diff --git a/src/hards/filesystem.py b/src/hards/filesystem.py
index 0f32e32..a82bb0e 100644
--- a/src/hards/filesystem.py
+++ b/src/hards/filesystem.py
@@ -196,7 +196,7 @@ def get_file(self, name: str) -> Path:
         return location
 
     def add_file(
-        self, file: Path, *, name: str | None = None, permissions: int | None = None
+        self, file: Path, *, name: str | None = None, permissions: int = 0o400
     ) -> None:
         super().add_file(file, name=name)
         if not file.exists() or not file.is_file:
@@ -209,9 +209,11 @@ def add_file(
         new_name = name or file.name
         destination = self._files_location / new_name
 
+        if destination.exists():
+            Path(destination).chmod(0o700)
+
         shutil.copy2(file, destination, follow_symlinks=True)
-        if permissions is not None:
-            Path(destination).chmod(0o400)
+        Path(destination).chmod(permissions)
         self._files.append(new_name)
 
 
diff --git a/tests/test_filesystem_dataset.py b/tests/test_filesystem_dataset.py
index 84c5764..5d8a552 100644
--- a/tests/test_filesystem_dataset.py
+++ b/tests/test_filesystem_dataset.py
@@ -70,12 +70,15 @@ def test_dataset_files_overwrite(filesystem_dataset, data_assets):
     assert filesystem_dataset.has_file("example_file.dat")
 
 
-def test_dataset_files_permissions(filesystem_dataset, data_assets):
-    filesystem_dataset.add_file(data_assets / "example_file.dat", permissions=0o400)
+def test_dataset_files_read_only(filesystem_dataset, data_assets):
+    filesystem_dataset.add_file(data_assets / "example_file.dat")
     assert filesystem_dataset.has_file("example_file.dat")
 
-    with pytest.raises(PermissionError):
-        filesystem_dataset.add_file(data_assets / "example_file.dat")
+    with (
+        pytest.raises(PermissionError),
+        filesystem_dataset.get_file("example_file.dat").open("w") as f,
+    ):
+        f.write("test")
 
 
 def test_dataset_files_rename(filesystem_dataset, data_assets):