diff --git a/custom_components/houseplan/http_api.py b/custom_components/houseplan/http_api.py
index f166e5c1..27c98745 100644
--- a/custom_components/houseplan/http_api.py
+++ b/custom_components/houseplan/http_api.py
@@ -451,6 +451,7 @@ class HouseplanUploadView(HomeAssistantView):
                     partial(
                         check_quota, files_root, tmp_path.stat().st_size,
                         MAX_FILES_BYTES, MAX_FILES_COUNT, exclude=tmp_path,
+                        additional_disk_bytes=0,
                     ),
                 )
             except QuotaError as err:
diff --git a/custom_components/houseplan/plans.py b/custom_components/houseplan/plans.py
index 8a02b10b..fa7be5a6 100644
--- a/custom_components/houseplan/plans.py
+++ b/custom_components/houseplan/plans.py
@@ -222,9 +222,20 @@ def dir_usage(path: Path, *, exclude: Path | None = None) -> tuple[int, int]:
 
 
 def check_quota(
-    path: Path, incoming: int, max_bytes: int, max_files: int, *, exclude: Path | None = None,
+    path: Path,
+    incoming: int,
+    max_bytes: int,
+    max_files: int,
+    *,
+    exclude: Path | None = None,
+    additional_disk_bytes: int | None = None,
 ) -> None:
-    """Raise QuotaError unless `incoming` more bytes fit (`exclude`: see dir_usage).
+    """Raise QuotaError unless `incoming` fits the store and disk.
+
+    `exclude` is omitted from current store usage but still charged as
+    `incoming`. `additional_disk_bytes` is the part not physically written yet;
+    by default all incoming bytes still need disk space. A staged file already
+    under `path` passes zero because promotion only renames it (#554).
 
     Deliberately not an age rule. Files are never removed for getting old — that
     cost real plans twice — so the limit sits where a decision is being made
@@ -245,7 +256,8 @@ def check_quota(
         free = shutil.disk_usage(str(path if path.is_dir() else path.parent)).free
     except OSError:
         return
-    if free - incoming < MIN_FREE_BYTES:
+    disk_incoming = incoming if additional_disk_bytes is None else additional_disk_bytes
+    if free - disk_incoming < MIN_FREE_BYTES:
         raise QuotaError("low_disk_space", f"only {free // 1024 // 1024} MB free on the disk")
 
 
diff --git a/docs/CHANGELOG.md b/docs/CHANGELOG.md
index 0fe945d4..8766c68e 100644
--- a/docs/CHANGELOG.md
+++ b/docs/CHANGELOG.md
@@ -2,6 +2,11 @@
 
 ## Unreleased
 
+- Uploading an attachment no longer reports low disk space merely because its
+  completed staging file was counted a second time. The 512 MiB safety reserve
+  itself is unchanged
+  ([#554](https://github.com/Matysh/houseplan-card/issues/554)).
+
 - Range and zone radar sources now stay live in the setup preview and use the
   same complete source list for Home Assistant read-permission checks as for
   frame calculation
diff --git a/docs/CHANGELOG.ru.md b/docs/CHANGELOG.ru.md
index dc085e19..a30d9d4d 100755
--- a/docs/CHANGELOG.ru.md
+++ b/docs/CHANGELOG.ru.md
@@ -8,6 +8,11 @@
 
 ## Не выпущено
 
+- Загрузка вложения больше не сообщает о нехватке места только потому, что уже
+  записанный staging-файл посчитали повторно. Сам резерв безопасности 512 МиБ
+  не изменился
+  ([#554](https://github.com/Matysh/houseplan-card/issues/554)).
+
 - Источники диапазонов и зон радара теперь обновляются в предпросмотре
   настройки, а проверка прав Home Assistant использует тот же полный список
   источников, что и расчёт данных радара
diff --git a/docs/TESTING.md b/docs/TESTING.md
index 15ba8312..ae3523c5 100644
--- a/docs/TESTING.md
+++ b/docs/TESTING.md
@@ -1652,6 +1652,13 @@ separately promised workflows:
       returns the newest 60 with a total
       [auto: unit: test_check_quota_counts_the_whole_store_not_one_request,
       backend test_uploads_are_bounded_by_a_store_quota]
+- [ ] An attachment already written to staging is not reserved twice: with the
+      real 512 MiB disk reserve intact its same-filesystem promotion succeeds,
+      while one byte below the reserve still fails. Uploads checked before any
+      bytes are written continue to reserve their full incoming size
+      [auto: backend test_issue_554_low_disk_reserve_distinguishes_staged_and_unwritten_bytes
+      + test_issue_554_upload_uses_actual_free_space_after_staging; mutation:
+      quota-reserves-staged-bytes-twice-on-disk]
 - [ ] Square canvas migration (v1.48.0): after the upgrade every existing plan
       looks exactly as before, just with margins where the canvas was extended.
       Measure a wall in the plan editor — the length in cm is unchanged. Marker
diff --git a/scripts/mutation-gate.mjs b/scripts/mutation-gate.mjs
index 8ff9dadc..c5ff8520 100644
--- a/scripts/mutation-gate.mjs
+++ b/scripts/mutation-gate.mjs
@@ -8873,6 +8873,19 @@ const MUTANT_DEFINITIONS = [
       replace: '        if item.name.startswith(TMP_PREFIX):  # mutant: every staged file is invisible\n',
     }],
   },
+  {
+    id: 'quota-reserves-staged-bytes-twice-on-disk',
+    guard: 'node scripts/backend-test-guard.mjs '
+      + 'issue_554_upload_uses_actual_free_space_after_staging '
+      + 'tests_backend/test_ha_upload.py',
+    because: 'the attachment body already occupies disk space inside files_root; subtracting its '
+      + 'size again rejects a rename even when the real reserve is intact (#554 AC5)',
+    patches: [{
+      file: 'custom_components/houseplan/http_api.py',
+      find: '                        additional_disk_bytes=0,\n',
+      replace: '                        # mutant: staged bytes are reserved a second time\n',
+    }],
+  },
   {
     id: 'support-palette-copies-any-key',
     guard: 'node scripts/backend-test-guard.mjs '
diff --git a/tests_backend/test_ha_upload.py b/tests_backend/test_ha_upload.py
index 243ca693..e60875e9 100644
--- a/tests_backend/test_ha_upload.py
+++ b/tests_backend/test_ha_upload.py
@@ -150,6 +150,38 @@ async def test_issue_498_concurrent_uploads_still_count_each_other(
     assert stored <= 1000
 
