"I made this decision because I felt it was time to return to the private sector," she said in a statement. "I am proud of the groundbreaking work the department has done during my tenure to root out corruption, increase transparency, overhaul the building code and increase safety for workers and the public alike."
On Monday, Mayor Michael Bloomberg had taken the unusual step of singling out a city agency, saying, "I don't think anybody should be fully satisfied with the Department of Buildings."
Thirteen people have died in construction accidents this year, more than the total for 2007.
Seven died in a crane collapse last month that leveled a townhouse and damaged several other buildings. Last week, Lancaster acknowledged that the department improperly approved a construction permit for the 46-story condominium that was being built there.
The department has been criticized before for ineffective inspections, including at a skyscraper near ground zero where two firefighters were killed in a fire last year. Inspectors had not noticed a months-old problem with the former Deutsche Bank tower's standpipe system that hampered firefighting efforts.
In March, Lancaster acknowledged that department officials failed to process paperwork that had declared a vacant East Harlem apartment building to be unsafe more than a month before it collapsed near a Metro-North train station.
Critics said the department has been a mess since the 1990s, when it created a "self-certification" system to streamline the permit process and drastically reduced its inspections staff. Lancaster has been credited for raising the number of inspectors from less than 300 to more than 400 in recent years.
Lancaster, the first woman to head the buildings department, served two years as deputy commissioner for design and construction in the Department of General Services for Bloomberg's predecessor, Rudy Giuliani.
