The 4 Types of IRS Audits: What They Are and What to Expect

Written by Sam Brotman, J.D., LL.M. (Taxation), MBA — Managing Attorney, Brotman Law  ·  Updated May 2026

What Is a Tax Audit?

The short version is that the IRS is checking your math — and your supporting documentation. It is a legal proceeding, not an accounting review. That distinction matters because it affects what rights you have, who should be handling the response, and what the consequences of getting it wrong look like.

The IRS audits approximately 0.5% of individual returns in a given year. But that average covers a wide range — the audit rate for high-income returns (over $10 million in income) runs closer to 8%, and certain return types (Schedule C sole proprietors, cash-intensive businesses, returns with large charitable deductions) draw more scrutiny than others.

The IRS has three years from the return's filing date to assess additional tax under IRC § 6501(a). If gross income was understated by more than 25%, that window extends to six years under IRC § 6501(e)(1). For fraud or a return that was never filed, there is no limitations period at all.

Not all tax audits are the same. The type the IRS initiates tells you a lot about how much attention they're paying, how many years they're looking at, and how seriously you need to take the response.

The 4 Types of IRS Audits

There are four main types of IRS audits, each with a different scope, format, and risk profile. Most taxpayers will only ever encounter a correspondence audit. Field audits are reserved for more complex cases. The TCMP audit is rare and essentially random.

Here is how they break down — each covered in detail below.

Correspondence Audit

Roughly 70% of all IRS audits are correspondence audits. If you've received a letter from the IRS questioning a specific line item — a charitable deduction, a business expense, a reported 1099 that doesn't match what the IRS has on file — this is likely what you're dealing with.

How a Correspondence Audit Works

The IRS sends an audit notice by mail — usually a CP2000 (automated notice proposing changes based on information return mismatches) or a Letter 525 (General 30-Day Letter, formally proposing adjustments after an examination). The letter identifies the specific item under review and asks you to either agree to the proposed change or submit documentation to support your original return.

You respond by mail. If your documentation is adequate, the IRS closes the case. If not, they may assess the additional tax or escalate to a more formal examination.

Typical Triggers

• A 1099 or W-2 that doesn't match what you reported (the IRS's automated matching program catches these)
• A large charitable deduction relative to your income
• Home office deductions or vehicle expense claims on Schedule C
• Unreported income from a side business or freelance work

How Long Does a Correspondence Audit Take?

Most correspondence audits resolve in 3 to 6 months, depending on how quickly you respond and whether a follow-up request is needed. The IRS gives you 30 days from the notice date to respond; extensions are generally available on request.

A correspondence audit that is not handled carefully can turn into something larger. If your response raises new questions, or if the IRS's automated systems flag additional items while the case is open, the scope can expand. That is one reason why, even for a "simple" correspondence audit, it is worth reading what the IRS is actually asking before assuming it's routine.

Office Audit (Desk Audit)

Office audits are less common than correspondence audits but more involved. The IRS has identified specific issues it wants to examine in person, and it has scheduled time with an examining agent to do it.

How an Office Audit Works

You receive an audit notice — typically a Letter 2205 or a similar scheduling letter — identifying the issues under review and the documents the IRS wants to see. You bring those records to the local IRS office and meet with an agent. The agent reviews the documentation, asks follow-up questions, and either closes the audit or requests additional information.

You can bring a representative with you. In most cases, if you have an attorney or enrolled agent handling the matter, they can attend the meeting in your place (with a valid Form 2848 Power of Attorney on file).

Typical Triggers

• Business income and expense deductions on Schedule C, particularly in cash-intensive industries
• Rental income and depreciation claims
• Casualty or theft loss deductions
• Returns with multiple flagged issues that are too complex to resolve by mail

How Long Does an Office Audit Take?

Office audits typically run 6 to 12 months from the initial notice to resolution, though that varies significantly based on how well-organized your records are and whether follow-up requests are needed. A well-prepared, organized response can shorten this considerably. Poor documentation almost always extends it.

Field Audit

Field audits are reserved for complex returns — typically business returns, high-income individuals, partnerships and S-corps, or situations where the IRS believes there is significant unreported income or fraudulent activity. If you are receiving a field audit, the IRS has already decided this is worth the cost of sending a revenue agent to you.

