diff --git a/tests/test_extensions.py b/tests/test_extensions.py
new file mode 100644
index 00000000..7f76f681
--- /dev/null
+++ b/tests/test_extensions.py
@@ -0,0 +1,174 @@
+from typing import Optional
+
+import pytest
+
+
+@pytest.fixture
+def temp_extension_registry():
+    from wsimod.extensions import extensions_registry
+
+    bkp = extensions_registry.copy()
+    extensions_registry.clear()
+    yield
+    extensions_registry.clear()
+    extensions_registry.update(bkp)
+
+
+def test_register_node_patch(temp_extension_registry):
+    from wsimod.extensions import extensions_registry, register_node_patch
+
+    # Define a dummy function to patch a node method
+    @register_node_patch("node_name", "method_name")
+    def dummy_patch():
+        print("Patched method")
+
+    # Check if the patch is registered correctly
+    assert extensions_registry[("node_name", "method_name", None, False)] == dummy_patch
+
+    # Another function with other arguments
+    @register_node_patch("node_name", "method_name", item="default", is_attr=True)
+    def another_dummy_patch():
+        print("Another patched method")
+
+    # Check if this other patch is registered correctly
+    assert (
+        extensions_registry[("node_name", "method_name", "default", True)]
+        == another_dummy_patch
+    )
+
+
+def test_apply_patches(temp_extension_registry):
+    from wsimod.arcs.arcs import Arc
+    from wsimod.extensions import (
+        apply_patches,
+        extensions_registry,
+        register_node_patch,
+    )
+    from wsimod.nodes import Node
+    from wsimod.orchestration.model import Model
+
+    # Create a dummy model
+    node = Node("dummy_node")
+    node.dummy_arc = Arc("dummy_arc", in_port=node, out_port=node)
+    model = Model()
+    model.nodes[node.name] = node
+
+    # 1. Patch a method
+    @register_node_patch("dummy_node", "apply_overrides")
+    def dummy_patch():
+        pass
+
+    # 2. Patch an attribute
+    @register_node_patch("dummy_node", "t", is_attr=True)
+    def another_dummy_patch(node):
+        return f"A pathced attribute for {node.name}"
+
+    # 3. Patch a method with an item
+    @register_node_patch("dummy_node", "pull_set_handler", item="default")
+    def yet_another_dummy_patch():
+        pass
+
+    # 4. Path a method of an attribute
+    @register_node_patch("dummy_node", "dummy_arc.arc_mass_balance")
+    def arc_dummy_patch():
+        pass
+
+    # Check if all patches are registered
+    assert len(extensions_registry) == 4
+
+    # Apply the patches
+    apply_patches(model)
+
+    # Verify that the patches are applied correctly
+    assert (
+        model.nodes[node.name].apply_overrides.__qualname__ == dummy_patch.__qualname__
+    )
+    assert (
+        model.nodes[node.name]._patched_apply_overrides.__qualname__
+        == "Node.apply_overrides"
+    )
+    assert model.nodes[node.name].t == another_dummy_patch(node)
+    assert model.nodes[node.name]._patched_t == None
+    assert (
+        model.nodes[node.name].pull_set_handler["default"].__qualname__
+        == yet_another_dummy_patch.__qualname__
+    )
+    assert (
+        model.nodes[node.name].dummy_arc.arc_mass_balance.__qualname__
+        == arc_dummy_patch.__qualname__
+    )
+    assert (
+        model.nodes[node.name].dummy_arc._patched_arc_mass_balance.__qualname__
+        == "Arc.arc_mass_balance"
+    )
+
+
+def assert_dict_almost_equal(d1: dict, d2: dict, tol: Optional[float] = None):
+    """Check if two dictionaries are almost equal.
+
+    Args:
+        d1 (dict): The first dictionary.
+        d2 (dict): The second dictionary.
+        tol (float | None, optional): Relative tolerance. Defaults to 1e-6,
+            `pytest.approx` default.
