Animal production inherently involves odour emissions, but due to the nature of the source of these emissions, end-of- pipe methods are usually not applicable in this area. In order to reduce odour nuisance, it is crucial to select the right location for farms, taking into account the results of the odour impact forecast. An important element of such forecasts is the way in which the operation of the ventilation system in livestock buildings is represented. The literature lacks guidelines on the acceptable degree of simplifying assumptions regarding the operation of the ventilation system for which the forecast results are still reliable. The article assesses the impact of various assumptions regarding ventilation operation on the determination of odour nuisance areas, analysing the case of a planned pig farm in Poland. Thirty- three modelling variants were compared, differing in the way the fan operating modes were determined. The results indicate that the total ventilation efficiency is of key importance, while the details of the fan configuration are less important. It was also found that it is acceptable to use the maximum simplification of the description of the ventilation system of livestock buildings, based on the continuous operation of all fans at a constant capacity, while, in order to maintain a safety margin, it was recommended to assume a capacity lower than the nominal value.