There is no requirement in California to file a prenuptial agreement before obtaining a license. There is a requirement for disclosure of property and finances, 7 days minimum to review the agreement and being represented by a lawyer or waiving representation, specifically:
The party against whom enforcement is sought was represented by
independent legal counsel at the time of signing the agreement or,
after being advised to seek independent legal counsel, expressly
waived, in a separate writing, representation by independent legal
counsel. The advisement to seek independent legal counsel shall be
made at least seven calendar days before the final agreement is
signed.
It is possible for a couple to write their own agreement without hiring separate lawyers, but the law is written so as to presume that if not advised by independent counsel, the agreement is unenforceable. The presumption can be overcome by carefully following the requirements of §1615, for example, the party against whom the agreement is to be enforced was
fully informed of the terms and basic effect of the agreement as well
as the rights and obligations the party was giving up by signing the
agreement, and was proficient in the language in which the explanation
of the party’s rights was conducted and in which the agreement was
written. The explanation of the rights and obligations relinquished
shall be memorialized in writing and delivered to the party prior to
signing the agreement. The unrepresented party shall, on or before the
signing of the premarital agreement, execute a document declaring that
the party received the information required by this paragraph and
indicating who provided that information.