Indoor layout reconstruction has broad applications in various scenarios, such as smart homes and virtual reality. However, optical sensors like cameras and LiDAR can raise privacy concerns and often struggle in challenging conditions, such as darkness or smoke. This raises the question: can we achieve accurate indoor layout reconstruction using a single, static millimeter-Wave (mmWave) wireless device? mmDiff addresses this challenge by proposing an efficient method that leverages a single static mmWave sensor—the same type of signal used in 5G and next-generation WiFi—for accurate and privacy-preserving indoor layout reconstruction.