The main objective in TIMBRE’s Work Package 6 (WP6) is to develop a customisable open-source web based site assessment and re-use planning tool (SAT), which enables end-users to develop and evaluate their own ideas of the future uses of brownfield sites in terms of a decision support system (DSS). The objective of this tool is to provide better support to projects in the screening stage, where possibilities with regard to future use of the sites need to be explored, holistically evaluated and communicated. In order to provide European-wide use and benefit of the tool, the goal was to provide a user friendly web based graphical user interface (GUI). The interface comprises diverse models and methods (hereafter also denoted as ‘modules’) and provides the necessary geographic information system (GIS) functionalities, e.g. for creation of land use allocation maps, definition of use-oriented and risk-based remediation targets, analyses of conflicts between environmental quality targets/thresholds, and existing contamination level, remediation cost and market value estimation, sustainability assessment, etc. This report is one part of deliverable D6.1 and serves as accompanying document to the web based tool itself, which forms the other part of deliverable D6.1. The report explains the conceptual background of the web based tool and gives an overview on capabilities, limitations and data requirements of the tool. Second, the report describes development stages of the tool and its modules, including (i) the general design, which partly builds upon and modifies existing assessment modules, (ii) the provision of access via internet, (iii) the management of uploaded project data, and (iv) the respective user management. Two further deliverables will complement the documentation on the web based site assessment and re-use planning tool (SAT): • D6.2 provides a guideline for the development of add-ons to the web based tool, including the detailed description of one exemplary add-on. • D6.3 describes the application of the web based tool at the TIMBRE model sites in Romania, Poland and Germany. While the web based site assessment and re-use planning tool (SAT) is still being further developed, it is available directly at http://89.38.157.5:8089/timbre/. Upon finishing, the public version of the tool will be moved to a TIMBRE server and made publically available (e.g., http://www.timbre-project.eu/SAT.html).