Urbanization increases impervious surfaces, generating higher stormwater volumes and pollutant loads that challenge conventional drainage systems. While nature-based solutions (NBS) are widely used for stormwater management, compact stormwater treatment systems (CSTS) provide a decentralized alternative suited to space-limited urban areas. However, their evaluation is hindered by inconsistent international testing protocols. This study compares three major CSTS performance standards, the British Water Code of Practice (BW CoP), the New Jersey Corporation for Advanced Technology (NJCAT) program, and the Deutsches Institut für Bautechnik (DIBt) protocol, to identify methodological differences and testing implications. Based on this comparison, a modified DIBt protocol is developed, expanded to include nutrient (phosphorus and nitrogen) tests, was applied to a filter chamber under controlled laboratory conditions. Results showed particle removal ranging from 25% at high flow to 97% at low flow conditions, hydrocarbon removal remaining consistently high (98–99%), and metal removal efficiencies varying with flow, copper increasing from 19 to 95% and zinc from slightly negative (release) to 88%. Nutrient tests indicated phosphorus removal from 32 to 84% and negative nitrogen removal rates. These findings highlight the need for harmonized testing frameworks reflecting regional conditions to improve CSTS evaluation and implementation