As urbanisation progresses, open space becomes structured as units of progressively smaller sizes and with more pronounced physical and functional boundaries. This paper analyses these Open Space Units (OSUs) in Flanders, and seeks how size of open space units, hence also spatial fragmentation, affects the evaluation of these units. The results clearly confirm a “fragmentation bias”, meaning a lower valuation of smaller units, which leads to a strategic gap and land use uncertainty concerning large stretches of area with high degree of fragmentation. This valuation is confronted with the contrasting and positive values expressed in a strategic open space project by local stakeholders about a typical peri-urban remnant open space unit. Overcoming the ‘fragmentation bias’ in open space valuation is a continuing challenge in planning and open space policies, especially in highly urbanized environments.