The design of heating, ventilation and air conditioning systems (HVAC) is critical for buildings. The capacity of the main generation sources, such as boilers, chillers and ventilation plants, plays a vital role in the building’s performance and energy consumption. Many installed existing HVAC systems are oversized. Different approaches to sizing exist, either starting from a peak demand load or catering for already installed equipment. To understand the sizing requirements and to outline the design process, a detailed literature review was conducted to investigate the current practices and assumptions in the sizing of HVAC systems. In the early stages of design, various assumptions are commonly made regarding building fabric details, temperature set points, equipment use, occupancy, operation schedules and internal gains are typically made in practice, irrespective of the design methods. This paper explores the impact of how these design assumptions influence the sizing calculation. A healthcare building case study is used to compare demand, installed capacity, and the results of different approaches to sizing to illustrate the resulting overdesign.