diff --git a/backend/tests/test_webdav_backend.py b/backend/tests/test_webdav_backend.py
new file mode 100644
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--- /dev/null
+++ b/backend/tests/test_webdav_backend.py
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+"""
+Tests for WebDAVBackend and NextcloudBackend (Plan 05-04).
+
+TDD RED phase — all tests fail until backend/storage/webdav_backend.py and
+backend/storage/nextcloud_backend.py are implemented.
+
+Covers:
+  - WebDAVBackend subclasses StorageBackend (all 7 abstract methods present)
+  - All 7 methods are async coroutines
+  - SSRF guard: construction with private/localhost URL raises ValueError
+  - presigned_get_url and generate_presigned_put_url raise NotImplementedError
+  - _make_path constructs correct WebDAV path
+  - NextcloudBackend subclasses WebDAVBackend
+  - NextcloudBackend inherits SSRF guard
+  - NextcloudBackend has list_folder (async)
+"""
+from __future__ import annotations
+
+import inspect
+
+import pytest
+
+pytestmark = pytest.mark.asyncio
+
+
+class TestWebDAVBackendStructure:
+    """Static structure tests — no network calls required."""
+
+    def test_webdav_backend_importable(self):
+        from storage.webdav_backend import WebDAVBackend  # noqa: F401
+
+    def test_webdav_backend_is_storage_backend_subclass(self):
+        from storage.base import StorageBackend
+        from storage.webdav_backend import WebDAVBackend
+
+        assert issubclass(WebDAVBackend, StorageBackend)
+
+    @pytest.mark.parametrize(
+        "method",
+        [
+            "put_object",
+            "get_object",
+            "delete_object",
+            "presigned_get_url",
+            "health_check",
+            "generate_presigned_put_url",
+            "stat_object",
+        ],
+    )
+    def test_all_7_methods_are_async(self, method):
+        from storage.webdav_backend import WebDAVBackend
+
+        fn = getattr(WebDAVBackend, method)
+        assert inspect.iscoroutinefunction(fn), f"{method} is not async"
+
+
+class TestWebDAVBackendSSRF:
+    """SSRF guard tests — construction with blocked URL must raise ValueError."""
+
+    def test_localhost_raises(self):
+        from storage.webdav_backend import WebDAVBackend
+
+        with pytest.raises(ValueError):
+            WebDAVBackend("http://localhost/dav", "user", "pass")
+
+    def test_127_x_raises(self):
+        from storage.webdav_backend import WebDAVBackend
+
+        with pytest.raises(ValueError):
+            WebDAVBackend("http://127.0.0.1/dav", "user", "pass")
+
+    def test_10_x_raises(self):
+        from storage.webdav_backend import WebDAVBackend
+
+        with pytest.raises(ValueError):
+            WebDAVBackend("http://10.0.0.1/dav", "user", "pass")
+
+    def test_192_168_raises(self):
+        from storage.webdav_backend import WebDAVBackend
+
+        with pytest.raises(ValueError):
+            WebDAVBackend("http://192.168.1.1/dav", "user", "pass")
+
+    def test_169_254_raises(self):
+        from storage.webdav_backend import WebDAVBackend
+
+        with pytest.raises(ValueError):
+            WebDAVBackend("http://169.254.169.254/dav", "user", "pass")
+
+
+class TestWebDAVBackendNotImplemented:
+    """presigned methods must raise NotImplementedError (D-14)."""
+
+    async def test_presigned_get_url_raises(self):
+        from storage.webdav_backend import WebDAVBackend
+
+        backend = WebDAVBackend.__new__(WebDAVBackend)
+        backend._server_url = "https://8.8.8.8/dav"
+        with pytest.raises(NotImplementedError):
+            await backend.presigned_get_url("some/key")
+
+    async def test_generate_presigned_put_url_raises(self):
+        from storage.webdav_backend import WebDAVBackend
+
+        backend = WebDAVBackend.__new__(WebDAVBackend)
+        backend._server_url = "https://8.8.8.8/dav"
+        with pytest.raises(NotImplementedError):
+            await backend.generate_presigned_put_url("some/key")
+
+
+class TestWebDAVMakePath:
+    """_make_path must produce percent-encoded WebDAV paths (Pitfall 2)."""
+
+    def test_make_path_basic(self):
+        from storage.webdav_backend import WebDAVBackend
+
+        backend = WebDAVBackend.__new__(WebDAVBackend)
+        path = backend._make_path("user-uuid", "doc-uuid", ".pdf")
+        assert path == "docuvault/user-uuid/doc-uuid.pdf"
+
+    def test_make_path_encodes_special_chars(self):
+        from storage.webdav_backend import WebDAVBackend
+
+        backend = WebDAVBackend.__new__(WebDAVBackend)
+        # UUIDs are alphanumeric and hyphens — no encoding needed for typical values
+        # This test ensures the encoding is applied (result should not contain raw spaces)
+        path = backend._make_path("user id", "doc id", ".pdf")
+        assert " " not in path
+        assert "user%20id" in path or "user+id" in path or "user%2520id" in path
+
+
+class TestNextcloudBackendStructure:
+    """NextcloudBackend structure tests."""
+
+    def test_nextcloud_importable(self):
+        from storage.nextcloud_backend import NextcloudBackend  # noqa: F401
+
+    def test_nextcloud_is_webdav_subclass(self):
+        from storage.nextcloud_backend import NextcloudBackend
+        from storage.webdav_backend import WebDAVBackend
+
+        assert issubclass(NextcloudBackend, WebDAVBackend)
+
+    def test_nextcloud_is_storage_backend_subclass(self):
+        from storage.base import StorageBackend
+        from storage.nextcloud_backend import NextcloudBackend
+
+        assert issubclass(NextcloudBackend, StorageBackend)
+
+    @pytest.mark.parametrize(
+        "method",
+        [
+            "put_object",
+            "get_object",
+            "delete_object",
+            "presigned_get_url",
+            "health_check",
+            "generate_presigned_put_url",
+            "stat_object",
+        ],
+    )
+    def test_all_7_methods_async_on_nextcloud(self, method):
+        from storage.nextcloud_backend import NextcloudBackend
+
+        fn = getattr(NextcloudBackend, method)
+        assert inspect.iscoroutinefunction(fn), f"{method} is not async"
+
+    def test_list_folder_present_and_async(self):
+        from storage.nextcloud_backend import NextcloudBackend
+
+        assert hasattr(NextcloudBackend, "list_folder")
+        assert inspect.iscoroutinefunction(NextcloudBackend.list_folder)
+
+
+class TestNextcloudBackendSSRF:
+    """SSRF guard inherited from WebDAVBackend.__init__."""
+
+    def test_10_x_raises_inherited(self):
+        from storage.nextcloud_backend import NextcloudBackend
+
+        with pytest.raises(ValueError):
+            NextcloudBackend("http://10.0.0.1/dav", "user", "pass")
+
+    def test_localhost_raises_inherited(self):
+        from storage.nextcloud_backend import NextcloudBackend
+
+        with pytest.raises(ValueError):
+            NextcloudBackend("http://localhost/dav", "user", "pass")