As part of the UKAEA Spherical Tokamak for Energy Production (STEP) fusion power station programme, a novel breeding blanket design was assessed. A conceptual design of STEP, which will be an innovative plan for a commercially-viable fusion power station, will be completed by 2024. The final aim of the current assessment is to find a conceptual design that can be manufactured with existing techniques without compromising the fuel self-suciency, the heat removal and the shielding functions. The proposed design is based on encapsulating the breeding material into pebbles of a gas cooled packed bed. The generated tritium in the breeding material, as well as Helium, will be extracted from the pebbles and directly removed from the blanket by the cooling gas. A frontal multiplier as well as a back reflector have also been studied to maximize the tritium generation. This paper, which is a decided first step towards a possible commercial breeding blanket for STEP, covers the material design properties and their compatibility, the design configuration, the neutronics analysis, the thermal-hydraulics analysis, a preliminary structural analysis, safety aspects and waste segregation and disposal