The design of controlled low-strength materials (CLSM) is generally based on experimental approaches without considering an efficient use of the component materials. The present study proposes a general methodology for the design of optimised CLSM that includes the definition of the mechanical requirements through numerical simulations with FEM and an experimental procedure to define the mix by optimising the aggregate skeleton, the content of cement and the use of admixtures and additions. Moreover, the methodology is applied to the backfill of narrow trenches.