For a service provider, stochastic demand growth along with expansion lead times and economies of scale may encourage delaying the start of expansion until after some shortages have been accumulated. Assuming demand follows a geometric Brownian motion, we define the service level in terms of the proportion of demand satisfied, which is then analytically evaluated using financial option pricing theory. Under a stationary expansion policy, an infinite time horizon discounted expansion cost is minimized under the service level constraint, where the expansion timing and size parameters are the decision variables. With the current formulation, the problem seems to be unbounded.