To start with, make sure your lease or rental agreement includes a "Limits on Use and Occupancy" clause that states that the rental unit is the residence of the tenants and their minor children only. Back this up, by making sure that every adult who will live in the rental unit has signed the lease or rental agreement and has legal status of a tenant and a direct relationship with you, the landlord.
Your occupancy limits clause should specify that the tenant may not move anyone else into the rental unit or add a roommate without your consent, and that doing so will be considered a breach of the agreement; violation of the occupancy limits clause gives you grounds to terminate the tenancy. Make sure that your occupancy limits are legal and do not violate housing laws that prohibit discrimination against families. See Can I limit the number of occupants in a rental? for details. Keep in mind that some states may restrict your ability to limit occupants (assuming you are not violating local rules governing overcrowding). New York's "Roommate Law" (RPL § 235-f), for example, allows tenants to move in relatives and other qualified individuals.
Another way to limit the number of people staying in your rental property is to include a time limit on guest stays in the "Limits on Use and Occupancy" clause of your lease or rental agreement. For example, you may allow guests to stay up to 10 consecutive days in any six-month period, with additional stays requiring your permission. A lease clause limiting guest stays will help prevent a tenant moving in a friend or relative for a month or more, calling that person a guest, who turns into a full-time resident
who has not been screened or approved. To avoid discrimination charges, any restrictions on guests should not be based on the age or sex of the occupant or guest.
Nolo's books Every Landlord's Legal Guide and The California Landlord's Law Book: Rights and Responsibilities include detailed discussions of setting occupancy limits and provide lease forms with appropriate clauses on the subject. Nolo's downloadable leases also include relevant language on occupancy limits.
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