The hope was that the Bullitt Center would spawn countless office buildings like it. That did not happen, in part because such projects are more complicated, with the permitting process alone often taking longer. The projects are also 5 percent to 25 percent more expensive to build, according to companies that have worked on them. (The buildings can be less expensive to operate, however, especially when they generate their own power and collect their own water.) But now that the technology for things like solar power has improved, and the incremental costs of going green have declined, a growing number of companies are tackling such projects, with some going “deep” on one or two aspects of a development.