Cob is an earthen building material made by soil, fibres and water used for millennia. However, cob construction disappeared out during the nineteenth century. These last years, it is experiencing a renaissance in Northwestern France and Southern England. Due to a limited technical knowledge, the investigation of engineering properties is important for modern design practice and code requirements. Moreover, to ensure building properties, it is necessary to have same quality mix along the building phases. The aim of this study is to determine material variation during the monitoring of a cob prototype building in Normandy (France). This study investigated structural cob mix composition, water content, density, mechanical properties and thermal conductivity. Specimens shape used were cylindrical 110 x H220 mm and prismatic 300 x 300 x 70 mm. Results indicated a variation in cob mix (water content, materials proportions) between three different lifts. These variations lead to different densities and, consequently, to variables compressive strengths: 0.99 to 1.38 MPa and thermal conductivities from 0.610 - 0.816 W.m-1∙K-1.