A sales tax audit is a state review of whether your business collected, reported and paid the right sales and use tax for a set period, usually the last three or four years. You handle it best by confirming the scope in writing, pulling the records the auditor lists before the first meeting, agreeing on the sampling method early, and checking every proposed adjustment before the appeal deadline on the notice runs out.
Most states treat a sales tax audit as routine. The Texas Comptroller tells businesses in its August 2026 audit notice that "we audit many taxpayers regularly, so you shouldn't feel that we have singled out your account." This guide walks through each stage using the published procedures of California, Texas and Washington, which show how most states run the process.
An auditor's job is to find tax that was under-collected or under-reported, and in many states also tax you overpaid. The Texas Comptroller's audit process page says auditors "strive to identify both under- and overpaid tax, as taxpayers at times pay taxes on items that may be exempt."
Most reviews concentrate on four areas:
California's Department of Tax and Fee Administration (CDTFA) describes these tests in Publication 76, Audits. Its auditors may compare "total sales recorded on your books to the total sales reported on your sales tax returns" and "total sales recorded on your books to the total sales on your income tax returns."
The lookback period depends on the state and on whether you filed returns. Businesses that never registered, or that badly understated tax, can face a much longer window.
| State | Normal audit period | When it can be longer | Deadline to dispute the result |
|---|---|---|---|
| California | Three years for businesses that file returns | Eight years if no returns were filed | 30 days from the Notice of Determination |
| Texas | Four years, the most recent four-year period | Any period if a fraudulent return was filed, tax due would rise by 25% or more, or no return was filed | The deadline printed on the audit notification |
| Washington | The past four calendar years plus the current year | Seven years plus the current year for an unregistered business the state discovers; no limit for evasion or misrepresentation | 30 days from the date you are notified of the audit findings |
Texas spells out its exceptions in the Notice of Routine Audit: tax "may be assessed for any period if" a false or fraudulent return was filed, a return "was filed erroneously and the tax due would increase by 25% or more," or "a return was not filed." The Washington Department of Revenue says on its audit process questions page that the audit period "is usually for the past four calendar years, plus the current year." Under Washington's rule for unregistered businesses, WAC 458-20-230, the department "will assess taxes, interest, and penalties for a period of seven years plus the current year" when it discovers a business that should have registered. A business that registers voluntarily before any contact is limited to four years plus the current year, provided it made a good faith attempt to report correctly.
If your business crossed an economic nexus threshold in a state years ago and never registered, the normal three or four year limit may not protect you there. An auditor may also ask you to sign a waiver that extends the statute of limitations. CDTFA describes its Waiver of Limitation as "a document extending the legal requirements" and says auditors may request one "for significant delays."
The first letter tells you what the state wants and how fast it wants it. Note the periods under review, the tax types involved, the auditor's name and contact details, and any questionnaire that must be returned. In Texas, the notice comes with Form 00-750, an audit questionnaire. If it is not returned within 30 days, the auditor will follow up, and if contact cannot be made, "a liability may be established by the best available method, including estimation."
Share the notice with your CPA or sales tax advisor the day it arrives. Both California and Texas allow an authorized representative to deal with the auditor directly. The Texas Comptroller also warns that the list of businesses selected for audit is public, so consultants may contact you.
If your federal return is also under review, keep the two processes separate. Our guide on how to prepare for a tax audit covers IRS examinations.
States publish the records they expect. The Texas audit page lists, among others, "sales invoices, along with any current resale and exemption certificates," purchase invoices and depreciation schedules, general ledgers, federal income tax information, bank statements, and "working papers and accounting data used to prepare tax reports." Washington's list adds reseller permits and "supporting documentation for all deductions and exemptions."
Organize the file by period and by category before the auditor arrives:
Exemption certificates deserve extra time. A missing or expired certificate turns an exempt sale into a taxable one in the auditor's schedule.
Keep everything for the full audit period even if the normal retention period has passed. CDTFA says that if you are being audited, you "should retain all records that cover the audit period until the audit is complete, even if that means keeping them longer than four years."
If records are missing, tell the auditor what you do have. CDTFA notes that missing records may need to be reconstructed, or the auditor "may need to estimate the missing information."
At the entrance conference, ask the auditor to explain the audit plan. The key question is whether the review will be detailed, covering every transaction, or sampled, where errors in a test period are projected across the whole audit period.
Texas explains that in a sample audit, "errors found in representative samples of business records are projected throughout the entire audit period." CDTFA says its auditors generally try to use a statistical sample and "should discuss the sampling elements with you before finalizing the plan."
Before you agree to a sample, check that the test period is typical of your business. A quarter that included a one-off bulk sale to an exempt customer, a system migration or a staff change can produce an error rate that does not reflect the other eleven quarters. If the proposed period is unusual, explain why and ask for a different one or a larger sample.
