This record contains a methodological concept note describing the European Garden Biodiversity Observatory as a privacy-conscious framework for citizen science in private gardens, school grounds, courtyards, balconies and urban green spaces. The document builds on Naturkompass as a multilingual biodiversity knowledge, education and participation platform. It defines the problem space, conceptual premise, governance model, potential research and evaluation questions, European scaling logic, implementation status and phased roadmap of the Observatory concept. The document is not a peer-reviewed journal article and does not present final empirical results. It should be understood as a citable concept note and technical report documenting the strategic and methodological framework of the Naturkompass approach.