The first thing to understand is how California treats contractors differently from most other businesses. Under CDTFA rules, contractors are generally the consumers of the materials they install rather than retailers reselling those materials to customers. You pay sales tax when you buy materials, and you don’t collect sales tax from your customer on those materials. The rule flips for fixtures, where you’re treated as the retailer and owe tax based on what you charged the customer.

The distinction between materials and fixtures matters in an audit. Materials lose their identity when installed, things like lumber, concrete, rebar, drywall, and paint. Fixtures retain their identity after installation, things like cabinets, water heaters, HVAC units, and light fixtures. Different tax treatment applies to each, and auditors will test whether you classified items correctly and paid or collected tax accordingly.

Pull together every material purchase invoice for the audit period. CDTFA auditors want to see that you paid California sales tax on materials you consumed in your contracts. If a supplier didn’t charge California tax, especially on out-of-state or online purchases, the auditor will look for a use tax accrual on your return. Internet purchases from out-of-state vendors are one of the most common triggers for assessments because contractors routinely forget to self-assess use tax.

Gather your resale certificates. If you issued a resale certificate to a supplier claiming you would resell the material, but you actually installed it as a contractor who’s the consumer, you owe use tax on that purchase. Auditors specifically look for this pattern. Resale certificates are useful for items you’ll sell as fixtures, not for materials you consume in a contract.

Match material purchases to specific jobs. Your job cost records should show which materials went to which projects and how they were used. If an auditor questions whether a purchase was for a taxable installation or something else, documentation of the job, the contract scope, and the application of the materials is what resolves it. This is one of the reasons good construction bookkeeping ties every material purchase back to a job number at the time of entry rather than reconstructing it later.

Review your fixture billings. When you install fixtures under a lump sum contract, tax is generally owed on your cost. Under a time and materials contract where you separately state the fixture price, tax is owed on the billed price. Auditors will compare your contracts, invoices, and tax collected to see whether fixtures were billed and taxed correctly.

Check your use tax returns for the entire audit period. California requires contractors to self-assess use tax on taxable purchases where sales tax wasn’t paid at the point of sale. This includes out-of-state and online purchases, items withdrawn from inventory for personal use, and purchases where a supplier incorrectly didn’t charge tax. Missing or understated use tax is one of the most common findings and one of the easiest for auditors to calculate.

Organize everything by year, vendor, and job before the auditor shows up. When records are organized, the audit moves faster and stays focused on specific questions. When records are incomplete, auditors use projections and sampling, and those estimates almost never favor the taxpayer. Respond to requests in writing, keep copies of everything you hand over, and don’t volunteer records that weren’t asked for.

If the audit produces proposed adjustments you disagree with, you can discuss them with the auditor, request a supervisor review, and ultimately appeal to the Office of Tax Appeals. Most disagreements get resolved at the auditor or supervisor level when you have documentation that supports your position.

Getting through a CDTFA audit really comes down to records. Material invoices with tax paid, resale certificates used correctly, use tax accrued on untaxed purchases, and job-level documentation showing how each item was used. If the records are in order, the audit is mostly a confirmation exercise. If they’re not, the gaps get filled with assessments. Working with Pasadena bookkeepers who understand how California handles contractor sales and use tax is cheaper than discovering the gaps during an audit.