Lancaster County-based engineering firm Rettew Associates Inc. is expanding into the West and will open its first office there this fall.

The company signed a lease for offices in Lakewood, Colo., part of the Denver metropolitan area, according to a news release. The office should be fully operational by mid-September.

The office will serve clients in the oil- and gas-extraction industries in Colorado, Montana, New Mexico, North Dakota, Utah and Wyoming, Rettew said.

Rettew entered the Marcellus Shale engineering services market early and has succeeded in expanding rapidly as a result. In April, it jumped 249 places on the Engineering News-Record’s annual Top 500 ranking of design firms, making it the fastest-growing firm on the list.

The company has 10 offices in Pennsylvania, New York and Ohio.