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 is staring at a tax bill most beneficiaries never see coming. The stretch IRA, the planning move that let prior generations spread inherited account withdrawals across 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 a set of final IRS regulations published in July 2024 that quietly made the math worse.

Because the parent died after reaching the required beginning date for RMDs, the heir does not get to wait until year 10 and pull 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 RMDs from 2021 through 2024 while the rules were being finalized, but annual RMDs from inherited accounts are now in force starting 2025.

The Tax Bomb in Plain Dollars

Assume the inherited 401(k) earns a 6% blended return inside the account. By year 10, the cumulative distributions plus residual balance approach roughly $1.6 million of taxable income spread across the decade. If the beneficiary is a high earning California resident still working in the 32% federal bracket, which begins at $201,776 for single filers and $403,551 for joint filers in 2026, and pays California’s 9.3% state rate, the combined marginal rate on inherited dollars sits north of 41%.

Run the cumulative tax over the full 10 years and the bill lands somewhere in the $375,000 to $450,000 range, a 40% to 50% haircut on the net inheritance. The exact number depends on growth and bracket placement, but the order of magnitude is the headline. Roughly half of the inheritance can be lost to income tax if the heir does nothing strategic.

Why the IRMAA Cliff Matters Here

Our 65 year old beneficiary is also a Medicare enrollee. Every distribution dollar lands in modified adjusted gross income and feeds the two year IRMAA lookback. The 2026 IRMAA Part B surcharges run from $81.20 to $487 per month per person, with Part D surcharges layered on top. Pulling a single large distribution can push a retiree two or three IRMAA tiers higher for the following year, adding thousands in premium surcharges that never recover.

Bracket Smoothing Is the Whole Game

Take partial distributions every year. Front load withdrawals in years the heir is still working only if those dollars stay inside the 24% bracket, which tops out at $201,775 for singles and $403,550 for joint filers in 2026. If retirement is planned for year 5, defer the heavier withdrawals to those lower bracket years between retirement and the start of the beneficiary’s own Social Security and RMDs.

A few rules that close off common workarounds:

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 is to convert the heir’s own pre tax accounts in low income years to make room.
2. The 9 month disclaimer. A beneficiary can disclaim the inheritance within 9 months, passing the account to the contingent beneficiary. This is worth considering if a lower bracket sibling or a grandchild is next in line.
3. Spouses are the exception. A surviving spouse can roll the account into their own IRA and use their own RMD schedule. Adult children cannot.
4. Charitable offset. Once the heir reaches 70 and a half, qualified charitable distributions from their own IRA can offset other taxable income, though QCDs are not permitted from an inherited account before that age.

What to Do This Week

Confirm the decedent’s required beginning date status with the plan administrator, since that determines whether annual RMDs are required during the 10 year window. Build a year by year distribution schedule that targets the top of the 24% bracket and avoids the next IRMAA tier two years forward. If projected lifetime taxes on the inheritance exceed $100,000, the planning fee for a CPA who models multi year bracket and IRMAA scenarios pays for itself several times over.