If an option to renew or extend the term is a valid provision of your lease, then the mere fact that the building has new ownership will neither void your option nor prevent you from exercising it.  Of course, when dealing with a commercial lease it is important to carefully review the entire document plus any additional agreements such as the Estoppel Certificate that you mention was signed in connection with the sale of the building.  In this situation, you or your lawyer should respond to the landlord's demand that you vacate the premises with a letter stating that under the terms of the lease you have an option to renew, which you intend to exercise at the appropriate time.  It may be advisable to actually give the required notice to exercise the option right away.  That may help to counter a possible claim by the landlord that you failed to exercise the option in a timely manner.  Since commercial leases and the surrounding circumstances raise many legal issues, and because a suit for eviction could put your business at serious risk, you should consult a lawyer who has expertise in commercial leases and related issues.