 
+async def test_issue_554_upload_uses_actual_free_space_after_staging(
+    hass: HomeAssistant, hass_client: ClientSessionGenerator, monkeypatch,
+) -> None:
+    """A completed staging file needs no second disk reserve before its rename."""
+    import shutil
+    from pathlib import Path
+
+    from custom_components.houseplan.const import FILES_DIR, MIN_FREE_BYTES
+    from custom_components.houseplan.plans import TMP_PREFIX
+
+    await _setup(hass)
+    client = await hass_client()
+    usage = type("Usage", (), {"free": MIN_FREE_BYTES})()
+    monkeypatch.setattr(shutil, "disk_usage", lambda _path: usage)
+
+    accepted = await client.post(
+        "/api/houseplan/upload", data=_pdf_form("at-reserve.pdf", 50),
+    )
+    assert accepted.status == 200, await accepted.text()
+
+    usage.free = MIN_FREE_BYTES - 1
+    refused = await client.post(
+        "/api/houseplan/upload", data=_pdf_form("below-reserve.pdf", 50),
+    )
+    assert refused.status == 507
+    assert (await refused.json())["error"] == "low_disk_space"
+
+    root = Path(hass.config.path(FILES_DIR))
+    assert not list(root.glob(TMP_PREFIX + "*"))
+    assert sorted(path.name for path in (root / "m1").iterdir()) == ["at-reserve.pdf"]
+
+
 def _svg_chain(length: int) -> bytes:
     defs = "".join(
         f'<linearGradient id="g{index}" href="#g{index + 1}"/>' for index in range(length - 1)
diff --git a/tests_backend/test_validation.py b/tests_backend/test_validation.py
index 8937df56..80c164d2 100644
--- a/tests_backend/test_validation.py
+++ b/tests_backend/test_validation.py
@@ -1574,6 +1574,48 @@ def test_check_quota_refuses_when_the_disk_is_nearly_full(tmp_path, monkeypatch)
     assert e.value.reason == "low_disk_space"
 
 
+def test_issue_554_low_disk_reserve_distinguishes_staged_and_unwritten_bytes(
+    tmp_path, monkeypatch,
+):
+    """Already-written staging needs no second reserve; pending bytes still do."""
+    import shutil
+
+    d = tmp_path / "files"
+    d.mkdir()
+    staged = d / (plans.TMP_PREFIX + "own")
+    staged.write_bytes(b"x" * 50)
+    usage = type("Usage", (), {"free": const.MIN_FREE_BYTES})()
+    monkeypatch.setattr(shutil, "disk_usage", lambda _path: usage)
+
+    plans.check_quota(
+        d,
+        50,
+        max_bytes=1000,
+        max_files=10,
+        exclude=staged,
+        additional_disk_bytes=0,
+    )
+
+    usage.free = const.MIN_FREE_BYTES - 1
+    with pytest.raises(plans.QuotaError) as staged_low:
+        plans.check_quota(
+            d,
+            50,
+            max_bytes=1000,
+            max_files=10,
+            exclude=staged,
+            additional_disk_bytes=0,
+        )
+    assert staged_low.value.reason == "low_disk_space"
+
+    usage.free = const.MIN_FREE_BYTES + 50
+    plans.check_quota(d, 50, max_bytes=1000, max_files=10)
+    usage.free -= 1
+    with pytest.raises(plans.QuotaError) as unwritten_low:
+        plans.check_quota(d, 50, max_bytes=1000, max_files=10)
+    assert unwritten_low.value.reason == "low_disk_space"
+
+
 class TestVacuum:
     """marker.vacuum (docs/VACUUM.md): optional everywhere, matrices strict."""