The collapse happened around 11:00 p.m. Tuesday in the 700 block of South 15th Street.
Falling debris even crushed a car parked on the 1400 block of Clymer.
Two people were inside the building at the time. One was fast asleep but both got out safely and without injury.
"I heard this very clear sound of something falling apart, a huge crumbling noise, said neighbor Hannah Vanderlan.
One owner and two occupants who rent rooms live here and this morning they were able to get back inside for suitcases.
The building owner told Action News he believes water from the recent rains seeped behind the bricks and caused the collapse.
Authorities earlier said the entire building will have to be demolished but, later in the morning, said further inspection revealed only part of the building will have to come down.
There was a third apartment in the building but it was unoccupied.
This was the second building collapse in Philadelphia on Tuesday. Earlier in the day, the wall of the old Central United Methodist Church came crashing down in the Frankford section.