There has been a history of low-energy design failing to translate into low measured energy consumption in domestic buildings. In part this failure can be attributed to occupant behaviour and household variation. It is therefore important to provide a method whereby such variation can be accounted for so that deviations from design values can be identified as natural variation rather than design failure. This paper addresses the likely range of occupant behaviour and the resultant impact on heating energy consumption for domestic Passivhaus buildings. Realistic, quasi-empirical, profiles for different occupancies, lighting, and appliance-use were applied to a set of 100 terraced Passivhaus units, and modelled in a dynamic building simulation programme. Strong correlations between the results and measured data from a large set of similar properties are shown. Multiple regression techniques were used to identify the relationship between space heating load and behavioural variables. This led to the development of a regression equation which can be used to estimate the likely space-heating requirements of a household given particular behavioural variables, and to the test the impact of certain behaviours on annual heating energy demand. It is found that in general passive houses are less sensitive to behaviour than anticipated.