Most leases prohibit people not listed on the lease living there. The landlord may or may not be willing to add your girlfriend to the lease but without her being on the lease while still living there, you are probably in breach of your lease and presumably are subject to being evicted if she does not leave.
Steps to Add a Tenant to an Existing Lease Acquire a Written Request to Add a Tenant. Check the Unit's Occupancy Limit. Require a Completed Rental Application. Screen and Approve/Deny the New Tenant. Review the Details With Each Tenant.
If the landlord agrees to add someone's name to a lease, it can be considered a “vacancy lease” even though no one is moving in or out and could result in a possible risk of losing any “preferential rent” agreement with the landlord. Tenants may wish to consult an experienced tenant lawyer before signing a new lease.
All you can do is ask the landlord, that pursuant to your lease, you'd like to add someone to the lease, and then the landlord can offer a new lease or not. And if the LL does offer it, it's up to you whether you want to accept it or not.
In general, things to include in a lease addendum might be: Your name. The rental property address. The tenant's name. Relevant policy/information (that complies with your state/municipal's rental laws) Consequences for breaking any contract agreements. Space to sign & date for landlord. Space to sign & date for tenant.
Basically anyone who is living there needs to be on the lease with your approval so that they are clearly held to the same responsibilities to you as any other tenant. You want as many people liable for your property as there are adults living there. Always.