How to add a roommate to an existing lease
Read the occupancy, subletting, and assignment clauses first. Some leases require written landlord approval; San Francisco rules may also affect how a landlord handles replacement roommates. Do not promise a room until you know the process.
Give the prospective roommate the rent, deposit, house rules, and lease facts. If the landlord approves, get a written addendum or other written confirmation stating the person’s status, move-in date, and financial responsibility.
Update the household agreement too: utilities, chores, keys, guests, and how the new person’s deposit is handled. Keep copies of the request, approval, and signed documents.
Sources: SF Rent Board guidance, California Civil Code §1946.2.