Municipal Services Building at 15 & JFK is the place to go for this type of record. The Zoning Board of Review seems to go back further chronologically and is worth checking, but that may not have complete plans. ZBR can be accessed online via City of Phila web site.
Before doing any of the above, contact your condo association and contact the original builder/developer.
Hannah Angert, Realtor & Conveyancer
215. 271. 3000 office
215. 869. 9571 cell
William Festa Realty
3001 S. Sydenham St.
Philadelphia, PA 19145