add missing 3.9

Author: alpha-beta-soup

src/content/docs/classification/class-3.mdx

@@ -267,5 +267,21 @@ This land-use type includes land uses dedicated to managing and processing vario
 
 </Tabs>
 
+### 3.9.0 Vacant and transitioning land
+
+This land-use type includes areas that are currently unused or undergoing a transition from one land use to another, but in this case with a clear transition towards or within other concepts under the built environment.
+
+<Tabs>
+  <TabItem label="3.9.1 Vacant land">
+    Includes derelict land and developed land that is idle.
+  </TabItem>
+    <TabItem label="3.9.2 Greenfield development">
+      Previously undeveloped or agricultural land zones for or undergoing new construction projects or urban expansion, typically involving the conversion of rural or natural areas into residential, commercial, industrial, or infrastructural uses.
+  </TabItem>
+    <TabItem label="3.9.3 Brownfield development">
+      Areas of active redevelopment of previously developed (often industrial) land that may be abandoned, contaminated, or economically under-utilised, with the aim of rehabilitating and repurposing these sites for new urban activities. May include residential areas undergoing infill development that increase housing density.
+  </TabItem>
+</Tabs>
+
 --- 
 *Last updated: October 2025*