A lease addendum is a separate document that adds to or changes the terms of an existing lease, signed by both the landlord and the tenant and kept with the original lease. It does not replace the lease, it attaches to it, so the two documents are read together as one agreement.
In practice
Say a tenant signs a 12-month lease at $1,800 a month, and three months in they ask to bring home a dog. Rather than rewrite the whole lease, you draft a one-page pet addendum: it names the animal, sets a $300 pet deposit, adds $25 a month in pet rent, and spells out who pays for any pet damage at move-out. Both of you sign and date it, you each keep a copy, and the original page goes in the same file as the lease. From that day the monthly rent owed is $1,800 plus the $25 pet rent, which is $1,825, and the held deposit rises by $300.
The mechanics that make an addendum stick are dull but specific. It references the original lease by its address and start date, it states only the terms that change, every party who signed the lease signs the addendum, and it carries its own date. A common one fixes the lease at its end date: a renewal addendum that extends the term another year and resets the rent saves you from drafting a fresh lease each cycle.
Why it matters to a small landlord
An addendum is how you change a deal in the middle without reopening every clause, and that is exactly when disputes start. The risk is the verbal side agreement: you tell a tenant a roommate can move in, money or responsibility shifts, and nothing is on paper. When the relationship frays, the unwritten change is the part that costs you. The same habit of writing things down that wins deposit fights also settles addendum questions, which is why a clean record of documented tenant interactions is worth more than memory. When the lease term itself is what you are changing, a lease renewal letter and a renewal addendum do the same job from two sides. What an addendum can and cannot override is set by your state's landlord-tenant statute, so read your local rules before you change a term that touches notice, entry, or deposits.
Most addenda a small landlord writes fall into a few buckets. Pets get a pet rent addendum that prices and bounds the animal. Adding a financial backstop is a guarantor addendum, which brings a co-signer onto the obligation. And when a tenant wants to hand the unit to someone else for part of the term, that is a sublease arrangement, which an addendum can permit or forbid in writing. Whatever the change, the rule holds: if it alters the lease, it gets signed, dated, and filed with it.