Tenants of the mammoth East River-facing American Copper Building apartment complex filed a class action lawsuit on Tuesday, alleging that its owner systematically overcharged renters in rent-stabilized units. Tenants allege that this rent concession system violates the types of increases that are allowed under the state rent stabilization laws. But as part of the lease, Cohen received a four-month rent concession, making his “net effective rent” $3,737.69. The lawsuit argues that the landlord should have used that lower, effective rent figure as the rent-stabilized amount it registered for Cohen’s unit. On his next lease renewal, the landlord completely withdrew the rent concessions, increasing Cohen’s rent by 12.4%.