Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2025

Summary of Significant Accounting Policies

The accounting and financial reporting policies of Columbia Banking System, Inc. conform to accounting principles generally accepted in the United States of America and with prevailing practices within the banking and securities industries. All references in this report to "Columbia," "we," "our," or "us" or similar references mean the Company and its subsidiaries, including the wholly-owned banking subsidiary Columbia Bank (the "Bank"). FinPac is a commercial equipment leasing company and a wholly-owned subsidiary of the Bank. The accompanying interim condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries, and the Bank's wholly-owned subsidiaries. All inter-company balances and transactions have been eliminated. The condensed consolidated financial statements have not been audited. A more detailed description of the Company's accounting and financial reporting policies is included in the Company's Annual Report on Form 10-K for the year ended December 31, 2024. These interim condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and related notes contained in the Company's Annual Report on Form 10-K for the year ended December 31, 2024.

Basis of Presentation - In preparing these condensed consolidated financial statements, the Company has evaluated events and transactions subsequent to September 30, 2025, for potential recognition or disclosure. In management's opinion, all accounting adjustments necessary to accurately reflect the financial position and results of operations on the accompanying financial statements have been made. These adjustments include those that are normal and recurring in nature considered necessary for a fair presentation. The results for interim periods are not necessarily indicative of results for the full year or any other interim period.

On August 31, 2025, Columbia closed its previously announced acquisition of Pacific Premier. The acquisition of Pacific Premier was accounted for using the acquisition method of accounting and was effectively an all-stock transaction accounted for as a business combination. The Company's financial results for periods ended prior to August 31, 2025 reflect only Columbia's standalone operations and are not directly comparable to the consolidated results for periods following the completion of the acquisition.

During the third quarter of 2025, management elected to change the presentation of the Company's financial statements and accompanying footnote disclosures from thousands to millions. The change in presentation had no material impact on previously reported financial information, but certain amounts reported for prior periods may differ by insignificant amounts due to the nature of rounding relative to the change in presentation. In addition, historical percentages and per share amounts presented may not add to their respective totals or recalculate due to rounding.

Application of New Accounting Guidance

Standard

Description

Effective Date

Effect on the Financial Statements or Other Significant Matters

ASU No. 2023-09, Income Taxes (Topic 740): Improvements to Income Tax Disclosures

The amendments are intended to provide more transparency about income tax information through improvements to income tax disclosures primarily related to the rate reconciliation and income taxes paid information. The ASU requires annual disclosure of the rate reconciliation of specific categories as well as additional information related to the reconciliation of certain items that meet a quantitative threshold and further disaggregation of income taxes paid.

Fiscal years beginning after December 15, 2024.

The Company adopted the guidance on January 1, 2025 for annual reporting purposes. Refined disclosures will be included in the 2025 10-K.

Significant Accounting Standards Issued but Not Yet Adopted

Standard

Description

Effective Date

Effect on the Financial Statements or Other Significant Matters

ASU No. 2025-07

Derivatives and Hedging (Topic 815) and Revenue from Contracts with Customers (Topic 606): Derivatives Scope Refinements

and Scope Clarification for Share Based Noncash Consideration from a Customer in a Revenue Contract

The amendments refine the scope of derivative accounting under ASC 815 by introducing a scope exception for certain contracts linked to the operations or activities of one of the parties to the contract. It also clarifies the guidance in ASC 606 regarding share-based noncash consideration (e.g., warrants or shares) received from a customer in exchange for goods or services.

These changes aim to reduce complexity, improve consistency in application, and better reflect the economics of such arrangements.

Fiscal years beginning after December 2026, including interim periods within those annual reporting periods. Early adoption is permitted.

The Company is currently evaluating the impact of this ASU on the Company's consolidated financial statements.

ASU No. 2025-06 Intangibles — Goodwill

and Other — Internal-Use Software

(Subtopic 350-40): Targeted

Improvements to the Accounting for

Internal-Use Software

The amendments modernize the accounting for internal-use software under ASC 350-40 by removing references to software development project stages and introducing a "probable-to-complete" recognition threshold. Entities may begin capitalizing costs once management has authorized and committed to funding the project and it is probable the software will be completed and used

as intended. The Update also consolidates guidance on website development costs into ASC 350-40.

Fiscal years beginning after December 15, 2027, including interim periods within those annual reporting periods. Early adoption is permitted.

The Company is currently evaluating the impact of this ASU on the Company's consolidated financial statements.

ASU No. 2024-03 Income Statement—Reporting Comprehensive Income—Expense Disaggregation Disclosures (Subtopic 220-40): Disaggregation of Income Statement Expenses

These amendments are aimed to enhance the transparency and usefulness of financial information by requiring entities to break down significant expense categories in the notes to the financial statements. The amendments focus on the disaggregation of income statement expenses and specifically address the need for more detailed disclosures about expense categories.

Fiscal years beginning after December 15, 2026, and for interim periods beginning after December 15, 2027. Early adoption is permitted.

The Company is currently evaluating the impact of this ASU on the Company's consolidated financial statements.