The four-story building had been a headache - and a biohazard bonanza, with mold, fungi, vermin, asbestos and other problems - for neighbors, including the Freire Charter School.
The high school shared a wall with the annex, and school administrators had pleaded for years with the city - the Redevelopment Authority had owned it since 1993 - to remediate or raze the hazardous hellhole.
At last week's groundbreaking, Freire's student-body president, Tyrone Williams, thanked the developers and politicians gathered, including Mayor Nutter, City Council President Darrell Clarke and Councilwoman Jannie Blackwell.
"In a world where money and profits seem to be the focus, it is refreshing when you see the focus being placed on bettering the community," Williams said.
Aquinas Realty Partners bought the site from the city for $800,000 and named the project AQ Rittenhouse.
The 12-story tower will include space for Freire, a bicycle-repair shop and storage, rooftop grills and fire pits, a center courtyard, a 24-hour doorman and concierge, a Wi-Fi cafe, a restaurant and other ground-level retail.
It's expected to be completed this fall.
Center City is Philadelphia's fastest-growing residential neighborhood, according to the Center City District. Residential population downtown grew by more than 10 percent from 2001 to 2011, the district found in a 2011 report.
On Twitter: @DanaDiFilippo