The Lloyds building in London was constructed in the early 1980s based on a vi­sion­ary design by Richard Rogers and Partners. Its planning and construction transformed the ideas of megastructure and intelligent environment into built reality. Mock-ups (prototypic full-size models) played a crucial role in this, enabling Rogers team to test and refine the necessary novel constructions and procedures to minimize the risks of such an ­ambitious project. Lloyds 1:1 is the first case study of one of the most important building projects of the late modern era. It showcases pre­viously unpublished ma­ter­ial from the ­archives of Rogers Stirk Harbour+Partners, Josef Gartner and Arup to reconstruct the planning process and demonstrate the working methods. Ex­amples of how mock-ups were used as research or planning ­instruments highlight their ­relevance for further developing building culture.