This document gives guidance on the structural design of underground modular systems for infiltration, attenuation and storage of surface water under various conditions of loading. The procedures are explained, with the appropriate variables in the design formulae, and provides graphical information on vehicle surcharge loadings. These modular systems are constructed from multiple cuboid shaped thermoplastic boxes generally with ancillary components such as inlet/outlet connectors, vents, and access/inspection provision. This guidance is for the design of modular systems conforming to EN 17152 1. The boxes, including integral components, are injection moulded, extruded or thermoformed thermoplastics, manufactured from polypropylene (PP) or unplasticized poly(vinyl chloride) (PVC-U), and are intended to be used as elements in a modular system where the manufacturer has clearly stated in the documentation how the components are assembled to create a complete infiltration, attenuation or storage system. Outside the scope of this document are the following conditions: - seismic loads, - lateral loads from adjacent structures and embankments, - influence of trees, - backfill materials not according to CEN/TR 17179 [1]. Geotextile and/or geomembrane used with modular systems are outside the scope of this document. NOTE – If reference is made in this document to Eurocode standards, the conditions in a national foreword or national annex are normally stated.