This study determines the effect of compact built-up residential areas (with water mains but without sewage systems) on selected indices of water quality in the winter and summer half-year. The assessment was based on an analysis of the Grodna stream draining catchment with an area of 598.21 ha. Water quality measurements were conducted before and after water flowed through the built-up part of the catchment. The obtained results unanimously point to a negative impact on surface water quality from the residential built-up area, which has no regular water supply or sewage disposal system. Mean annual concentrations of the analyzed indices of the stream water (except dissolved oxygen) always increased after the water flowed through the built-up area. The concentration increases were sometimes very high for some indices (NH₄ over 30-fold, NO₂ and PO₄ almost 10-fold). Water quality was also assessed using a direct method.