In studies of Renaissance architecture, little attention is normally given to the organization of building sites and to the way in which the practicalities of mechanics and material supplies influenced the finished works. This article describes Inigo Jones's site organization at St. Paul's Cathedral where, during the 1630s, he undertook the large and complex task of refacing the existing Gothic structure. Here practical site constraints influenced Jones's design at least as much as theoretical objectives. This study throws light on Jones's relationship with his assistant, John Webb, who oversaw much of the day-to-day work, and illustrates such details as Jones's use of craftsmen to make and amend models in perfecting his designs on-site. In this article the work's accounts are studied in order to reinterpret the few known surviving working drawings and contemporary views that record one of Jones's most important achievements.