Textile Reinforced Mortar and Fiber Reinforced Cementitious Matrix composites (FRCM) are nowadays considered as a viable solution to enhance in-plane strength of masonry walls where they are bonded to. Such composites are usually applied to the entire surface of the wall, bonded to one side of the wall or to both sides. The past experimental activity demonstrated that there is a significant improvement of the in-plane strength even applying a single layer of material to the sides of the walls, but not wide consensus has been found on the modelling of these benefits for code and design guidelines purpose. Great effort has been put in defining simple models, practitioner oriented, and the present paper aims at comparing the ACI549 and novel RILEM proposal. The main crucial issue is the evaluation of an effective contribution of the strengthening solution to the as built in-plane capacity of masonry walls. It is in fact expected that different substrates (both in terms of materials as in terms of thickness and basic properties of the wall) has an impact on the efficiency of a strengthening system (characterized by another variability of parameters like as fiber type, grid and mortar matrix geometrical and mechanical properties). In this effort, main parameters governing the in-plane response are first recognized; hence a simple mechanical model is proposed to fit the typical average experimental behaviour of TRM/FRCM strengthened masonry walls in-plane loaded. Finally the impact of uncertainty and variability of results is analysed to propose appropriate coefficients for design uses.