diff --git a/apps/admin-ui/src/api/sites.ts b/apps/admin-ui/src/api/sites.ts
index d4f484b..d027287 100644
--- a/apps/admin-ui/src/api/sites.ts
+++ b/apps/admin-ui/src/api/sites.ts
@@ -38,7 +38,7 @@ export async function updateSiteConfig(
   siteId: string,
   body: Partial<SiteConfig>,
 ): Promise<SiteConfig> {
-  const { data } = await apiClient.put<SiteConfig>(`/sites/${siteId}/config`, body);
+  const { data } = await apiClient.patch<SiteConfig>(`/sites/${siteId}/config`, body);
   return data;
 }
 
diff --git a/apps/admin-ui/src/test/sites-api.test.ts b/apps/admin-ui/src/test/sites-api.test.ts
new file mode 100644
index 0000000..fce94e6
--- /dev/null
+++ b/apps/admin-ui/src/test/sites-api.test.ts
@@ -0,0 +1,31 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+
+const apiClient = {
+  get: vi.fn(),
+  post: vi.fn(),
+  patch: vi.fn(),
+  put: vi.fn(),
+  delete: vi.fn(),
+};
+
+vi.mock('../api/client', () => ({
+  default: apiClient,
+}));
+
+describe('sites API client', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it('partially updates site config with PATCH so omitted fields are preserved', async () => {
+    apiClient.patch.mockResolvedValue({ data: { id: 'cfg-1' } });
+
+    const { updateSiteConfig } = await import('../api/sites');
+    await updateSiteConfig('site-1', { banner_config: { displayMode: 'corner_popup' } });
+
+    expect(apiClient.patch).toHaveBeenCalledWith('/sites/site-1/config', {
+      banner_config: { displayMode: 'corner_popup' },
+    });
+    expect(apiClient.put).not.toHaveBeenCalled();
+  });
+});
diff --git a/apps/admin-ui/src/types/api.ts b/apps/admin-ui/src/types/api.ts
index cbe23bb..2ef4a9b 100644
--- a/apps/admin-ui/src/types/api.ts
+++ b/apps/admin-ui/src/types/api.ts
@@ -125,7 +125,7 @@ export interface SiteConfig {
   banner_config: BannerConfig | null;
   privacy_policy_url: string | null;
   terms_url: string | null;
-  default_language: string | null;
+  default_language?: string | null;
   consent_expiry_days: number;
   scan_enabled: boolean;
   scan_frequency_hours: number;