+    """
+    for key in d1.keys():
+        assert d1[key] == pytest.approx(d2[key], rel=tol)
+
+
+def test_path_method_with_reuse(temp_extension_registry):
+    from wsimod.arcs.arcs import Arc
+    from wsimod.extensions import apply_patches, register_node_patch
+    from wsimod.nodes.storage import Reservoir
+    from wsimod.orchestration.model import Model
+
+    # Create a dummy model
+    node = Reservoir(name="dummy_node", initial_storage=10, capacity=10)
+    node.dummy_arc = Arc("dummy_arc", in_port=node, out_port=node)
+
+    vq = node.pull_distributed({"volume": 5})
+    assert_dict_almost_equal(vq, node.v_change_vqip(node.empty_vqip(), 5))
+
+    model = Model()
+    model.nodes[node.name] = node
+
+    @register_node_patch("dummy_node", "pull_distributed")
+    def new_pull_distributed(self, vqip, of_type=None, tag="default"):
+        return self._patched_pull_distributed(vqip, of_type=["Node"], tag=tag)
+
+    # Apply the patches
+    apply_patches(model)
+
+    # Check appropriate result
+    assert node.tank.storage["volume"] == 5
+    vq = model.nodes[node.name].pull_distributed({"volume": 5})
+    assert_dict_almost_equal(vq, node.empty_vqip())
+    assert node.tank.storage["volume"] == 5
+
+
+def test_handler_extensions(temp_extension_registry):
+    from wsimod.arcs.arcs import Arc
+    from wsimod.extensions import apply_patches, register_node_patch
+    from wsimod.nodes import Node
+    from wsimod.orchestration.model import Model
+
+    # Create a dummy model
+    node = Node("dummy_node")
+    node.dummy_arc = Arc("dummy_arc", in_port=node, out_port=node)
+    model = Model()
+    model.nodes[node.name] = node
+
+    # 1. Patch a handler
+    @register_node_patch("dummy_node", "pull_check_handler", item="default")
+    def dummy_patch(self, *args, **kwargs):
+        return "dummy_patch"
+
+    # 2. Patch a handler with access to self
+    @register_node_patch("dummy_node", "pull_set_handler", item="default")
+    def dummy_patch(self, vqip, *args, **kwargs):
+        return f"{self.name} - {vqip['volume']}"
+
+    apply_patches(model)
+
+    assert node.pull_check() == "dummy_patch"
+    assert node.pull_set({"volume": 1}) == "dummy_node - 1"
diff --git a/wsimod/extensions.py b/wsimod/extensions.py
new file mode 100644
index 00000000..6d4a1be4
--- /dev/null
+++ b/wsimod/extensions.py
@@ -0,0 +1,117 @@
+"""This module contains the utilities to extend WSMOD with new features.
+
+The `register_node_patch` decorator is used to register a function that will be used
+instead of a method or attribute of a node. The `apply_patches` function applies all
+registered patches to a model.
+
+Example of patching a method:
+
+`empty_distributed` will be called instead of `pull_distributed` of "my_node":
+
+    >>> from wsimod.extensions import register_node_patch, apply_patches
+    >>> @register_node_patch("my_node", "pull_distributed")
+    >>> def empty_distributed(self, vqip):
+    >>>      return {}
+
+Attributes, methods of the node, and sub-attributes can be patched. Also, an item of a
+list or a dictionary can be patched if the item argument is provided.
+
+Example of patching an attribute:
+
+`10` will be assigned to `t`:
+
+    >>> @register_node_patch("my_node", "t", is_attr=True)
+    >>> def patch_t(node):
+    >>>     return 10
+
+Example of patching an attribute item:
+
+`patch_default_pull_set_handler` will be assigned to
+`pull_set_handler["default"]`:
+
+    >>> @register_node_patch("my_node", "pull_set_handler", item="default")
+    >>> def patch_default_pull_set_handler(self, vqip):
+    >>>     return {}
+
+If patching a method of an attribute, the `is_attr` argument should be set to `True` and
+the target should include the attribute name and the method name, all separated by 
+periods, eg. `attribute_name.method_name`.
