In your renovation considerations, the following approach is standard for a realistic and reliable renovation planning by an independent planning office: building condition analysis (assessment of the building fabric, identification of building damages, mostly moisture damage), building survey including the creation of current floor plans, determination of the renovation and repair effort with cost estimation, possibly a preliminary design for conversion, extension, and repurposing projects by the client, clarification of the building and planning law situation for these projects, planning and realization of the construction project based on the [HOAI (Honorarordnung für Architekten und Ingenieure)] including tendering and construction management. The renovation measures should be planned and carried out in the context of the entire building (building damage remediation, energy-efficient renovation, modernization, renewal of building services, etc.)
In your case, a detailed building survey is a basic prerequisite for as realistic a cost estimation and financing plan as possible. If no development plan exists, as in your case, §34 of the Building Code applies: a building inquiry must be made. The planned construction measure must be adapted to the type and scope of development customary for the surroundings. For the renovation and construction measures, you need an independent planner/architect. In any case, it is advisable to avoid a renovation as an uncoordinated collection of individual craftsmen’s measures conceived by the executing companies themselves.