PHILADELPHIA — At 40th and Chestnut Streets on the west side of this city, a block of six red brick Victorian rowhouses is interrupted by an incongruous stretch of new student housing with gray walls and beige metal window frames.

The units were built over the last two years on land that was made available by the demolition of two historic houses, among the latest casualties of Philadelphia’s boom in commercial real estate.

As demand for commercial property surges with a strengthening downtown economy, preservationists and the city authorities are searching for ways to defend the fabric of this historic city from the wrecker’s ball.

The city is adding staff to its Historical Commission, which designates buildings as historic and must approve any demolition permits for those buildings. And Mayor Jim Kenney has set up a task force on historic preservation to recommend ways to foster economic growth without sweeping away the past, aided by the National Trust for Historic Preservation, which this year listed Philadelphia’s historic neighborhoods as among its “national treasures.”