This paper presents an experimental programme aimed at investigating the use of limestone filler additions to modify the mechanical properties of commercial premixed hydraulic lime based mortars. The influence of adding recycled limestone filler was evaluated from the mechanical point of view, i.e. by comparing the experimental compressive and flexural strengths of five different mortar mixes with variable filler contents. The comparison of results shows that, up to a certain amount, the addition of filler provides an improvement of the mortar’s mechanical properties. Beyond this optimum limit, strengths tend to decrease and eventually stabilize. Hence, mortars tested as part of this experimental campaign covered a range of strengths. Consequently, adding limestone filler can serve to adapt the applicability of hydraulic lime mortars to different practical applications, from the conservation of historical buildings to laboratory research on masonry.