**5. Conclusions**

This study presented a cost-benefit analysis to evaluate the socio-economic viability of GI, which was considered as a climate change adaptation option in the cities of Badalona and Barcelona. The GI planning of the two cities is significantly different: Barcelona proposed a widespread GI implementation plan while Badalona proposed a much lower degree of implementation. CBA is relevant for decision making of urban drainage planning and is useful for comparing different scenarios: in this case a business as usual (BAU) and the GI scenario were compared. Multiple benefits derived from GI implementation were considered and they were grouped into three different categories: benefits derived from flood damage reduction, from water quality improvements and from additional benefits. For each categories both direct and indirect tangible (that can be monetized) benefits were defined and quantified. The largest share of GI benefits in Barcelona was from reduced flood damages (56%), while in Badalona was from additional benefits like added value of properties and habitat provision (89%). The GI benefits derived from reduced sewage treatment costs; from reduced indirect damages to coastal economies; from air quality improvement and from reduction of the heat island effect and energy consumption resulted in the range of 0–1% playing an insignificant role in the socio-economic assessment. The calculated cumulative net present value (NPV) in Barcelona became positive after 10–11 years considering all benefits, whereas in Badalona was mostly negative. Overall, this study presented and quantified how different multiple benefits that can contribute to net present value as part of CBA. The details provided in this paper guarantee the replicability of the presented CBA to other case studies.

**Author Contributions:** Conceptualization, L.L. and M.G.; methodology, L.L., M.G., E.M.-G. and B.R.; software, L.L. and D.S.; validation, L.L. and D.S.; investigation, L.L. and M.G.; resources, M.M.; data curation, L.L., E.M.-G. and B.R.; writing—original draft preparation, L.L.; writing—review and editing, L.L., M.G., B.R. and E.M.-G.; visualization, L.L.; project administration, L.L., M.M. and B.R.; funding acquisition, M.M. and B.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the BINGO European H2020 project, Grant Agreement No.641739 and the RESCCUE European H2020 project, Grant Agreement No. 700174.

**Acknowledgments:** The authors thank AMB, BCASA and the Municipalities of Badalona and Barcelona for their collaboration.

**Conflicts of Interest:** The authors declare no conflict of interest.
