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CoastMath models a death as happening at a partner’s life expectancy. What follows depends on the projection horizon. By default the projection ends with you, and only a partner who dies before you is modeled. With the household horizon it runs until the longer-lived partner’s life expectancy, whichever of you dies first.
In both, the year of death keeps the plan’s filing status and later years use the one you choose for the survivor. The deceased partner’s income stops, their accounts become the survivor’s, and their taxable accounts can get a basis step-up.
| At your longevity age | At the last survivor’s longevity age | |
|---|---|---|
| Last projected year | The year before you reach your life expectancy | The year the longer-lived partner reaches theirs |
| If you die first | The projection ends; your partner’s later years aren’t shown | The projection continues for your partner |
| Partner’s modeled death | Their birth month, in the year they reach their life expectancy | December 31 of the year they reach it |
| Their income stops | At the end of the month before their birth month | At the end of that year |
| Their accounts pass to the survivor | January of the death year | January after the death year |
| Joint accounts | Stay joint | Become the survivor’s |
| Filing status after death | Any of the four options | Single or Head of household |
| Survivor Social Security | Not available | Optional |
The household horizon needs the partner’s birth year and life expectancy; without a life expectancy the projection won’t run. If you both reach your life expectancies in the same year, no one is treated as a survivor and the projection ends that year.
Year
Alex’s accounts become Sam’s in January 2034, before that year’s growth and withdrawals.
The year of death keeps the plan’s filing status, so a married couple files one last joint return, with both partners’ ages counting toward the standard deduction. From the next year, CoastMath uses Filing status after partner’s death, which defaults to Single. The brackets, standard deduction and Social Security taxation thresholds follow that status; see Federal brackets, deductions and indexing.
With the default horizon you can also choose Married filing jointly (remarried) or Married filing separately for the later years. No new partner is added; the option only changes the tax tables.
Each account the deceased partner owned alone becomes the survivor’s. Balances and Roth contribution and conversion records, including the five-year clocks, carry over unchanged. From then on:
- Required minimum distributions and the age-59½ penalty test use the survivor’s age, as if the survivor had treated the account as their own. See Required minimum distributions.
- With the household horizon, the deceased partner’s 401(k) accounts keep their balances and distribution rules but receive no new employee or employer contributions.
- With Step up cost basis on partner-owned taxable accounts at their death checked (the default), the cost basis of the deceased partner’s taxable brokerage and cryptocurrency accounts resets to the balance at the transfer. Pre-tax and Roth accounts have no basis step-up.
| Balance at the transfer | $300,000 | |
|---|---|---|
| Alex’s cost basis | $120,000 | |
| → | Sam’s cost basis after the step-up | $300,000 |
| = | Gain removed by the step-up | $180,000 |
A joint brokerage account worth $100,000 with $60,000 of basis passes to Sam with its $60,000 basis unchanged.
Every income stream whose Income recipient is the deceased partner stops at the time shown in the table above. That includes their Social Security; a survivor benefit replaces it only if you turn it on (see Spousal and survivor Social Security). The survivor’s own streams continue.
Per-person health costs stop at the deceased partner’s modeled death. Other expenses don’t change on their own: household spending continues at the amounts you entered.
Estate tax and what heirs receive are calculated once, at the end of the projection, not at the first death; see Estate tax and net estate.