The paper presents the results of the study of the building techniques and construction of the baptistery of the church of San Lorenzo at Lomaso (Trento, Italy) dated between the end of the 12th and the beginning of the 13th century. Firstly, the general characteristics of the site and the monument are considered, describing the building techniques. The masonry is characterised by Romanesque opus quadratum in small squared blocks arranged in horizontal rows with bonding courses. Then the stone materials and the tools used by the artisans to work them are identified. Thirdly, a method of quantitative estimation is proposed with regard to the supply of the materials, their processing and the labour involved in the construction. In this way, the number of teams of artisans at work, their degree of specialisation and the construction time are hypothesised, based on previous studies and manuals from the 18th to the 20th century. The final objective is to hypothesise the phases of the entire building process.