DENVER—The Mile High City’s skyline is getting a glassy new addition, the latest sign that the U.S. office sector is strengthening beyond just large coastal markets.

Hines, a Houston-based developer, on Tuesday broke ground on 1144 Fifteenth, a planned 40-story office tower one block southeast of Denver’s historic Larimer Square. The 640,000-square-foot structure will feature 10-foot ceilings and glass walls, open floor layouts, several ledge terraces, a 5,000-square-foot fitness center, and a mezzanine level with a fireplace and seating for informal gatherings.

The addition comes as Denver’s vacancy rate was 12.9% in the first quarter, down 0.56 percentage point from its year-ago level and near its lowest point since 2008, according to CBRE, a commercial real-estate company.

Hines is so confident of the project’s success that it is building on a speculative basis, meaning it has yet to sign a single tenant to the property it will spend more than $300 million to build.

“We’ve already had inquiries from finance, energy, technology and professional firms,” said Barry Dorfman, president for the Rocky Mountain region at JLL, the real-estate firm marketing 1144 Fifteenth for lease. “I’m confident there’s a very strong market for this property, especially with the quality of the location and the amenities that Hines is putting into it.”