If instead, the deed describes the property by using metes and bounds (vectors described as degrees from north and south to east and west and distances typically measured in feet), then you may not be in a plan. It is also possible in the City of Pittsburgh that you are in a plan but your particular parcel is not exactly according to the lots as drawn on the plan (part of a single lot or perhaps a whole lot and a part of a second lot).
Unfortunately, in either case, the plan or the metes and bounds description are typically only going to show the boundaries of the land and won't show where the house or any other structures on the property are located relative to the boundaries. You may have to contact a surveyor for that information. Before you do, however, I'd suggest going back to the real estate agent that helped you purchase the property and see if there was a survey from the previous owner that might be available.
Also, don't waste your time with the assessment office. They only have tax maps which are not suited for your purpose.