This paper explores urban growth mechanisms in the Reunion Island based on a modelisation approach that uses the notion of geographical scope. MUTE (Urban Growth Model) is a geographical model that calculates the propensity for urban growth of a given territory. The assessment stage highlights two urban growth patterns: formal and informal. The informal pattern, or postagricultural pattern, allows a different and non-academic interpretation of rapid, urban growth mechanisms. Far from being the consequence of a dysfunctional land policy, it appears more likely to be a historical legacy of a still-effective, post-colonial, agricultural policy based on social exclusion.