As a result of different works on the buildings wall (insulation, installing billboards or mailboxes, etc.) the access to the existing benchmarks may be hindered and as a result it may be difficult to perform geometric levelling using classical measuring equipment. For this reason, the authors have developed and created a prototype of a mechanical benchmark snap-in. Its construction and principle of operation are presented in this paper. As part of the research and experimental works carried out under the laboratory conditions the instrument was calibrated and its functionality and accuracy were designated. On the basis of height differences between benchmarks which were determined using the benchmark snap-in, it was found that the average accuracy of the height difference designation under the laboratory conditions, where the length of the line of sight is 4 m, does not exceed ± 0.01 mm. Keywords: Geometric levelling Engineering measurements of elevation Benchmark snap-in