After the merger of Northrup, Inc. into ARCO, Northrup became engaged in real estate development, including the assembly of one of the largest tracts of land in downtown Dallas, where he entered into a joint venture with James Rouse’s Enterprise Development Company to build a festival marketplace.
 This project was sold to a Belgian investment group who failed to pursue the project with Rouse.
Northrup helped fund the establishment of Rouse's Enterprise Foundation affordable housing projects in Dallas.
He also assembled tracts on 1½ miles of shoreline on Lewisville Lake, a part of which later became the city of The Colony.
 Northrup acquired 5,000 acres outside San Antonio, Texas, which was put into a trust for his family, and assembled land now occupied by the regional Rolex headquarters, among others.

Northrup subsequently started American Limestone, an innovator in using surface quarried Texas limestone in building facades, using a patented method that employs thin panels of limestone as a veneer, attached to a metal grid without mortar.
This popularized the use of rough cut limestone as a thin veneer in residential and commercial applications, including store fronts, such as Brooks Brothers, churches, bank buildings and municipal buildings.
 At the other extreme in size, Northrup has utilized massive blocks of limestone for their passive heating / cooling characteristics, and free-standing structural qualities, most notably donating the stone and devising the construction technique for the Cistercian Chapel in Irving, Texas, which reflects both a classic design aesthetic and construction technique of mortar-less masonry  - a thoroughly modern building built in the rational style of Cistercian architecture.
The architect, Gary Cunningham, said "We wanted to build a church that would literally last for the next 900 years".
After the merger of Northrup, Inc. into ARCO, Northrup became engaged in real estate development, including the assembly of one of the largest tracts of land in downtown Dallas, where he entered into a joint venture with James Rouse’s Enterprise Development Company to build a festival marketplace.  This project was sold to a Belgian investment group who failed to pursue the project with Rouse. Northrup helped fund the establishment of Rouse's Enterprise Foundation affordable housing projects in Dallas. He also assembled tracts on 1½ miles of shoreline on Lewisville Lake, a part of which later became the city of The Colony.  Northrup acquired 5,000 acres outside San Antonio, Texas, which was put into a trust for his family, and assembled land now occupied by the regional Rolex headquarters, among others.
Northrup subsequently started American Limestone, an innovator in using surface quarried Texas limestone in building facades, using a patented method that employs thin panels of limestone as a veneer, attached to a metal grid without mortar. This popularized the use of rough cut limestone as a thin veneer in residential and commercial applications, including store fronts, such as Brooks Brothers, churches, bank buildings and municipal buildings.  At the other extreme in size, Northrup has utilized massive blocks of limestone for their passive heating / cooling characteristics, and free-standing structural qualities, most notably donating the stone and devising the construction technique for the Cistercian Chapel in Irving, Texas, which reflects both a classic design aesthetic and construction technique of mortar-less masonry  - a thoroughly modern building built in the rational style of Cistercian architecture. The architect, Gary Cunningham, said "We wanted to build a church that would literally last for the next 900 years".
Leonard L. Northrup Jr.