Buildings are responsible for approximately half of the energy used in the UK and are therefore responsible for significant quantities of CO2 emissions. Thermal assessment methods enable the building designer to reduce the likely energy consumption of those buildings. Dynamic thermal simulation programs can be used for thermal assessment but are generally considered difficult and time consuming to use. The advantages of such methods are high levels of accuracy and the range of information that can be obtained. Simplified methods are intended to be quicker and easier to use than dynamic thermal simulation. However, they are often of low accuracy and limited in the number of building and operational parameters that can be altered. This thesis describes a simplified thermal assessment method that combines some of the flexibility and accuracy of dynamic thermal simulation, while being quick and easy to use. Steady state and transient heat flow algorithms are combined with a new calibration process that determines the effect on space heating energy use of the thermal mass of different types of construction element. All major building and occupancy parameters can be altered and the use of hourly calculations enables the user to explore the effect on thermal performance of various strategies such as the use of night shutters, or increased ventilation rates during the day. A discussion of the further development of the method focuses on results that suggest that the calibration of different constructions will not be an open ended process. There is a high correlation between the effective heat capacity and the actual heat capacity of the constructions tested to date, although further work will be required in this area. The effective heat capacity is currently determined as part of the calibration process. The accuracy of annual heating energy prediction is good and accuracy of overheating prediction reasonable.