diff --git a/ogc/bblocks/metadata-schema.yaml b/ogc/bblocks/metadata-schema.yaml
index 3dfa651..f92b025 100644
--- a/ogc/bblocks/metadata-schema.yaml
+++ b/ogc/bblocks/metadata-schema.yaml
@@ -161,3 +161,22 @@ properties:
     type: array
     items:
       type: string
+  extends:
+    description: |
+      Used to declare that the schema for this Building Block extends another one. The schemas will be joined
+      with an "allOf" declaration.
+    oneOf:
+      - description: Identifier of the base Building Block to extend
+        type: string
+      - type: object
+        required:
+          - itemIdentifier
+        properties:
+          itemIdentifier:
+            description: Identifier of the base Building Block to extend
+            type: string
+          path:
+            description: |
+              Property path where the schema for this Building Block will be inserted, in the format "a.b.c.d".
+              If a property is an array, "prop[]" notation can be used (e.g., "a.b[].c.d[]").
+            type: string
diff --git a/ogc/bblocks/util.py b/ogc/bblocks/util.py
index 282ad27..e5282ab 100644
--- a/ogc/bblocks/util.py
+++ b/ogc/bblocks/util.py
@@ -10,6 +10,7 @@
 from typing import Any, Sequence, Callable
 
 import jsonschema
+import networkx as nx
 from ogc.na.annotate_schema import SchemaAnnotator, ContextBuilder
 from ogc.na.util import load_yaml, dump_yaml, is_url
 
@@ -149,6 +150,7 @@ def jsonld_context_contents(self):
             self._lazy_properties['jsonld_context_contents'] = load_file(self.jsonld_context)
         return self._lazy_properties['jsonld_context_contents']
 
+
 class BuildingBlockRegister:
 
     def __init__(self,
@@ -191,8 +193,11 @@ def __init__(self,
                 print('=========', file=sys.stderr)
 
         if find_dependencies:
+            dep_graph = nx.DiGraph()
+
             for bblock in self.bblocks.values():
                 found_deps = self.find_dependencies(bblock)
+                dep_graph.add_edges_from([(d, bblock.identifier) for d in found_deps])
                 deps = bblock.metadata.get('dependsOn')
                 if isinstance(deps, str):
                     found_deps.add(deps)
@@ -200,6 +205,11 @@ def __init__(self,
                     found_deps.update(deps)
                 if found_deps:
                     bblock.metadata['dependsOn'] = list(found_deps)
+            cycles = list(nx.simple_cycles(dep_graph))
+            if cycles:
+                cycles_str = '\n - '.join(' -> '.join(reversed(c)) + ' -> ' + c[-1] for c in cycles)
+                raise BuildingBlockError(f"Circular dependencies found: \n - {cycles_str}")
+            self.bblocks = {b: self.bblocks[b] for b in nx.topological_sort(dep_graph) if b in self.bblocks}
 
     def find_dependencies(self, bblock: BuildingBlock) -> set[str]:
         if not bblock.schema.is_file():
@@ -230,6 +240,13 @@ def walk_schema(schema):
 
         walk_schema(bblock_schema)
 
+        extends = bblock.metadata.get('extends')
+        if extends:
+            if isinstance(extends, str):
+                deps.add(extends)
+            elif isinstance(extends, dict):
+                deps.add(extends['itemIdentifier'])
+
         return deps
 
 
diff --git a/requirements.txt b/requirements.txt
index 4554c92..3485222 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -9,3 +9,4 @@ pyparsing~=3.0.9
 pyshacl~=0.21.0
 rdflib~=6.3.2
 jsonref~=1.1.0
+networkx