Assume a retailer reported $2,400,000 of sales over a three-year audit period, or 12 quarters of about $200,000 each. The auditor samples one quarter with $200,000 of sales and finds $6,000 of sales recorded as exempt with no certificate on file. The combined tax rate is 8.25%.
| Item | Before certificates are found | After $4,000 of certificates are produced |
|---|---|---|
| Unsupported sales in sample | $6,000 | $2,000 |
| Error rate ($ unsupported / $200,000 sampled) | 3% | 1% |
| Projected unreported taxable sales ($2,400,000 x error rate) | $72,000 | $24,000 |
| Tax due at 8.25% (before interest and penalties) | $5,940 | $1,980 |
Producing valid certificates for two thirds of the flagged sales in the test quarter cuts the tax by $3,960 across the full period. Every dollar fixed in the sample is multiplied by the ratio of total sales to sampled sales, which is 12 in this case. Clearing exceptions during fieldwork is therefore the most productive work in a sales tax audit.
Respond to document requests on time and in writing. Texas auditors are instructed to avoid gaps of more than 30 days with no audit activity. If records are not provided, the auditor will issue a written request "giving the taxpayer thirty (30) days to produce the records," and a further extension, not to exceed 90 days, is at the auditor's discretion. Records that never arrive lead to an estimated audit.
Answer questions truthfully and explain weak periods, such as a year of staff turnover. Keep a log of every request, the date you responded and what you sent.
If you discover errors yourself during the audit, ask how amended returns are treated. CDTFA says amended returns filed after an account is assigned for audit are treated as additional records, and payments made with them are credited to the audit, "thus stopping interest accrual on the amount paid."
Some states offer a managed audit, where the business reviews its own records under an agreed plan. In Texas, the request must reach the audit manager within 60 days of the date on the audit notification letter. In California, a business that completes a managed audit pays interest at one-half of the normal rate on the amount due.
When fieldwork ends, the auditor presents schedules of proposed adjustments. CDTFA says its auditors "are required to provide you copies of all the audit working papers they prepared."
Check each schedule line by line:
If you disagree, say so promptly and in writing. In California, a business that still disagrees after meeting the audit supervisor receives a letter giving it ten days to book a meeting with a CDTFA representative, and silence is treated as agreement. Texas offers a reconciliation conference with the audit manager or supervisor, and an independent audit review conference with a third party for issues still in dispute after that.
The final notice starts a short clock. In California, within 30 days of the Notice of Determination you must either pay or file a petition for redetermination. Miss that window and CDTFA adds "a penalty equal to ten percent of the unpaid taxes or fees due," and the right to file a petition is lost. Washington gives 30 days from the date you are notified of the audit findings to appeal. In Texas, a Statement of Grounds must be received by the deadline on the notice to contest the assessment without paying first.
Interest keeps running while you dispute. CDTFA suggests considering paying the billed amount, or at least the undisputed portion, because interest continues to accrue during a petition for redetermination. If the petition succeeds, any overpaid tax is refunded with interest.
A finished sales tax audit gives you a list of the weak spots in your process. Most findings trace back to a small number of recurring sales tax mistakes: untaxed sales with no certificate, incorrect product taxability settings, local rate errors and use tax never accrued on internal purchases.
Write a short corrective plan within a month of closing the audit. Typical fixes include collecting certificates at onboarding before the first exempt invoice, reconciling reported sales to the ledger every filing period, and reviewing taxability settings whenever you add a product line. A sales tax compliance checklist for small businesses gives you a monthly routine to follow, and sales tax automation software can handle rates and certificate tracking at volume.
If the audit revealed that you owe tax in other states where you never registered, deal with those states before they contact you. A sales tax voluntary disclosure agreement usually limits the lookback period and may reduce penalties compared with waiting for an audit there.
What triggers a sales tax audit? Many audits are routine selections. The Texas Comptroller lists methods such as reviewing the largest sales tax payers, revisiting businesses whose prior audits found $25,000 or more due, random selection by industry, information sharing with other agencies and analysis of return data.
How long does a sales tax audit take? It depends on the size of the business, its records and whether findings are disputed. CDTFA notes that most petitions for redetermination are decided in less than a year, though some take several years.
Can I be audited for a state where I never filed returns? Yes. If you had nexus and were required to register, the normal limit usually does not apply. California allows eight years for businesses that do not file returns, Texas allows assessment for any period in which a required return was not filed, and Washington assesses seven years plus the current year when it discovers an unregistered business.
Should I pay a sales tax audit assessment I plan to appeal? Paying at least the undisputed portion stops interest on that amount. In California, a successful petition brings a refund of any overpaid tax with interest, and a business that misses the 30-day petition deadline can still pay and then file a claim for refund for each payment.
Do I need a representative for a sales tax audit? California and Texas do not require one. Texas says professional representation "is not required" and is "solely a business decision." A representative who knows the state's sampling and appeal procedures can still save time, and CPA firms with many multi-state clients often use sales tax outsourcing to keep records audit-ready.
Madras Accountancy helps US businesses and CPA firms prepare for and respond to state sales tax reviews, including record reconstruction, exemption certificate clean-up, reconciliation of reported sales to the ledger and ongoing multi-state filing support. If you have received an audit notice or want your records ready before one arrives, contact our team.
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