A development firm involving Pittsburgh-based engineer Louis P. Astorino is planning a nine-story condo building on the 2100 block of Walnut Street near Rittenhouse Square that incorporates a historic 1869 rowhouse into its design.

The plan by Astoban Investments LLC of Philadelphia preserves the four-story Frank Furness-designed brownstone at 2108 Walnut St., adding a five-story overbuild to the structure and incorporating a new nine-story addition to be constructed on an adjacent empty lot at 2110 Walnut St.

The 2108-2110 Walnut St. complex will encompass nine dwellings over 46,000 square feet, with 10 parking spots and commercial space on its first floor, according to a release from Cecil Baker & Partners, the project's architect.

Astoban Investments counts as a partner Astorino, whose family's architecture firm designed Children's Hospital of Pittsburgh before being sold to CannonDesign of Niagara Falls, N.Y.