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, those 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. Consider a high-earning California resident still working and sitting in the 32% federal bracket. Per IRS Revenue Procedure 2025-32, that bracket covers single filers with taxable income between $201,776 and $256,225, and the 35% rate takes over above that ceiling. Joint filers enter the 32% bracket at $403,551. 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. That is 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 each year, 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 stands at $202.90 per month, up $17.90 (roughly 9.7%) from the 2025 rate of $185.00. Surcharges begin at $109,000 MAGI for single filers and $218,000 for joint filers, adding between $81.20 and $487.00 per month to Part B, with Part D surcharges of $14.50 to $91.00 per month on top.

The system operates as a cliff: one dollar over any threshold triggers the full tier premium with no phase-in. A single filer crossing into the first IRMAA tier at $109,001 pays an extra $81.20 per month in Part B plus $14.50 per month in Part D surcharges, and that exposure does not ease until income falls back below $109,000 in a future lookback year. The second tier begins at $137,001 for single filers ($274,001 for joint filers), where Part B surcharges jump to $202.90 per month. A couple crossing into the first tier pays roughly $2,297 more per year in combined Part B and Part D premiums compared to staying just below the threshold. Those costs accumulate quickly 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 $105,701 to $201,775 for single filers and $211,401 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 gap 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 starting in 2026 under the One Big Beautiful Bill Act (signed July 4, 2025), 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. The law also created a new above-the-line deduction for non-itemizers of up to $1,000 per single filer ($2,000 for joint filers) on cash gifts to public charities, but that benefit is limited and does not reach the scale available through a QCD. Because QCDs are excluded from income rather than deducted, they sidestep both the 0.5% floor and the 35% cap entirely. For retirees with charitable goals, the direct IRA-to-charity transfer is now the clearly superior route.

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. Retirement itself qualifies as a triggering life event. A Roth conversion does not.

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. There is no extension for beneficiaries who missed distributions during the waiver period.

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 a missed distribution is steep, and the window for corrective action shrinks with every passing year.

Editor’s note: This pass corrected the 2026 joint 32% bracket entry threshold to $403,551 (from $403,550), added the IRMAA second-tier threshold ($137,001 single / $274,001 joint) and the Part B second-tier surcharge of $202.90 per month, noted the Part B premium increase as approximately 9.7%, and added context on the One Big Beautiful Bill Act’s new non-itemizer charitable deduction of up to $1,000 (single) or $2,000 (joint) that took effect in 2026.

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