From the date of confirmation of the plan of said park or parkway, in an entirely built-up section of said city, until the expiration of said period of five years, or until the passage of an ordinance to appropriate to public use the land within the lines of such park or parkway, the owners of said land may alter their buildings thereon, in accordance with plans approved by the Bureau of Building Inspection and other municipal authorities having jurisdiction thereof; and, upon the assessment of damages for the appropriation of their property to public use, they shall be entitled to compensation therefor, including the buildings so altered, computed as of the date of said appropriation.
53 P.S. § 14135