Filed an extension on your 2025 taxes? You are not alone — and with the October 15 deadline approaching, this is the moment to run a final check rather than rushing the return in the last week. Most extension filers overpay or miss deductions for the same handful of avoidable reasons. Here is the checklist I run through before signing any 2025 return.
1. Confirm your standard deduction vs. itemizing
For 2025, the federal standard deduction is $15,000 (single), $30,000 (married filing jointly), and $22,500 (head of household). Add up your mortgage interest, state and local taxes (capped at $10,000), and charitable gifts — if the total does not beat the standard amount, do not itemize. Roughly 9 in 10 filers take the standard deduction, but the 1 in 10 who should itemize and do not leaves real money behind.
2. Verify you used the right brackets
The 2025 single-filer brackets run 10% to $11,925, 12% to $48,475, 22% to $103,350, 24% to $197,300, 32% to $250,525, 35% to $626,350, and 37% above that. The single most common error I see: people apply their marginal rate to their entire income. Only the dollars inside each bracket are taxed at that bracket's rate — your effective rate is always lower than your marginal rate.
3. Max out what you still can
The extension deadline does not extend every contribution window, but two still matter:
- SEP IRA / solo 401(k) for the self-employed: if you filed an extension, you generally have until October 15 to fund these for 2025 — one of the last remaining levers to lower 2025 taxable income.
- HSA contributions: 2025 limits were $4,300 (self-only) and $8,550 (family), with an extra $1,000 if you are 55 or older. HSAs are triple tax-advantaged and often underfunded.
Note that traditional IRA contributions for 2025 ($7,000, or $8,000 if 50+) had to be made by the April deadline — that window is closed even on extension.
4. Do not forget the state return quirks
Federal accuracy means little if the state return is wrong. The traps are state-specific: California taxes HSA contributions that are federally deductible; Pennsylvania sorts income into eight classes that cannot offset each other; Ohio cities levy their own income tax on top of the state's. If you moved states in 2025, check part-year residency rules — and keep a day-count log, because states like New York audit departures aggressively.
5. Remember: extension to file is not extension to pay
This is the expensive one. An extension gives you until October 15 to file, but any tax you owed was due in April. Interest has been accruing since then, plus failure-to-pay penalties. If you have not paid yet, pay now — every week of delay costs you. If you cannot pay in full, file anyway and set up an installment agreement; the failure-to-file penalty is much worse than the failure-to-pay penalty.
The bottom line
Extension filers usually fall into two camps: people with genuinely complex returns, and people who procrastinated. Either way, the fix is the same — work the checklist top to bottom, confirm the state-level quirks for your state with our state tax guides, and file before October 15. Then set a calendar reminder for January: the best 2026 tax move is the one you plan before the year ends, not after it.