California Evidence Code 403 governs conditionally relevant evidence: proof that only matters if some underlying fact turns out to be true. The statute requires the party offering the evidence to produce enough support for that underlying fact that a reasonable jury could find it exists. If the proponent clears that low bar, the evidence comes in, and the jury then decides for itself whether the foundational fact is actually true. The judge screens; the jury decides.
The Four Situations Section 403 Covers
Section 403 does not reach every preliminary fact question. It applies in four specific categories where the proponent must lay foundation before the jury hears the evidence:
- Conditional relevance. The evidence only matters if some other fact is true. A letter is relevant to prove notice only if the recipient actually received it.
- Personal knowledge. A witness may testify only about what they personally observed. The preliminary fact is whether the witness has firsthand knowledge of the subject.
- Authenticity of a writing. A document is admissible only if the proponent shows it is what it purports to be — a genuine contract, a real email, an authentic business record.
- Statements or conduct of a particular person. When evidence consists of something a specific person said or did, the proponent must first establish that the person actually made the statement or performed the act.
Anything outside those four categories falls under a different provision, Section 405, which uses a different standard and a different procedure. That distinction matters, and it is one of the most commonly muddled points in California trial practice.
What “Sufficient to Sustain a Finding” Means
The Section 403 standard is not preponderance of the evidence. The statute requires only that the court find “evidence sufficient to sustain a finding of the existence of the preliminary fact.”1California Legislative Information. California Evidence Code Section 403 The judge is not asking whether the preliminary fact is more likely than not. The judge is asking a narrower question: has the proponent produced enough evidence that a reasonable jury could find the preliminary fact exists?
This is a screening standard, not a weighing standard. The judge does not assess credibility, resolve conflicts in the evidence, or make the ultimate factual call. If a reasonable jury could go either way on the foundational fact, the evidence comes in and the jury sorts it out. Applying the wrong standard — treating a Section 403 question as if it required preponderance — can lead the court to exclude evidence that should have gone to the jury.
Who Decides What: Judge and Jury
The division of labor under Section 403 is unusual, and understanding it is essential.
The Judge as Gatekeeper
When a party offers evidence that depends on a preliminary fact, the judge examines whether the proponent has produced enough foundational proof. If a reasonable jury could find the preliminary fact exists, the judge admits the evidence. If the proponent has produced nothing, or the showing is so thin no reasonable jury could find the foundational fact, the judge excludes it. The judge can also hear argument on admissibility outside the jury’s presence under Section 402, though that procedure is not automatic for Section 403 issues the way it is for confessions.2California Legislative Information. California Evidence Code Section 402
The Jury as Fact-Finder
Once the evidence is admitted, the jury takes over. Under Section 403(c), the court may instruct the jury to determine whether the preliminary fact exists and to disregard the evidence if it concludes the fact does not. If either party requests that instruction, the court must give it.1California Legislative Information. California Evidence Code Section 403 The jury is not simply weighing evidence the judge blessed. It is independently deciding the foundational question the judge only screened.
The judge holds a backstop role as well. If, after all the evidence is in, the judge determines that no reasonable jury could find the preliminary fact exists, the judge must instruct the jury to disregard the evidence entirely.1California Legislative Information. California Evidence Code Section 403 That prevents conditionally admitted evidence from swaying a verdict when the foundation never materialized.
Conditional Admission and Connecting Up
Section 403(b) is the statute’s most practical feature. The court can let evidence in before the foundational proof is fully established, on the condition that the proponent supplies it later during trial.1California Legislative Information. California Evidence Code Section 403 Lawyers call this “connecting up.” The evidence comes in now, and the foundation catches up later.
This happens constantly. A party may need to introduce a document early in the case through one witness but can only authenticate it through a different witness scheduled later. The judge admits it conditionally, and the proponent is expected to close the gap before the case reaches the jury.
