diff --git a/tests/sample_data/extra_facets/override/test6-01.yml b/tests/sample_data/extra_facets/override/test6-01.yml
new file mode 100644
index 0000000000..3f375d1314
--- /dev/null
+++ b/tests/sample_data/extra_facets/override/test6-01.yml
@@ -0,0 +1,12 @@
+---
+PROJECT1:
+  Amon:
+    tas:
+      source_var_name: "t2m"
+      cds_var_name: "temperature_2m"
+    uas:
+      source_var_name: "u10n"
+      cds_var_name: "10m_u-component_of_neutral_wind"
+    vas:
+      source_var_name: "v10n"
+      cds_var_name: "10m_v-component_of_neutral_wind"
diff --git a/tests/sample_data/extra_facets/override/test6-02.yml b/tests/sample_data/extra_facets/override/test6-02.yml
new file mode 100644
index 0000000000..7cf8a552cd
--- /dev/null
+++ b/tests/sample_data/extra_facets/override/test6-02.yml
@@ -0,0 +1,6 @@
+---
+PROJECT1:
+  Amon:
+    vas:
+      source_var_name: "10v"
+      cds_var_name: "v-component_of_neutral_wind_at_10m"
diff --git a/tests/sample_data/extra_facets/simple/test6-01.yml b/tests/sample_data/extra_facets/simple/test6-01.yml
new file mode 100644
index 0000000000..d940b8c96b
--- /dev/null
+++ b/tests/sample_data/extra_facets/simple/test6-01.yml
@@ -0,0 +1,5 @@
+---
+PROJECT1:
+  Amon:
+    tas: {source_var_name: "2t", cds_var_name: "2m_temperature"}
+    psl: {source_var_name: "msl", cds_var_name: "mean_sea_level_pressure"}
diff --git a/tests/unit/test_config.py b/tests/unit/test_config.py
index e5f5fde4b0..86c218673e 100644
--- a/tests/unit/test_config.py
+++ b/tests/unit/test_config.py
@@ -1,6 +1,10 @@
 import pytest
 
-from esmvalcore._config._config import _deep_update
+from esmvalcore._config._config import (
+    _deep_update,
+    _load_extra_facets,
+    importlib_files,
+)
 
 TEST_DEEP_UPDATE = [([{}], {}), ([dict(a=1, b=2), dict(a=3)], dict(a=3, b=2)),
                     ([
@@ -15,3 +19,33 @@ def test_deep_update(dictionaries, expected_merged):
     for update in dictionaries[1:]:
         merged = _deep_update(merged, update)
     assert expected_merged == merged
+
+
+BASE_PATH = importlib_files('tests') / 'sample_data' / 'extra_facets'
+
+TEST_LOAD_EXTRA_FACETS = [
+    ('test-nonexistent', tuple(), {}),
+    ('test-nonexistent', (BASE_PATH / 'simple', ), {}),
+    ('test6', (BASE_PATH / 'simple', ),
+     dict(PROJECT1=dict(Amon=dict(
+         tas=dict(cds_var_name='2m_temperature', source_var_name='2t'),
+         psl=dict(cds_var_name='mean_sea_level_pressure',
+                  source_var_name='msl'))))),
+    ('test6', (BASE_PATH / 'simple', BASE_PATH / 'override'),
+     dict(PROJECT1=dict(Amon=dict(
+         tas=dict(cds_var_name='temperature_2m', source_var_name='t2m'),
+         psl=dict(cds_var_name='mean_sea_level_pressure',
+                  source_var_name='msl'),
+         uas=dict(cds_var_name='10m_u-component_of_neutral_wind',
+                  source_var_name='u10n'),
+         vas=dict(cds_var_name='v-component_of_neutral_wind_at_10m',
+                  source_var_name='10v'),
+     )))),
+]
+
+
+@pytest.mark.parametrize('project, extra_facets_dir, expected',
+                         TEST_LOAD_EXTRA_FACETS)
+def test_load_extra_facets(project, extra_facets_dir, expected):
+    extra_facets = _load_extra_facets(project, extra_facets_dir)
+    assert extra_facets == expected