No tax. The Indian and Mongolian missions are not subject to taxation, pursuant to a Notice issued by the State Department, pursuant to its authority under the Foreign Missions Act. The Notice establishes an exemption from real property taxes on property owned by foreign governments and used to house the staff of permanent missions to the United Nations or the Organization of American States or of consular posts. The Second Circuit held that the State Department had acted within its power in issuing the Notice.
The decision in City of New York v. Permanent Mission of India to the United Nations can be found here.