Tenant Estoppels for Apartment Complex

[Wes Short]

Hi all, have a buyer client wanting to get estoppels filled out from all the current tenants of the property they are purchasing, in the due diligence phase right now. Wanted to ask if getting the estoppel signed by each tenant during this phase is common?

Also, wouldn't most leases signed by a tenant transfer over to the new owner? Know this depends on wordage in the lease and will check the leases in place but wanted to see if that is common practice. 

Thanks for any help!