The coexistence of a mixed use, in a significant part of the buildings, with a part being used for housing purposes and another part for economic activities (commercial, services, etc.) creates situations that are frequently a reason for complaints due to acoustic annoyance. In Portugal, the assessment of this type of discomfort is carried out through sound measurements, comparing the value of the difference between the total sound and the residual sound with the legal limits set in national legislation. This comparison is made without considering the uncertainty value of that difference. If the value of uncertainty is not depicted when declaring compliance in test reports, the level of risk associated with the decision rule (such as false accept and statistical assumptions) can take values between 2.5% and 50%. This risk assumes the highest value when the obtained difference is equal to the established limit value. This paper presents a formulation that allows obtaining the value of the uncertainty for that difference, based on the Monte Carlo simulation method. The estimation of the uncertainty value led to physically more plausible values, when compared to those normally obtained with the usual formulation based on the propagation of uncertainty law.