California Lease Agreement Addendum: Types, Rules, and Signing

A lease addendum in California is a written document that adds new terms to an existing lease without replacing it. Civil Code Section 1698 allows a written contract to be modified by another written agreement, which is exactly what an addendum does.¹ Once every party who signed the original lease signs the addendum, its terms are as enforceable as the lease itself.

Landlords and tenants use the words “addendum” and “amendment” interchangeably, and nothing legally turns on the label. California courts look at what the document does, not what it’s called.

What Makes an Addendum Legally Valid

Two statutes matter. Section 1698 lets a written contract be modified by another written contract.¹ For leases longer than one year, Section 1624 separately requires the agreement to be in writing to be enforceable at all.² Even for month-to-month tenancies, where the Statute of Frauds doesn’t strictly require a writing, putting the change in writing is the only reliable way to avoid a later dispute over what was agreed.

Every party to the original lease has to sign. A landlord cannot impose an addendum unilaterally mid-lease, and a tenant cannot force new terms on a landlord. If four people signed the lease, four signatures need to appear on the addendum. A missing signature from one co-tenant can leave the addendum unenforceable against that person, which usually defeats the purpose.

You don’t need new consideration. Under Section 1698(a), a written modification of a written contract doesn’t require a separate exchange of value. The mutual agreement to sign is enough.¹ Consideration only becomes an issue when parties try to modify a written lease orally, which the statute handles separately.

What to Put in the Document

A well-drafted addendum should stand on its own when read alongside the original lease. Include:

A clear title such as “Addendum to Lease Agreement.”
Reference to the original lease: signing date, property address, and the full legal names of all landlords and tenants.
The new or modified terms, spelled out concretely. Vague language like “the parties agree to adjust the pet policy” invites disagreement. Specify breed, weight, deposit amount, dates, or whatever applies.
The effective date, whether that’s the signing date or a later date.
A supremacy clause stating that if the addendum conflicts with the original lease, the addendum controls.
Signature and date lines for every party who signed the original lease.
A line confirming that all other terms of the original lease remain in full force and effect.

Common Addenda and the Rules That Constrain Them

Pet Addenda and the Deposit Cap

Adding a pet is one of the most common reasons for an addendum. The document usually covers allowed species, any breed or weight restrictions, tenant responsibility for damage, and deposits. This is where California’s deposit rules trip landlords up. Since July 1, 2024, most landlords can collect a total security deposit of no more than one month’s rent, and that cap includes any pet deposit. There’s no separate allowance for pets on top of the security deposit.³

Small landlords — natural persons or all-natural-person LLCs who own no more than two rental properties with a combined four units — can collect up to two months’ rent as a total deposit.³ If a tenant already paid a full month at move-in, there’s no room to add a pet deposit unless the landlord qualifies for the small-landlord exception and hasn’t already collected two months.

Rent Concessions

A rent concession addendum reduces the monthly payment for a defined period, often to keep a tenant during construction or a soft market. State the reduced amount, the start and end dates, and what happens if the tenant defaults during the concession period. Under the Tenant Protection Act, any concession the tenant accepts is excluded when calculating base rent for future rent-cap purposes, so list the gross rent and the concession separately.⁴

Required Disclosures Added Later

Some addenda exist because a disclosure was missing at signing or a rule changed during the tenancy. Federal law requires a lead-based paint disclosure for housing built before 1978.⁵ Since 2017, California landlords must provide written bed bug information before creating a new tenancy.⁶ Commercial leases require a Certified Access Specialist (CASp) disclosure about disability accessibility.⁷

Other Common Types

Parking addenda assign specific spaces, set guest rules, or establish fees. Co-tenant addenda add or remove a roommate; because all named tenants are jointly responsible for rent, this matters for liability. Guarantor addenda bring in a third party who agrees to cover the tenant’s obligations if the tenant defaults.

Rent Increase Addenda: Caps and Notice

If the addendum raises rent, two rules apply. Under the Tenant Protection Act, most residential landlords cannot raise rent more than 5% plus the local Consumer Price Index change, or 10%, whichever is lower, over any 12-month period. The cap runs on a rolling 12-month window, and any increase can be split into at most two increments.⁴ The Act exempts several categories, including newer buildings, most single-family homes and condos, owner-occupied duplexes, and deed-restricted affordable housing. Even exempt properties may face stricter local rent ordinances in places like Los Angeles, San Francisco, and Oakland.

Notice timing depends on the size of the increase. For increases of 10% or less within a 12-month period, the landlord must give at least 30 days’ written notice. For increases above 10%, the notice period is 90 days.⁸ An addendum that raises rent without proper notice isn’t enforceable until the notice period has run, regardless of when the tenant signs.

Fixed-Term vs. Month-to-Month Tenancies

The mechanics of changing terms differ by tenancy type. During a fixed-term lease, neither side can change terms without the other’s consent, and the full addendum process — signatures from everyone — applies.

Month-to-month tenancies work differently. Under Civil Code Section 827, a landlord can change the terms of a periodic tenancy by written notice, without needing the tenant’s signature on an addendum.⁸ If the tenant stays after the notice takes effect, the new terms become part of the tenancy. The tenant’s options are to accept, negotiate, or give notice and move out.

When a Tenant Can Refuse to Sign

During a fixed-term lease, a tenant has every right to refuse an addendum. The lease is a binding contract. A landlord who retaliates against a tenant for refusing — by raising rent, cutting services, or starting eviction — risks violating California’s anti-retaliation protections.

Things change when the fixed term ends. Under Civil Code Section 1946.2, if a tenant has been in occupancy for 12 months or more, a landlord who offers a renewal with similar terms can treat a refusal to sign as at-fault just cause for eviction.⁹ The controlling word is “similar.” A landlord cannot use this provision to force materially different terms and then claim just cause when the tenant balks.

Electronic Signatures

California’s Uniform Electronic Transactions Act, at Civil Code Sections 1633.1 through 1633.17, gives electronic signatures the same standing as ink signatures. A contract cannot be denied enforceability just because it was signed electronically, and an electronic signature satisfies any law that requires a signature.¹⁰ Standard residential and commercial lease modifications fall within the Act. Platforms like DocuSign and HelloSign produce valid signatures. Use one that generates a timestamped audit trail, and keep the electronic records in a format that stays accessible.

Signing and Storing the Addendum

Every person who signed the original lease signs and dates the addendum. Every party gets a fully executed copy. Attach it to the original lease so the complete set of obligations lives in one place. Two years later, when a dispute reaches court, the landlord or tenant who can produce a clean signed addendum attached to the original lease is in a fundamentally better position than one searching email threads to prove what was agreed.

1
California Legislative Information. California Code Civil Code Section 1698
2
California Legislative Information. California Code Civil Code 1624
3
California Legislative Information. California Code Civil Code Section 1950.5
4
California Legislative Information. California Code Civil Code Section 1947.12
5
U.S. Environmental Protection Agency. Lead-Based Paint Disclosure Rule Section 1018 of Title X
6
California Legislative Information. California Code Civil Code Section 1954.603
7
California Legislative Information. California Code Civil Code Section 1938
8
California Legislative Information. California Code Civil Code Section 827
9
California Legislative Information. California Code Civil Code 1946.2
10
California Legislative Information. California Code Civil Code Title 2.5 – Uniform Electronic Transactions Act