The main contract is ready to sign, but one issue remains: a special discount, an investor’s reporting rights, or a promise to waive a fee. Someone suggests putting it in a side letter. That can be practical, but a short document can still create significant obligations. For California businesses and individuals, the important question is not what the document is called, but how it changes the deal. This article explains how to structure a side letter agreement, resolve conflicts, and check authority before signing.
What is a side letter agreement?
A side letter agreement is a separate document that supplements, clarifies, or changes terms connected to another agreement. It may be signed with the main contract or negotiated afterward. Common examples include special pricing under a supply agreement, additional information rights for an investor, or a temporary exception to a contractual requirement.
The label does not create a special legal category. A side letter must satisfy the contract requirements applicable to its substance. Calling a document “informal” or “for reference” does not settle whether its promises are enforceable.
Distinguish an actual agreement from a statement of intent. If the document says the parties “may discuss” a future concession, that is different from a specific obligation to provide it. Clear language should identify:
- Who is making each promise and who receives the benefit.
- Which main agreement the side letter concerns.
- When the obligations begin and end.
- Whether the parties intend the document to be binding.
How does the side letter fit with the main contract?
Start by reading the main agreement’s integration, amendment, and priority clauses. An integration clause generally states that the contract contains the parties’ complete agreement. An amendment clause describes how changes must be approved. A priority clause identifies which document controls if terms conflict.
California Civil Code § 1642 provides that several contracts relating to the same matters, between the same parties, and made as parts of substantially one transaction are to be taken together. A side letter may therefore need to be read alongside the main agreement, rather than in isolation.
California Civil Code § 1698(a) permits a written contract to be modified by a written contract. Subdivisions (b)–(d) also recognize oral modifications to the extent executed by the parties, oral modifications supported by new consideration unless the contract expressly provides otherwise, and doctrines such as waiver and estoppel. Applicable statute-of-frauds requirements still matter. Electronic records and signatures may satisfy writing and signature requirements under applicable law; an unsigned document may also have legal significance depending on the parties’ agreement, conduct, and other circumstances. Neither an email nor an unsigned letter automatically creates an effective modification.
A practical drafting approach is to identify the affected sections, state precisely what changes, and confirm that all other terms remain in effect. If the side letter controls only on one issue, say so. Reviewing both documents as part of the same business contract review can expose inconsistencies before signature.
Terms that deserve more than a short sentence
A concession can create questions beyond its headline benefit. “Customer receives a discount,” for example, leaves open the amount, eligible purchases, duration, and whether the discount survives renewal.
Give particular attention to these provisions:
- Payment and pricing: State amounts, calculation methods, payment dates, and any conditions for earning a credit or discount.
- Duration: Specify whether the side letter expires on a date, ends with the main contract, or survives termination for a limited purpose.
- Confidentiality: Identify permitted disclosures, including disclosures to advisers and those required by law. Confidentiality is not a substitute for required disclosure.
- Transfer: Explain whether benefits pass to a buyer, successor, or permitted assignee.
- Disputes: Clarify whether the main contract’s governing-law, forum, arbitration, and attorney-fee provisions apply.
If the letter adds a personal guaranty, do not bury that obligation in routine business language. Identify the guarantor and the covered obligations clearly. Under California Civil Code § 1624(a)(2), a promise to answer for another person’s debt, default, or miscarriage generally must be in a writing subscribed by the guarantor or the guarantor’s authorized agent, subject to applicable exceptions.
Check authority, approvals, and other affected parties
A signature is only part of the review. Confirm that the person signing has actual or apparent authority to bind the relevant company, and identify approvals required by applicable law, the entity’s governing documents, or the transaction documents. Depending on the entity and transaction, that may involve managers, directors, members, or shareholders. Apparent authority depends on the company’s conduct, not merely the signer’s claim of authority. Failure to obtain an internal approval may create issues between the company and its agent or under its governing documents, but does not automatically make the side letter unenforceable against a third party.
Also examine obligations to people who are not signing the side letter. A special investor benefit could interact with another investor’s contractual rights. A pricing concession could trigger a most-favored-customer clause. A lender’s documents may restrict changes to important commercial agreements.
These consequences depend on the actual documents; they are not automatic rules for every side letter. Review consent requirements and disclosure obligations rather than assuming a private arrangement affects only its signatories. A side letter ordinarily cannot impose contractual duties on a nonparty without that person’s assent, but it may confer enforceable rights on an intended third-party beneficiary under California Civil Code § 1559. Other legal doctrines may apply depending on the circumstances.
A pre-signing checklist for your side letter agreement
Before signing, compare the side letter against the final version of the main agreement—not an earlier draft. Small changes in section numbers, defined terms, or effective dates can create avoidable uncertainty.
- Confirm identities: Use the correct legal names and distinguish the company from its owners.
- Identify the deal: Include the main agreement’s title, date, and parties.
- Resolve conflicts: Specify which provisions change and which document controls.
- Check conditions: Record required approvals, signatures, notices, and supporting obligations.
- Keep the records together: Store the executed documents together and track deadlines.
If a side letter already exists and the parties disagree about its meaning, preserve drafts, emails, and performance records. Do not assume that either the main contract or the side letter automatically prevails.
Talk to a California business attorney
Itkin Law offers a free consultation to discuss a proposed side letter or a dispute over how it affects your agreement. Schedule a free consultation or call (424) 603-8888.
This article is attorney advertising and provides general information only. It is not legal advice and does not create an attorney–client relationship. The law changes, and this article reflects the law as of its publication date. Every situation is different — contact us to discuss how the law applies to your exact circumstances. See our full disclaimer.
