Missed deductions

File Form 1040-X within three years of the original return due date to claim missed deductions or credits. Common ones: missed mileage (if you have records), missed home-office deduction, missed retirement contributions reported but not deducted, missed QBI deduction, missed self-employed health insurance deduction.

Missed depreciation

Missed depreciation from prior years is corrected via Form 3115 (change in accounting method) with a Section 481(a) catch-up adjustment in the year of change — not via Form 1040-X. The catch-up can produce a substantial one-time deduction.

Unfiled returns

The IRS is generally lenient with non-filers who voluntarily come into compliance — file the missing returns, work out a payment plan if needed, and the failure-to-file penalty (which is the largest of the penalties) can sometimes be abated for first-time non-filers under the First Time Abate program.

Missed S-corp election

Rev. Proc. 2013-30 grants automatic late-election relief for S-corp elections missed within 3 years and 75 days, provided the entity has been operating consistently with the intended election. File Form 2553 with the magic language "FILED PURSUANT TO REV. PROC. 2013-30" and attach a reasonable cause statement.

Missed retirement contributions

SEP-IRA contributions can be made as late as the extended due date of the return — including amended returns claiming the contribution. Solo 401(k) elective deferrals require the plan to have been in place by year-end and cannot be made retroactively. Traditional IRA contributions for a prior year cannot be made after the original April 15 deadline (no extension applies to IRA contributions).

How experienced filers approach this

Experienced self-employed filers and the CPAs who advise them treat this question as a recurring planning exercise rather than a one-time decision. They model the multi-year tax impact rather than just the current year, document the reasoning in a short workpaper that survives staff turnover and software changes, and revisit the analysis annually as facts and laws change. The discipline is not difficult — a half-day in January with last year's return, the current-year IRS publications, and a spreadsheet — but it is rare among DIY filers, which is precisely why it produces outsized results.