How a Field Audit Works

A revenue agent contacts you (or your representative) to schedule an initial meeting at your place of business or home. They will send an Information Document Request (IDR) listing the records they want to review — often covering multiple years and multiple issues simultaneously. The agent works through those records over a series of meetings, asking follow-up questions and sometimes requesting additional documents.

The agent can also interview employees, review bank records, examine computer records, and in some cases request third-party records. At the end of the examination, they issue a Form 4549 (Income Tax Examination Changes) detailing any proposed adjustments. You have the right to disagree, and if you do, the case moves to the IRS Appeals process.

Typical Triggers

• Business returns with significant deductions, complex ownership structures, or related-party transactions
• High-income returns where the amounts at stake justify the investment of agent time
• Returns flagged through a related-party audit (e.g., the IRS is auditing your business partner and your return is pulled into scope)
• Returns where prior correspondence or office audits revealed the need for broader examination
• Eggshell audits — where the IRS suspects potential criminal issues but hasn't yet referred the case to Criminal Investigation

How Long Does a Field Audit Take?

Field audits are measured in months to years, not weeks. A straightforward field audit covering a single business year might resolve in 12 to 18 months. Complex examinations covering multiple years, multiple entities, or significant unreported income can run two to three years. The IRS's own workload and the availability of the assigned revenue agent affect the timeline as much as the complexity of your case.

One thing worth knowing: the IRS generally cannot extend the three-year assessment statute of limitations without your consent. Agents routinely request that taxpayers sign a Form 872 (Consent to Extend the Time to Assess Tax). You are not required to sign it, but declining has its own strategic implications — worth discussing with counsel before you decide either way.

TCMP / National Research Program Audit

TCMP audits are rare. If you are selected, it is not because the IRS found something wrong — it is because your return was drawn in a statistical sample. The IRS uses these audits to update the DIF (Discriminant Information Function) scoring model, which is the computer system that flags returns for examination.

How a TCMP / NRP Audit Works

The process is essentially a field audit with one significant addition: the IRS must verify every line of your return, not just the lines it selected for scrutiny. This means documentation for every source of income, every deduction, every credit. There is no shortcut and no way to narrow the scope by being responsive on one issue.

These audits are conducted by specially trained revenue agents. The thoroughness is by design — the data collected from NRP audits is used to refine the IRS's targeting models for years afterward.

How Long Does a TCMP/NRP Audit Take?

These audits are among the most time-intensive. The comprehensive, line-by-line nature of the review means that even a relatively straightforward return takes significantly longer to examine than a targeted field audit. Expect 12 to 24 months at minimum for a completed examination.

How the IRS Selects Returns for Audit

The whole question here hinges on the DIF score. Every return that runs through IRS processing receives a DIF score that compares your deductions, income, and credits to statistical norms for your income bracket and return type. Returns that deviate significantly from those norms score higher and are more likely to be pulled for review.

DIF scoring is not the only mechanism. The IRS also selects returns through:

• Automated Underreporter Program (AUR) — the IRS cross-references your return against third-party information returns (W-2s, 1099s, K-1s). A mismatch generates a CP2000 notice, which is often the opening move in a correspondence audit.
• Related-party audits — if the IRS is examining your business partner, your investor, or someone you transacted with, your return may be pulled into scope through the related examination. This is one of the less-anticipated audit triggers for business owners.
• Industry and occupational programs — the IRS maintains active audit programs targeting specific industries where noncompliance is historically common: cash-intensive businesses, restaurant and food service, construction, real estate.
• Informant tips — Form 211 allows anyone to report suspected tax underpayment. The IRS Whistleblower Office processes these referrals, and if the tax at issue is large enough, the IRS may investigate.
• Random selection — a small percentage of returns are chosen purely at random, most commonly for NRP/TCMP research purposes.

The practical takeaway: most audits are triggered by something specific on your return or in your industry, not by bad luck. Understanding the trigger matters for understanding how to respond.

