In addition to the applicable underlying street tree planting requirements, all changes of use within the same or to other Use Groups of at least 50 percent of the floor area of an existing building, or alterations above 30 percent of the building value of an existing building, pursuant to the applicable articles of the Building Code of the City of New York, within Area B, shall provide street trees as set forth in Section 26-41 (Street Tree Planting), except that for a zoning lot frontage on Houston Street such mandatory trees may alternatively be located on the median traffic island of Houston Street.