The best time to build is when the church reaches 85% of its worship capacity for the first time. While the “85% capacity” rule-of-thumb is sound and proven, the more important thing is upon reaching it for the first time.
Growth patterns, whether positive or negative, tend to feed on themselves. If you enlarge your facilities as you approach saturation, you permit growth to continue. If you don’t permit enlargement, you impose an upper limit on that growth and upon reaching your “facility saturation point”, the growth rate slows and eventually stops. Frequently, decline sets in, and the church begins a “roller coaster” growth curve.