Without the proper consent, a landlord is likely to consider an assignee (i.e. the new tenant) to be an unauthorized occupant of the property. In this case, landlords often have the option to evict or remove the unauthorized occupant, terminate the original lease agreement, and hold the original tenant liable for any breached lease terms. However, keep in mind that in this situation landlord rights may vary by state.
Generally, landlords can only refuse consent under reasonable circumstances—a subjective term that courts may define based on the facts of a particular situation. In some states, a tenant may have the option to terminate their lease early if their landlord unreasonably withholds consent to a lease assignment.
If a tenant believes their landlord unreasonably withheld consent, they may be able to seek legal remedies such as filing a claim for damages or getting a court order for consent to be given.