+
+It should be noted that the patched function should have the same signature as the
+original method or attribute, and the return type should be the same as well, otherwise
+there will be a runtime error. In particular, the first argument of the patched function
+should be the node object itself, which will typically be named `self`.
+
+The overridden method or attribute can be accessed within the patched function using the
+`_patched_{method_name}` attribute of the object, eg. `self._patched_pull_distributed`. 
+The exception to this is when patching an item, in which case the original item is no
+available to be used within the overriding function.
+
+Finally, the `apply_patches` is called within the `Model.load` method and will apply all
+patches in the order they were registered. This means that users need to be careful with
+the order of the patches in their extensions files, as they may have interdependencies.
+
+TODO: Update documentation on extensions files.
+"""
+from typing import Callable, Hashable
+
+from .orchestration.model import Model
+
+extensions_registry: dict[tuple[str, Hashable, bool], Callable] = {}
+
+
+def register_node_patch(
+    node_name: str, target: str, item: Hashable = None, is_attr: bool = False
+) -> Callable:
+    """Register a function to patch a node method or any of its attributes.
+
+    Args:
+        node_name (str): The name of the node to patch.
+        target (str): The target of the object to patch in the form of a string with the
+            attribute, sub-attribute, etc. and finally method (or attribute) to replace,
+            sepparated with period, eg. `make_discharge` or
+            `sewer_tank.pull_storage_exact`.
+        item (Hashable): Typically a string or an integer indicating the item to replace
+            in the selected attribue, which should be a list or a dictionary.
+        is_attr (bool): If True, the decorated function will be called when applying
+            the patch and the result assigned to the target, instead of assigning the
+            function itself. In this case, the only argument passed to the function is
+            the node object.
+    """
+    target_id = (node_name, target, item, is_attr)
+    if target_id in extensions_registry:
+        raise ValueError(f"Patch for {target} already registered.")
+
+    def decorator(func):
+        extensions_registry[target_id] = func
+        return func
+
+    return decorator
+
+
+def apply_patches(model: Model) -> None:
+    """Apply all registered patches to the model.
+
+    TODO: Validate signature of the patched methods and type of patched attributes.
+
+    Args:
+        model (Model): The model to apply the patches to.
+    """
+    for (node_name, target, item, is_attr), func in extensions_registry.items():
+        starget = target.split(".")
+        method = starget.pop()
+
+        # Get the member to patch
+        node = obj = model.nodes[node_name]
+        for attr in starget:
+            obj = getattr(obj, attr)
+
+        # Apply the patch
+        if item is not None:
+            obj = getattr(obj, method)
+            obj[item] = func(node) if is_attr else func.__get__(node, node.__class__)
+        else:
+            setattr(obj, f"_patched_{method}", getattr(obj, method))
+            setattr(
+                obj, method, func(node) if is_attr else func.__get__(obj, obj.__class__)
+            )
diff --git a/wsimod/orchestration/model.py b/wsimod/orchestration/model.py
index 8e890175..50507249 100644
--- a/wsimod/orchestration/model.py
+++ b/wsimod/orchestration/model.py
@@ -157,6 +157,8 @@ def load(self, address, config_name="config.yml", overrides={}):
             config_name:
             overrides:
         """
+        from ..extensions import apply_patches
+
         with open(os.path.join(address, config_name), "r") as file:
             data = yaml.safe_load(file)
 
@@ -191,6 +193,8 @@ def load(self, address, config_name="config.yml", overrides={}):
         if "dates" in data.keys():
             self.dates = [to_datetime(x) for x in data["dates"]]
 
+        apply_patches(self)
+
     def save(self, address, config_name="config.yml", compress=False):
         """Save the model object to a yaml file and input data to csv.gz format in the
         directory specified.