Variations in sound insulation are a problem for lightweight constructions, since it demands a high safety margin to the legal requirements on acoustical performance, if the variations are large. The building costs can be lowered if these variations can be characterised and identified. This paper describes an investigation of how the vibro-acoustical properties of nominally identical dwellings change during the construction phases. The objective is to find out whether acoustical deviations in the field can be traced to the earlier stages of construction. It also gives an indication of how the variations grow during the process. Throughout the investigation, all measurements were made on the same building elements. The building technique under study is a lightweight timber system consisting of industrially produced prefabricated volumes. Acceleration level measurements have been performed in the factory on building elements at different stages of completion; plates attached to beams, floor with gypsum board covering, the whole volume without floor parquet and the finished volume. An ISO tapping machine was used as excitation source and accelerometers were placed along the edges of the floors and across the surface. Field measurements were performed in the finished building. In addition to the analysis of acceleration level, airborne and impact sound insulation were measured in situ. Acoustical deviations were found for frequencies above 400 Hz, but these could not be traced back to the earlier construction stages.