Residential development water testing supports commissioning, investigation and verification for defined systems and purposes. Individual houses, shared storage, communal distribution and vacant units have different conditions. Identify supplies and boundaries, and clarify private-source or onward-distribution arrangements with the relevant authority. Pressure testing, microbiological analysis and operational monitoring are different forms of evidence. Agree commissioning requirements with the relevant water company. Northumbrian Water guidance illustrates that ordinary new domestic-house service pipes are not normally required to be disinfected, while specified circumstances require disinfection and bacteriological analysis. Do not transfer its conditions or validity windows universally. Select microbiological and chemical tests for the applicable specification or concern. HSE states that Legionella testing is not usually required for domestic hot and cold water systems and health and safety law does not require landlords to obtain a Legionella test certificate. Risk management remains necessary, and communal installations or uncertain controls may require another approach. This differs from commissioning drinking-water analysis. Shared plant needs operational checks and maintenance, not just laboratory samples. Separate closed heating-water analysis from drinking-water assessment. Manage phased occupation and vacant homes between commissioning and handover. Sampling requires representative locations, defined methods, suitable containers, transport and traceability. Check laboratory and sampling accreditation for the actual services; DWI provisions for regulated private supplies should not be applied indiscriminately to all mains-fed developments. Interpret methods, units, reporting limits and criteria in context. Not detected is not proof of absolute absence, and one result cannot guarantee an entire scheme indefinitely. Investigate findings, document corrective actions and provide drawings, commissioning records, reports and responsibilities at handover. Reviewed 7 October 2026 using HSE, DWI and Northumbrian Water guidance.