The risk is obvious. Sometimes the proponent never connects up. When that happens, the opposing party should move to strike, and the court is required to instruct the jury to disregard the evidence. Failing to make that motion can be a serious tactical error. Conditionally admitted evidence sitting in the record without objection may be treated as properly before the jury. The burden falls on both the court and the opposing party, but experienced litigators do not rely on the judge to catch it unprompted.
Section 403 Compared With Section 405
California divides preliminary fact questions between two statutes, and the difference is fundamental. Section 403 handles the four conditional-relevance categories above. Section 405 is the catch-all: it covers every other preliminary fact not governed by Sections 403 or 404.3California Legislative Information. California Evidence Code EVID 405
Under Section 405, the judge, not the jury, decides whether the preliminary fact exists. The burden of proof is whatever standard the underlying rule of law implies, typically preponderance of the evidence. Hearsay exceptions, privilege, the voluntariness of a confession, and expert qualification are all resolved here. The judge hears the dispute, makes the call, and admits or excludes accordingly.
Under Section 403, the judge only screens. The jury makes the ultimate determination. The standard is lower — sufficient evidence to sustain a finding rather than preponderance — and the jury receives explicit instructions about its role in deciding the foundational fact.
Mixing up the two statutes is not a minor procedural error. If a court applies the Section 405 preponderance standard to a Section 403 question, it takes over a decision the legislature assigned to the jury. If it applies the Section 403 screening standard to a Section 405 question, like the voluntariness of a confession, it lets the jury decide something the law requires the judge to resolve. Either mistake can be grounds for reversal.
Challenging Evidence Offered Under Section 403
Opponents have several tools, and timing and specificity matter enormously.
Foundation Objections
The most direct challenge is an objection that the proponent has not produced sufficient foundational evidence. When a party offers a document the opponent believes has not been authenticated, or elicits testimony from a witness who may lack personal knowledge, the opponent objects on foundation grounds. The judge then evaluates whether the proponent has met the screening standard. If not, the evidence stays out until the proponent offers more foundation.
Foundational Voir Dire
Rather than simply objecting, an attorney can ask permission to conduct a brief examination of the witness, called a foundational voir dire, to expose gaps before the judge rules. The typical sequence: the proponent offers the evidence, the opponent objects, and the opponent then asks the court for permission to question the witness on the narrow issue of whether the foundational fact has been established. The technique works especially well against document authentication and personal knowledge, where a few pointed questions can quickly reveal that the witness cannot supply the preliminary fact the proponent needs.
Motions to Strike When the Proponent Fails to Connect Up
When the court conditionally admits evidence under Section 403(b), the opposing party must stay alert. If the foundation never arrives, a motion to strike is the proper remedy, and the court must then instruct the jury to disregard the evidence.1California Legislative Information. California Evidence Code Section 403 Waiting too long, or assuming the judge will act on their own, is a gamble. Courts expect litigants to police conditionally admitted evidence and may treat silence as waiver.
Preserving the Issue for Appeal
California requires objections to be timely and specific. The objection must be raised when the evidence is offered, and it must state the specific ground. A general objection, or one raised for the first time after the evidence is already before the jury, risks waiver. If the issue is not preserved at trial, an appellate court will typically decline to review it. Even a clearly improper admission under Section 403 may stand if no one objected at the right time in the right way.
Section 352 Still Applies
Clearing the Section 403 threshold does not end the admissibility inquiry. Section 352 gives the court discretion to exclude evidence whose probative value is “substantially outweighed by the probability” that it will cause undue prejudice, confuse the issues, or waste time.4California Legislative Information. California Evidence Code EVID 352 The two provisions ask different questions. Section 403 asks whether the foundational fact has enough support for the evidence to be relevant at all. Section 352 asks whether relevant evidence should still be kept from the jury because its costs outweigh its benefits. An opponent who loses on foundation can immediately pivot to a Section 352 argument, and the two are evaluated on their own terms.