Selecting an IPO Auditor — Big Four or National? How to Decide

Selecting your PCAOB-registered auditor is one of the first and most consequential decisions in the IPO preparation process. The audit committee owns this decision — not the CFO, not the CEO. And it must be made at least 18 months before your anticipated S-1 filing date to allow sufficient time for the required 2–3 year audit history.

What the SEC Requires

The S-1 registration statement must include audited financial statements for the two most recent fiscal years (three years for large accelerated filers). The auditor must be registered with the PCAOB. If your current auditor is not PCAOB-registered, you will need to re-audit those years with a qualified firm — a process that typically adds 6–9 months to your preparation timeline.

Big Four vs. National PCAOB Firms

The Audit Committee's Role

The audit committee — not management — selects and oversees the external auditor. This is a legal requirement under Sarbanes-Oxley, not a convention. In practice, the CFO typically manages the bake-off process and presents recommendations, but the final decision and engagement letter are the audit committee's. This distinction matters for: fee negotiation (the audit committee negotiates directly), scope disputes (the audit committee resolves them), and partner rotation (the audit committee approves).

How to Run the Auditor Bake-Off

Questions to Ask Every Audit Firm

• How many S-1 audits did your team complete in the last 12 months? For companies in our sector?
• Who specifically will be the engagement partner and senior manager — and what is your firm's policy on mid-engagement staffing changes?
• What is your timeline for completing a 2-year re-audit on our historical financials, and what are the key risks to that timeline?
• How do you handle complex accounting areas such as [ASC 606 / stock-based compensation / lease accounting] specific to our business?
• What is your fee estimate and what are the primary drivers that could cause it to increase?
• What is your process for managing SEC consultation when we encounter a novel accounting question?
• Describe a recent situation where you identified a material accounting issue during an IPO audit — how was it resolved?

What the IPO Auditor Does

The PCAOB-registered auditor's scope for an IPO company covers more than just the audit of the financial statements:

• Audit of financial statements: The core work — auditing 2 (EGC) or 3 (non-EGC) fiscal years of financial statements included in the S-1. The audit must comply with PCAOB standards (not AICPA standards, which apply to private company audits). If the company previously used an AICPA audit firm, a PCAOB-registered firm must be engaged and may need to re-audit prior years.
• Consent letter: The auditor issues a consent letter included in the S-1 at every amendment — consenting to the inclusion of their audit report in the registration statement. This consent is re-issued with each amendment and at effectiveness.
• Comfort letter: Issued to the underwriters at closing, the comfort letter confirms specific financial information in the S-1 that is not covered by the audit opinion. Auditors perform "agreed-upon procedures" to support this letter.
• Review of quarterly financials: After the IPO, the auditor reviews (does not audit) each quarterly 10-Q financial statement. Reviews are less intensive than audits but require the auditor to inquire, read, and perform analytical procedures.
• SOX 404(b) attestation (when required): For accelerated filers that are not EGCs, the auditor issues an attestation report on management's ICFR assessment. This requires the auditor to independently test internal controls — significantly more work than a financial audit alone.

Big Four vs. National Firms — A Detailed Comparison

Transitioning From a Private Company Auditor

Most pre-IPO companies use smaller regional audit firms that are not PCAOB-registered — and must transition to a PCAOB-registered firm before the IPO S-1 can be filed. This transition requires careful management:

• Timing: Engage the new PCAOB auditor at least 18 months before the target IPO date. The new auditor needs to audit at least two full fiscal years of financial statements — ideally performing a re-audit of years already completed by the prior firm.
• Re-audit scope: The new PCAOB auditor cannot rely on work performed by the prior non-PCAOB firm. They must re-perform audit procedures for prior years — a scope that adds cost and time but is non-negotiable.
• Predecessor auditor cooperation: The prior audit firm must cooperate with the new firm's audit procedures, providing access to work papers and responding to inquiries. This cooperation is typically required by professional standards but can be delayed by disputes over fees or client retention.
• Disclosure in S-1: The change in auditors during the pre-IPO period is disclosed in the S-1 — the auditor report covers the years the new firm audited, and the prior firm's report (if required for periods they audited) is also included.

Auditor Independence — The Critical Constraint

PCAOB auditor independence rules are one of the most complex and consequential constraints in the pre-IPO process. The auditor is prohibited from providing certain services to audit clients — which means the company must hire separate advisors for those services:

• Prohibited services from the auditor: Bookkeeping or other accounting services; financial information system design and implementation; appraisal or valuation services; actuarial services; internal audit outsourcing; management functions; human resource functions; broker-dealer or investment advisory services; legal services.
• Why this matters for IPO preparation: The auditor cannot help design the ASC 606 revenue recognition methodology, cannot set up the SOX control framework, cannot advise on how non-GAAP metrics should be defined, and cannot prepare the technical accounting memos that support the S-1 disclosures. All of this requires a separate accounting advisory firm — and this is precisely why accounting advisory firms exist.
• The independence analysis: Before engagement, the audit firm conducts a formal independence analysis — checking all known business relationships between the firm and the company, its affiliates, and its major shareholders (including VC investors who often have board seats at multiple audit clients). This analysis must be completed before the auditor can begin work.

The PBC List — Prepared By Client

Once engaged, one of the auditor's first deliverables is the "Prepared By Client" (PBC) list — a comprehensive schedule of financial records, schedules, reconciliations, and supporting documentation that the company must provide to support the audit. For a first-year PCAOB audit of a pre-IPO company, the PBC list is typically 50–150 items covering:

• General ledger trial balances for each period being audited
• Revenue schedules — contract-level revenue recognition support, deferred revenue rollforwards, ARR schedules
• Equity rollforwards — all shares issued, options granted, SAFEs converted, and preferred stock outstanding for each period
• All 409A valuation reports and option grant documentation
• Bank statements, debt agreements, and borrowing base certificates
• Significant contracts — all material customer and vendor contracts that affect revenue recognition or disclosure
• Board and audit committee minutes for each period
• Intercompany transaction schedules for companies with subsidiaries

Companies that are not organized to produce these materials quickly will experience significant audit delays. The accounting advisory firm typically helps prepare and organize PBC materials in advance of the audit.

Why 18-Month Lead Time Is Not Negotiable

The PCAOB auditor cannot audit financial statements they helped prepare — they can only audit statements that management has prepared and taken responsibility for. This creates a strict sequencing requirement:

1. The company must complete the transition to GAAP-compliant accounting (ASC 606, ASC 718, ASC 842, etc.) for the periods to be audited
2. Only after that can the auditor perform its audit fieldwork
3. The S-1 can only be filed after the audit is complete

If the S-1 target filing date is 18 months away and the company is currently on a modified cash basis with minimal accounting infrastructure, 18 months is the absolute minimum to engage. Companies in this situation should engage both the auditor and accounting advisory firm simultaneously — the advisory firm builds the accounting infrastructure; the auditor audits the result.