There is no single definition for what constitutes an Altbau in Germany. The term is generally used to refer to residential buildings that were built before the Second World War. They generally have high ceilings, wood flooring, often a small pantry or maid’s room, stucco ceilings, and single or double casement windows. The Altbau period ends around 1949, when construction styles shifted to double glazing and concrete walls and ceilings. Unlike the term Altbau, however, we have a very precise definition for what constitutes a Neubau: buildings that have recently been completed and are ready for their first occupancy.
Saskia Blank of the Ideas 2020 team answered this question.