Large areas along Java’s north coast are threatened by subsidence and abrasion. Subsidence is even faster than sea level rise. Although forbidden by law, farmers have cleared the mangrove forest almost up to the coastline, thus reducing sedimentation and increasing exposure to coastal abrasion. Four interventions are needed: reduce groundwater' abstraction, protect the residual coastal mangrove, give up ponds along sea and rivers for mangrove recovery, and improve aquaculture (using a field school approach). The Building with Nature project, funded by the Indonesian and Dutch governments, and partners of the Ecoshape consortium, is currently implementing the latter three interventions in Demak district together with Indonesian institutions and villages. For Tambakbulusan, a village in Demak regency, north-east of Semarang, covering about 750 ha, we estimated the benefits of these actions with a Cost-Benefit-Analysis. We accounted, next to investments and profits including those for fisheries, the cost of destroyed houses and ponds, and of forgone benefits due to new mangrove forest and loss of land. Our baseline scenario assumes subsidence and abrasion similar as villages closer to Semarang, where most lands were gradually engulfed the last 25 years. For such a period, a no-intervention scenario would cost close to 3 million USD due to loss of land, infrastructure and livelihoods. Investing 88.000 USD on recovering mangrove-only or on improving aquaculture-only, would generate benefits of 7.8 and 1.0 million USD, resp.. Simultaneously investing on both mangrove recovery (climate change mitigation) and aquaculture improvement (adaptation) would yield almost double: 15 million USD.