Urban Form in Sight: A City-Wide 3D ISOVIST Analysis of Pedestrian-Level Visibility Across Building Height–Coverage Typologies

High-rise and high-density development is often assumed to reduce the visual quality of urban landscapes, yet building height alone does not fully explain what pedestrians can see at street level. In compact cities such as Seoul, taller buildings may obstruct views, but they may also reduce ground coverage and create more open space around buildings. This study examines how building height and building coverage jointly shape pedestrian-level visibility in high-density urban environments. Using Seoul as a case study, we developed a citywide 3D ISOVIST framework based on building footprint and height data. The city was divided into 500 m × 500 m grid cells, and road-based observation points were used to simulate pedestrian-level visual fields. Urban form was classified into nine height–coverage typologies, and visibility was measured using sky viewing ratio, building obstruction ratio, mean viewing distance, and viewing distance variance. The results show that pedestrian-level visibility varies systematically across height–coverage combinations. Visibility generally decreases as building coverage increases, while the effect of building height is not linear. In some high-rise conditions, reduced ground coverage can partly compensate for vertical obstruction and maintain visual openness at the pedestrian level. These findings suggest that urban landscape regulation should not rely on height control alone. Instead, building height, coverage, spacing, and ground-level openness should be considered together when evaluating the visual effects of dense urban development. This study contributes a scalable geometric visibility framework for urban morphology analysis while acknowledging that visual openness is only one dimension of broader urban landscape quality.