Building integrated solar thermal systems are considered an attractive alternative to traditional heating systems for water heating applications. They yield a high potential to efficiently and cost effectively meet a large portion of hot water demand in different building types. However, their performance to great extent depends on the climatic conditions, building water heating requirements and the optimal design and sizing of collectors. The present research attempts to identify a unified parameter for solar thermal sizing for various building types based on their hot water demand under a hot climate. The building types include residential, hospitals, schools, offices and industry. The analysis is conducted numerically through a conjugate heat transfer model executed through TRNSYS platform. The parameters investigated include the collector tilt angle (0o ≤ Φ ≤ 90o), azimuth angle (-90o ≤ β ≤ 90o), collector inlet temperature and collector inlet water flow rate. Eventually a unified design parameter of collector aperture area required per unit footprint area for each building type is determined. The unique parameter will help architects in optimal sizing of the collector for specific building types under a hot climate, taken as the climate of Abu Dhabi, United Arab Emirates.