diff --git a/toolchain/mfc/case_validator.py b/toolchain/mfc/case_validator.py
index 0eb8fc277..de2394cdd 100644
--- a/toolchain/mfc/case_validator.py
+++ b/toolchain/mfc/case_validator.py
@@ -81,6 +81,14 @@
         "math": r"m > 0, \quad n \geq 0, \quad p \geq 0",
         "explanation": ("The x-direction must have cells. Cannot have z without y. Cylindrical coordinates require odd p."),
     },
+    "check_domain_extents": {
+        "title": "Domain Extents Specified",
+        "category": "Domain and Geometry",
+        "math": r"m > 0 \Rightarrow x_{\mathrm{beg}}, x_{\mathrm{end}} \ \mathrm{set}",
+        "explanation": (
+            "Every dimension that has cells needs its physical extents so that the grid can be generated. Skipped on restarts (old_grid = T), where the mesh is read from the existing grid files."
+        ),
+    },
     "check_patch_within_domain": {
         "title": "Patch Within Domain",
         "category": "Domain and Geometry",
@@ -330,12 +338,6 @@ def check_parameter_types(self):
             f"recon_type must be one of {_recon_shown}",
         )
 
-        # Required domain parameters when m > 0
-        m = self.get("m")
-        if m is not None and m > 0:
-            self.prohibit(not self.is_set("x_domain%beg"), "x_domain%beg must be set when m > 0")
-            self.prohibit(not self.is_set("x_domain%end"), "x_domain%end must be set when m > 0")
-
     # Common Checks (All Stages)
 
     def check_simulation_domain(self):
@@ -1508,6 +1510,31 @@ def check_restart(self):
             f"num_patches must be <= {num_patches_max} (num_patches_max in m_constants.fpp)",
         )
 
+    def check_domain_extents(self):
+        """Checks that the physical extents of every active dimension are set (pre-process)
+
+        The grid generator needs (xyz)_domain%beg and (xyz)_domain%end for each
+        dimension that has cells. On a restart (old_grid = T) the mesh is read
+        from the existing grid files, so the extents are neither needed nor read.
+        """
+        if self.get("old_grid", "F") == "T":
+            return
+
+        m = self.get("m", 0)
+        if self._is_numeric(m) and m > 0:
+            self.prohibit(not self.is_set("x_domain%beg"), "x_domain%beg must be set when m > 0")
+            self.prohibit(not self.is_set("x_domain%end"), "x_domain%end must be set when m > 0")
+
+        n = self.get("n", 0)
+        if self._is_numeric(n) and n > 0:
+            self.prohibit(not self.is_set("y_domain%beg"), "y_domain%beg must be set when n > 0")
+            self.prohibit(not self.is_set("y_domain%end"), "y_domain%end must be set when n > 0")
+
+        p = self.get("p", 0)
+        if self._is_numeric(p) and p > 0:
+            self.prohibit(not self.is_set("z_domain%beg"), "z_domain%beg must be set when p > 0")
+            self.prohibit(not self.is_set("z_domain%end"), "z_domain%end must be set when p > 0")
+
     def check_qbmm_pre_process(self):
         """Checks QBMM constraints for pre-process"""
         qbmm = self.get("qbmm", "F") == "T"
@@ -2499,6 +2526,7 @@ def validate_pre_process(self):
         """Validate pre-process-specific parameters"""
         self.validate_common()
         self.check_restart()
+        self.check_domain_extents()
         self.check_qbmm_pre_process()
         self.check_parallel_io_pre_process()
         self.check_grid_stretching()
diff --git a/toolchain/mfc/params_tests/test_integration.py b/toolchain/mfc/params_tests/test_integration.py
index b7f4c6113..ceefc2f89 100644
--- a/toolchain/mfc/params_tests/test_integration.py
+++ b/toolchain/mfc/params_tests/test_integration.py
@@ -263,5 +263,40 @@ def test_validator_log_params_match_registry(self):
         self.assertEqual(validator_log_params, registry_log_params)
 
 
+class TestDomainExtents(unittest.TestCase):
+    """Tests for the pre-process domain extent requirement."""
+
+    # 2D case with no (xyz)_domain entries
+    BASE = {"m": 100, "n": 50, "num_patches": 1}
+
+    def _errors(self, params, stage):
+        from ..case_validator import CaseConstraintError, CaseValidator
+
+        validator = CaseValidator(params)
+        try:
+            validator.validate(stage)
+        except CaseConstraintError:
+            pass
+        return [e for e in validator.errors if "domain%" in e and "must be set" in e]
+
+    def test_missing_extents_rejected(self):
+        """Every dimension with cells needs its extents when generating a grid."""
+        self.assertEqual(len(self._errors(self.BASE, "pre_process")), 4)
+
+    def test_restart_does_not_need_extents(self):
+        """old_grid = T reads the mesh from the grid files, so the extents are not needed."""
+        params = {**self.BASE, "old_grid": "T", "old_ic": "T", "t_step_old": 7000, "num_patches": 0}
+
+        for stage in ("pre_process", "simulation", "post_process"):
+            with self.subTest(stage=stage):
+                self.assertEqual(self._errors(params, stage), [])
+
+    def test_extents_not_required_outside_pre_process(self):
+        """(xyz)_domain is a pre-process parameter only."""
+        for stage in ("simulation", "post_process"):
+            with self.subTest(stage=stage):
+                self.assertEqual(self._errors(self.BASE, stage), [])
+
+
 if __name__ == "__main__":
     unittest.main()