The paper determines the effect of daily precipitation, occurring in the sewerage catchment area located in the Jastków Commune, on the volume of wastewater, flowing into the treatment plant in Snopków. Daily and annual volumes of incidental water, caused by precipitation, flowing into the selected sewerage system were determined, and then the annual costs incurred for their treatment were calculated. The sewerage system selected for the study is located in the eastern part of Poland, in the Lublin Voivodeship, within the Jastków Commune. The design average daily inflow of wastewater to the classical treatment plant with activated sludge is 1200 m3·d-1. To the sewage network made of PVC with a length of 67.22 km domestic wastewater from 1119 single-family buildings from 3350 residents are discharged. The study was carried out in 2019–2022, and the daily precipitation totals and daily sewage inflows to the treatment plant were analyzed. The results of the calculations were related to the humidity classification of the year (dry, average and wet) under temperate climate conditions. The study showed that during precipitation events, the share of incidental water in the average daily volume of wastewater flowing into the treatment plant ranged from 3 to 26%. Annually, 7,400 to 10,325 m3 of rainwater flowed into the studied treatment plant, which accounted for 5.0 to 6.3% of the annual volume of wastewater. The incidental waters flowing into the sewage system resulted in the need to spend additional sums – from PLN 47,814 (€10,897) to PLN 71,232 (€16,234) per year. Thus, the annual cost of wastewater treatment in the studied system increased by about 5–6%. The results of the study are important and valuable information for the operators of the analyzed sewerage network and the wastewater treatment plant, as well as for other units that use similar systems. They will allow to initiate actions to detect and eliminate illegal connections of roof gutters and/or yard drains to sanitary sewers in the analyzed area, which would then allow to reduce the cost of wastewater treatment in the studied case.