In Subarea B4, as shown on Map 1 (Special Inwood District – Subdistricts and Subareas) in the Appendix to this Chapter, the height of a building within 10 feet of a street line shall not exceed 125 feet. Portions of buildings located beyond 10 feet of a street line may rise to a maximum height of 210 feet. Any development or enlargement with frontage on West 218th Street must provide a sidewalk widening with a minimum depth of five feet along such frontage. Any development or enlargement with frontage on Ninth Avenue must provide a sidewalk widening with a minimum depth of five feet along such frontage.