How to correctly write a lease for a call center?

[Irina Gav]

We got a building for our own business but have some room to lease. Call center would like to move in and they are just starting out so only expect 4 employees to start with. We have a rent set for that. But as they start growing the expenses will go up and we need to make sure we are not paying them to stay in. How would we structure the lease agreement if they have 15-20 employees?