Low impact development practices (LID), or what are known as sustainable urban drainage systems (SuDS) in the U.K., are expected to have a synergetic effect with the existing infrastructure to mitigate urban storm-water flooding. Designing and implementing effective LID practices require reliable full-scale data about their performance in different applications over an extended period of time; however, relatively few examples of LID practices have been monitored to provide such information. This study developed an innovative micro-monitoring system to assess the performance of porous pavement and rain gardens as retrofitting technologies. The system includes the installation of a V-notch weir at the outlet of each LID device to provide an accurate and reliable quantification of a wide range of discharges. In addition, a low flow sensor is installed downstream to check the readings. Extensive laboratory calibrations under different flows depicting the exact site conditions were performed. The micro-monitoring system is well suited for high-resolution temporal monitoring and enables accurate long-term evaluation of LID components’ performance.