When two or more people sign a lease or rental agreement together, they become co-tenants and share the same legal rights and responsibilities. Many leases contain language stating something to the effect of "tenants are jointly and severally liable for paying rent and adhering to the terms of the agreement." This means that each roommate is independently responsible for the entire rent – not just his or her share of the payment. In other words, if one roommate does not contribute to any month's rent, the other roommate(s) are still liable for the entire payment. A landlord has the legal right to hold all co-tenants responsible for the negative actions of one, and he or she can evict all tenants even if only one violated the lease or rental agreement.

Most landlords will require tenants to sign a written lease – a conveyance of a possessory estate in real property and a contract between the landlord and tenant. There are three different types of residential leases that express various terms for the rental agreement: 

  • tenancy for years 
  • tenancy from period to period 
  • tenancy at will. 

Each lease identifies a starting date (commencement date) for the tenancy and depending on the type of lease, an end date may or may not be specified. A fourth type of lease – a tenancy at sufferance – is never intentionally entered into.

If a renter wishes to add a roommate to the lease or rental agreement, the landlord will perform the usual credit and background checks for any additional tenants and will ask all tenants to sign a new lease or rental agreement. Because tenants are "jointly and severally liable" for paying the rent and any damages, the landlord will benefit from including all tenants on a new lease. The new lease also makes it clear to all tenants that each share the same legal rights and responsibilities.

Adding a roommate to a lease can sometimes result in an increase in rent, presumably to cover the additional wear and tear on the unit. The new lease signifies a new tenancy, so the landlord can increase the rent immediately (instead of giving any notice or waiting until the lease ends). The landlord also has a legal right to increase the security deposit on the unit. The amount of the security deposit is typically based on the rent. If the rent increases, the landlord can raise the security deposit correspondingly.

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