This chapter is dedicated to the modernisation process of existing buildings aiming to achieve the nearly zero-energy standard. The process is described from the designer's perspective. Related issues, requirements, constraints, design options, and local determinants are analysed, and optimal architectural solutions for selected case studies are also presented. The analysis is based on the KodnZeb project, which included the modernisation of two existing buildings, located in Warsaw (the Faculty of Building Services, Hydro- and Environmental Engineering and Student Housing Muszelka), which differ in architectural features function, location, and needs. Thus, two diverse approaches to the modernisation process are examined. The chapter concludes with general guidelines and recommendations for similar architectural projects.