The inclusion of construction inventories in Life Cycle Assessment (LCA) studies of wastewater treatment plants (WWTPs) involves the quantification of materials and energy required during the construction phase. Even though the most accurate option is to obtain such inventories from project drawings/budget, this is a tedious task and often a limited amount of information is available. An alternative is to estimate materials and energy from existing inventories. However, there are very few detailed inventories which can be used as a reference in these scaling processes. In addition, there is no knowledge in existing literature on which scaling approach is most suitable. Hence, in this study we provide detailed and comprehensive construction inventories from four small to medium conventional WWTPs. We also compare different manners of estimating construction inventories after scaling from Ecoinvent: i) extrapolation based on treatment flows; ii) extrapolation based on normalization of all materials masses to the volume of concrete. The results show that the potential environmental impacts estimated for construction were 95 ± 24 kg CO2eq/PE (climate change), 12 ± 3 mg CFC-11 eq/PE (ozone depletion), 20 ± 8 kg FE eq/PE (metal depletion), 26 ± 8 kg oil eq/PE (fossil depletion), and the process units dominating the impacts were secondary treatment and urbanization. Estimating inventories after scaling from Ecoinvent leads to large differences in masses of materials; between 2 and 30 times overestimation when scaling based on flows and between 1 and 2 times underestimation when scaling based on multiplication factors referred to concrete masses. Such differences in the estimates of masses translate to discrepancies in the potential environmental impacts ranging from 21% to 73% when using multiplication factors, and up to a factor of 26 for other scaling approaches. Finally, equations are provided to estimate material consumption for the construction of small-to-medium WWTPs as a function of plant capacity based on the four detailed and comprehensive inventories provided in this study.