It is Thursday night and the auditorium is packed. This midweek believers-oriented service—one of two held weekly—represents all that is going right these days for Willow Creek Community Church, the megachurch in South Barrington, Illinois, that is preparing for megagrowth.

As the prelude's last guitar riff fades, Bill Hybels steps on stage and the crowd applauds. The senior and founding pastor tells the crowd that the previous weekend's attendance topped 20,000, up from the usual 17,000 who pack six seeker-oriented worship services. The church has seen a 20 percent increase in attendance in the past five years.

These are exciting days, Hybels says. That's why Willow Creek needs to build.

In his annual vision-casting sermon in January, Hybels, 47, announced a new building project on the church's 155-acre suburban Chicago campus costing as much as $70 million.

Groundbreaking for a 49,000-square-foot office building is set for June. In late summer, plans will be unveiled for a classroom building for workshops and a new auditorium with approximately 7,000 seats, a 50 percent increase over seating in the 20-year-old worship center.

The project, coinciding with the church's twenty-fifth anniversary, is more than a building campaign, according to David Staal, director of communications. "We're calling it Chapter Two. The first chapter was incredible. This is our vision for the next 25 [years]—how we will reach people locally, regionally, and around the world."

Hybels told the church that the greatest limitation to its ministry is physical capacity. "This is our opportunity to maximize the use of our campus," Staal says.

"There are 750,000 unchurched people who live within a 30-to-60 minute drive of our campus. The drive ...