diff --git a/backend/src/app.ts b/backend/src/app.ts
index 91f8af6..1454ca9 100644
--- a/backend/src/app.ts
+++ b/backend/src/app.ts
@@ -3390,7 +3390,7 @@ app.post('/api/workspace/members', requireAuth, requireWriteAccess, requireWorks
 });
 
 app.put('/api/workspace/members/:memberId', requireAuth, requireWriteAccess, requireWorkspaceRole(['owner', 'assistant']), async (req: Request, res: Response, next: NextFunction) => {
-  const parsed = z.object({ role: workspaceRoleSchema.exclude(['owner']) }).safeParse(req.body);
+  const parsed = z.object({ role: workspaceRoleSchema }).safeParse(req.body);
 
   if (!parsed.success) {
     res.status(400).json({ error: 'Invalid flock member role payload', details: parsed.error.flatten() });
@@ -3400,6 +3400,12 @@ app.put('/api/workspace/members/:memberId', requireAuth, requireWriteAccess, req
   try {
     const billingEmail = req.auth!.workspace.billing_email ? normalizeEmail(req.auth!.workspace.billing_email) : '';
     const requesterIsBillingOwner = Boolean(billingEmail && billingEmail === normalizeEmail(req.auth!.user.email));
+
+    if (parsed.data.role === 'owner' && !requesterIsBillingOwner) {
+      res.status(403).json({ error: 'Only the billing owner can promote collaborators to owner.' });
+      return;
+    }
+
     const member = await updateWorkspaceMemberRole({
       memberId: req.params.memberId,
       workspaceId: req.auth!.workspace.id,
diff --git a/backend/src/repositories/workspaceRepository.test.ts b/backend/src/repositories/workspaceRepository.test.ts
index fca4660..645c0be 100644
--- a/backend/src/repositories/workspaceRepository.test.ts
+++ b/backend/src/repositories/workspaceRepository.test.ts
@@ -415,6 +415,57 @@ test('updateWorkspaceMemberRole does not let a non-billing owner change the bill
   assert.equal(member, null);
 });
 
+test('updateWorkspaceMemberRole lets the billing owner promote a non-owner to owner', async () => {
+  const { calls } = mockDb({
+    rowCount: 1,
+    rows: [
+      {
+        id: 'member-1',
+        workspace_id: 42,
+        user_id: 'user-2',
+        invite_email: 'helper@example.com',
+        name: 'Helper',
+        role: 'owner',
+        accepted_at: '2026-04-14T00:00:00.000Z',
+        created_at: '2026-04-14T00:00:00.000Z',
+      },
+    ],
+  });
+
+  const member = await updateWorkspaceMemberRole({
+    memberId: 'member-1',
+    workspaceId: 42,
+    role: 'owner',
+    requesterMemberId: 'billing-owner',
+    requesterIsBillingOwner: true,
+    requesterRole: 'owner',
+    billingEmail: 'billing@example.com',
+  });
+
+  assert.equal(member?.role, 'owner');
+  assert.deepEqual(calls[0].params, ['member-1', 42, 'owner', true, 'billing-owner', 'billing@example.com', 'owner']);
+  assert.match(calls[0].text, /\$3 <> 'owner'/);
+});
+
+test('updateWorkspaceMemberRole does not let a non-billing owner promote a member to owner', async () => {
+  mockDb({
+    rowCount: 0,
+    rows: [],
+  });
+
+  const member = await updateWorkspaceMemberRole({
+    memberId: 'member-1',
+    workspaceId: 42,
+    role: 'owner',
+    requesterMemberId: 'non-billing-owner',
+    requesterIsBillingOwner: false,
+    requesterRole: 'owner',
+    billingEmail: 'billing@example.com',
+  });
+
+  assert.equal(member, null);
+});
+
 test('deleteWorkspaceMember removes non-owner members without billing owner access', async () => {
   const { calls } = mockDb({
     rowCount: 1,
diff --git a/backend/src/repositories/workspaceRepository.ts b/backend/src/repositories/workspaceRepository.ts
index 565bcbd..e817262 100644
--- a/backend/src/repositories/workspaceRepository.ts
+++ b/backend/src/repositories/workspaceRepository.ts
@@ -415,6 +415,10 @@ export const updateWorkspaceMemberRole = async ({
      SET role = $3
      WHERE id = $1
        AND workspace_id = $2
+       AND (
+         $3 <> 'owner'
+         OR $4 = TRUE
+       )
        AND (
          role <> 'owner'
          OR (
diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index d0ef1b0..b8b6dec 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -7925,6 +7925,7 @@ function App() {
                         })
                       }
                     >
+                      {isBillingOwner ? <option value="owner">Owner</option> : null}
                       <option value="assistant">Assistant</option>
                       <option value="caregiver">Caregiver</option>
                       <option value="viewer">Viewer</option>
@@ -7949,6 +7950,7 @@ function App() {
                         activeMembership?.role === 'owner' &&
                         member.id !== activeMembership.id &&
                         (isBillingOwner || !memberIsBillingOwner);
+                      const canPromoteToOwner = member.role !== 'owner' && isBillingOwner && member.id !== activeMembership?.id;
                       const canRemoveMember = member.role !== 'owner' || canRemoveOwner;
 
                       return (
@@ -7969,7 +7971,7 @@ function App() {
                                 removingWorkspaceMemberId === member.id
                               }
                             >
-                              {member.role === 'owner' ? <option value="owner">Owner</option> : null}
+                              {member.role === 'owner' || canPromoteToOwner ? <option value="owner">Owner</option> : null}
                               <option value="assistant">Assistant</option>
                               <option value="caregiver">Caregiver</option>
                               <option value="viewer">Viewer</option>