DES MOINES — It took Wellmark Blue Cross and Blue Shield only about a year in 2007 to locate, assemble and buy a 6.6-acre parcel of land from nine owners for its new headquarters in this city’s downtown.

Next December, after 30 months of construction at a cost of $250 million, Wellmark is scheduled to complete the 600,000-square-foot five-story building, which will occupy three blocks of Grand Avenue and anchor a growing downtown office and entertainment district known as the Western Gateway. If that schedule is met, the project will have taken roughly four years from start to finish.

By the standards of most downtown construction — where land assembly alone can span half a decade or more — that is unusually fast, developers say. But Wellmark’s speedy pace is typical of recent commercial, residential and retail development in Des Moines, where public and private cooperation has produced a robust urban landscape matched by few cities its size.