Urban vegetation, such as vegetated roofs, is highly important for recovering the ecological balance and for providing visually pleasant environments. Also, it has the potential to reduce urban noise levels. The acoustic surface impedance of vegetated roofs can be determined by an in-situ two-microphone technique. However, different initial estimations of the impedance model parameters can retrieve completely different sets of material properties, but the same impedance, which makes no sense physically. The work as presented in this paper investigates the multiple-geometry approach with respect to its uniqueness in the determination of the acoustic impedance model parameters of vegetated roofs. The uniqueness of the new proposed method is first validated for typical porous materials where impedance tube measurements and physical measurements provide reference results. Then, a new proposed measurement system was implemented on three urban vegetated roofs. The results show that the proposed measurement system succeeded in uniquely characterising the acoustic impedance model parameters of vegetated roofs.