When you need it
You generally need Form 3115 to change from cash to accrual accounting (or vice versa), to change inventory methods (LIFO/FIFO/specific identification), to change depreciation methods, or to correct an impermissible method that has been used for more than two years.
Automatic vs non-automatic consent
Many common changes are listed in the IRS's Revenue Procedure 2024-23 (and predecessors) as "automatic consent" changes that the IRS approves without review when Form 3115 is filed in duplicate (one with the return, one separately). Non-automatic changes require IRS analysis and a user fee.
Section 481(a) adjustment
Changing methods often produces a "catch-up" income or deduction adjustment under Section 481(a) — the cumulative difference between the old and new methods. Net positive adjustments (income) are spread over four years; net negative adjustments (deduction) are taken in full in the year of change.
Common small-business uses
Form 3115 is most often used by small businesses to (a) switch from accrual to cash if newly eligible under the gross receipts test, (b) catch up on missed depreciation in prior years through a Section 481(a) adjustment, or (c) fix an impermissible inventory method.
Filing logistics
For automatic changes, attach the original Form 3115 to the timely-filed return for the year of change and mail a duplicate copy to the IRS Ogden office. The instructions list the specific mailing address and deadlines.
What it does not cover
Form 3115, Application for Change in Accounting Method does not stand alone — it lives inside a small ecosystem of supporting forms, schedules, and elections that together carry the full weight of a small-business return. Knowing what Form 3115, Application for Change in Accounting Method does not handle is just as important as knowing what it does. The instructions list the cross-references explicitly: items computed elsewhere (Schedule C net profit, Schedule SE self-employment tax, Form 4562 depreciation), items reported on a separate form (Form 8829 home office, Form 1099-NEC information returns), and items handled by an entirely different return entirely (Form 1120-S for S-corps, Form 1065 for partnerships). Drawing the boundary cleanly avoids double-counting and avoids leaving deductions on the table.