All 4 Types of IRS Audits: Side-by-Side Comparison

What to Do When You Receive an IRS Audit Notice

The specific letter matters. Here's what the most common audit initiation notices mean:

• CP2000 — the IRS's automated underreporter notice. It proposes changes based on a mismatch between your return and third-party information (1099s, W-2s, K-1s). You have 60 days to respond. This is not technically an audit, but if you don't respond or your response doesn't resolve the discrepancy, it can become one.
• Letter 525 (General 30-Day Letter) — formally proposes tax adjustments after an examination. You have 30 days to agree (sign the enclosed Form 4549 and pay) or file a protest requesting IRS Appeals review. Missing this deadline limits your options significantly.
• Letter 531 (Statutory Notice of Deficiency / 90-Day Letter) — the IRS's formal determination that you owe additional tax. You have 90 days to petition the U.S. Tax Court to dispute it before the IRS can assess the liability. This is the last off-ramp before assessment.
• Letter 2205 — initiates a field or office examination. It identifies the issues under review and the records the agent wants to see, and asks you to schedule a meeting.

Once you understand what type of audit you're facing and what the deadline is, the next steps are:

1. Gather the records that are relevant to the specific items flagged — not everything in your filing cabinet, just what the IRS asked about.
2. Do not volunteer information beyond what was requested. The scope of a correspondence audit is defined by the letter. Expanding it by providing additional information about unrelated issues is a mistake.
3. Respond within the deadline or request an extension. The IRS generally grants reasonable extension requests. A missed deadline is harder to recover from than a request for more time.
4. Consider whether you need representation. For most correspondence audits with clear documentation, you may not. For an office or field audit, or any audit where the adjustment amount is significant, representation is worth the cost.

When to Get a Tax Attorney vs. Handle It Yourself

Here's how I generally think about it:

Correspondence Audits — Maybe DIY

If the IRS is asking about a specific item and you have clear documentation — a 1099 that you did report, a charitable deduction with receipts, a business expense with invoices — a correspondence audit is often something you can handle yourself or with your CPA. The risk in a correspondence audit is not usually the legal complexity; it's making sure you're responding to exactly what was asked and not inadvertently expanding the scope.

An attorney adds real value in a correspondence audit when: (1) the proposed adjustment is legally wrong, not just documentarily wrong; (2) the amount at issue is large enough to warrant a formal protest; or (3) the correspondence audit looks like a precursor to a broader examination.

Office Audits — Probably Get Help

An office audit means the IRS has identified multiple issues and wants to examine records in person. That shifts the dynamic. The agent is going to ask follow-up questions, and the answers to those questions can open or close issues beyond what the original notice identified. Having someone experienced in IRS examination procedure at the table is generally worth it.

Field Audits — Get an Attorney

A field audit involving a revenue agent at your business is not something to handle on your own. The scope is broad, the agent has significant authority to expand the examination, and decisions made in the early meetings — what records to produce, what to say and not say, whether to sign a Form 872 extension — can significantly affect how the audit resolves. We have handled hundreds of field audits at Brotman Law, and the pattern is consistent: the earlier representation is in place, the more options the client has.

If the field audit has any indication of potential criminal issues — an eggshell situation where the IRS suspects fraud but hasn't said so — that's a different matter entirely. Those cases require experienced criminal tax defense counsel, and the calculus around document production and agent communications is more complicated.

Your Rights During an IRS Audit

These rights are not theoretical. They are enforceable, and the IRS is bound by them. The ones most relevant to an audit:

• Right to representation — you can bring a tax attorney, CPA, or enrolled agent to any audit meeting. If you are represented, you generally do not need to attend yourself. File Form 2848 (Power of Attorney and Declaration of Representative) to authorize someone to represent you before the IRS.
• Right to appeal — if you disagree with the IRS's proposed adjustments after an examination, you can request a conference with IRS Appeals before the agency assesses additional tax. Appeals is an independent function within the IRS, and settlements at Appeals are common. If Appeals doesn't resolve it, you can petition the U.S. Tax Court (for deficiency cases), or pay the assessment and sue for a refund in federal district court or the U.S. Court of Federal Claims.
• Right to finality — the IRS generally cannot re-examine a tax year that has already been closed through an examination. There are exceptions (fraud, mathematical errors, certain amended return scenarios), but the principle matters: a closed audit year is closed.
• Right to record the interview — you may record an IRS interview if you notify the agent in advance. This is underused and often strategically useful.

One practical note: the right to appeal the IRS's position exists at every stage of the audit, including after the IRS issues a Notice of Deficiency. The 30-day letter (Letter 525) and the 90-day letter (Letter 531) are both formal checkpoints — each one carries a response deadline and a corresponding set of rights. Missing those deadlines doesn't eliminate your options entirely, but it does narrow them.