Large DNA constructs (>10 kb) are invaluable tools for genetic engineering and the development of therapeutics. However, the manufacture of these constructs is laborious, often involving multiple hierarchical rounds of preparation. To address this problem, we sought to test whether Golden Gate assembly (GGA), an DNA assembly methodology, can be utilized to construct a large DNA target from many tractable pieces in a single reaction. While GGA is routinely used to generate constructs from 5 to 10 DNA parts in one step, we found that optimization permitted the assembly of >50 DNA fragments in a single round. We applied these insights to genome construction, successfully assembling the 40 kb T7 bacteriophage genome from up to 52 parts and recovering infectious phage particles after cellular transformation. The assembly protocols and design principles described here can be applied to rapidly engineer a wide variety of large and complex assembly targets.