the residual balance approach roughly $1.6 million of taxable income spread across the decade. Take a high-earning California resident who is still working and sitting in the 32% federal bracket. Per IRS Revenue Procedure 2025-32, that bracket begins at $201,775 for single filers and $403,550 for joint filers in 2026. Layer on California’s 9.3% state rate and the combined marginal rate on inherited dollars clears 41%.

Run the cumulative tax across all 10 years and the bill lands somewhere between $375,000 and $450,000, a 40% to 50% haircut on the net inheritance. The exact figure depends on account growth and where the heir sits in the bracket structure, but the order of magnitude tells the real story: roughly half the inheritance can disappear to income tax when the heir takes no strategic action.

Why the IRMAA Cliff Matters

The 65-year-old beneficiary is also a Medicare enrollee, and every distribution dollar lands in modified adjusted gross income and feeds the two-year IRMAA lookback. For 2026, IRMAA premiums are determined by 2024 income. The standard Medicare Part B premium is $202.90 per month, but surcharges kick in at $109,000 MAGI for single filers and $218,000 for joint filers, adding $81.20 to $487.00 per month to Part B, with Part D surcharges of $14.50 to $91.00 per month on top.

A single large distribution timed poorly can push a retiree two or three IRMAA tiers higher for the following year, a cost that persists until the lookback year drops off. Because this is a cliff system, a single dollar over any threshold triggers the full tier premium with no phase-in. A couple crossing into the first IRMAA tier pays roughly $2,300 more per year in combined Part B and Part D premiums compared to staying just below the line, and the costs accumulate if distributions are not carefully sequenced across the decade.

Bracket Smoothing Is the Whole Game

The core strategy is spreading withdrawals across every year of the window rather than lumping income into two or three years. Front-load distributions in years when the heir is still working only if those dollars stay inside the 24% bracket, which runs from $105,700 to $201,775 for single filers and $211,400 to $403,550 for joint filers in 2026. If retirement falls around year five, defer heavier withdrawals to the lower-bracket years between leaving work and the start of the beneficiary’s own Social Security and RMDs. That window is typically the most tax-efficient stretch of the entire decade.

Several rules close off workarounds that heirs commonly assume are still available:

1. No Roth conversion on the inherited balance. The IRS does not permit converting an inherited non-spouse 401(k) or IRA to a Roth. The only Roth path available is converting the heir’s own pre-tax accounts during low-income years, which frees up room in future brackets to absorb the inherited distributions.
2. The 9-month disclaimer window. A beneficiary can disclaim the inheritance within 9 months of the original owner’s death, passing the account to the contingent beneficiary. This option deserves careful evaluation when a lower-bracket sibling or a grandchild is next in line.
3. Spouses receive different treatment. A surviving spouse can roll the account into their own IRA and draw down under their own RMD schedule. Adult children and other non-spouse heirs do not have this option.
4. Charitable offset after age 70 and a half. Once the heir reaches age 70½, qualified charitable distributions from the heir’s own IRA can offset other taxable income. QCDs are also available from an inherited IRA once the heir clears that same age threshold. The 2026 QCD limit is $111,000 per person, up from $108,000 in 2025. QCDs grew more valuable in 2026 under the One Big Beautiful Bill Act, which imposed a new 0.5% of AGI floor on itemized charitable deductions and capped the deduction value at 35% for taxpayers in the top 37% bracket. Because QCDs are excluded from income rather than deducted, they sidestep both of those new restrictions entirely, making the direct IRA-to-charity transfer the superior route for most retirees with charitable goals.

One additional tool deserves mention: the SSA-44 life-changing event appeal. When a beneficiary’s income falls sharply after the IRMAA lookback year, filing Form SSA-44 with income documentation can prompt the Social Security Administration to substitute a more recent, lower-income year for the IRMAA calculation. That substitution can eliminate or meaningfully reduce premium surcharges for the affected year.

What to Do This Week

Start by confirming the decedent’s required beginning date status with the plan administrator. That single fact determines whether annual RMDs are mandatory throughout the 10-year window or whether the beneficiary controls the timing of every withdrawal. With that confirmed, build a year-by-year distribution schedule targeting the top of the 24% bracket while keeping projected MAGI below the next IRMAA tier two years forward.

If you inherited in 2020 through 2023, you are already mid-window. The 10-year clock started the year after death, and the IRS waiver for missed RMDs in 2021 through 2024 did not push out the final deadline. A beneficiary who inherited in 2022 must empty the account by December 31, 2032, and was required to begin taking annual RMDs in 2025 if the decedent had reached their required beginning date before death.

When projected lifetime taxes on the inheritance exceed $100,000, the cost of hiring a CPA to model multi-year bracket and IRMAA scenarios is almost always worthwhile. The math is layered, the penalty for missed distributions is steep, and the window for corrective action shrinks with every year that passes without a plan.

Editor’s note: This pass added the 2026 standard Medicare Part B premium ($202.90 per month) for context against the IRMAA surcharge ranges, clarified that QCDs from an inherited IRA become available once the heir reaches age 70½, and sharpened the description of the One Big Beautiful Bill Act’s charitable deduction changes to specify the new 0.5% of AGI floor and the 35% cap on deduction value for top-bracket itemizers, both confirmed per multiple tax authorities including the IRS, Fidelity Charitable, and Northern Trust.

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