WFMZ-TV reports the blaze broke out at 3:45 a.m. Tuesday at Legacy at the Papermill, an under-construction senior housing complex at Paper Mill Road and Spring Ridge Drive in Lower Heidelberg Township.
Witnesses said huge flames and smoke from the inferno could be seen for miles around.
It took crews several hours to get the upper hand. The blaze was finally reported under control by 6:00 a.m. There were no immediate reports of injuries.
Fire officials said there was extensive damage at the construction site.
The cause is under investigation.