New buildings along Fifth Avenue on the Upper East Side are hard to come by, but a rare development prospect may be in the works for a small site on a dormant stretch of the avenue in Carnegie Hill. Building permits filed yesterday detail the construction of a five-story residential addition atop an existing eight-story, 30-foot-wide townhouse at 1143 Fifth Avenue, between 95th and 96th Streets. The brick and limestone building designed by J.E.R. Carpenter, was erected in 1923, and up until recently, housed staffers of the French Embassy. Last June, the 16,000-square-foot, seven-unit property was unloaded by the French government for $36.4 million and picked up by a buyer listed as 1143 5th Ave LLC.