diff --git a/lib/hooks/promote-sidebar.react.js b/lib/hooks/promote-sidebar.react.js
--- a/lib/hooks/promote-sidebar.react.js
+++ b/lib/hooks/promote-sidebar.react.js
@@ -18,6 +18,25 @@
 import { useServerCall, useDispatchActionPromise } from '../utils/action-utils';
 import { useSelector } from '../utils/redux-utils';
 
+function canPromoteSidebar(
+  sidebarThreadInfo: ThreadInfo,
+  parentThreadInfo: ?ThreadInfo,
+): boolean {
+  const canChangeThreadType = threadHasPermission(
+    sidebarThreadInfo,
+    threadPermissions.EDIT_PERMISSIONS,
+  );
+  const canCreateSubchannelsInParent = threadHasPermission(
+    parentThreadInfo,
+    threadPermissions.CREATE_SUBCHANNELS,
+  );
+  return (
+    sidebarThreadInfo.type === threadTypes.SIDEBAR &&
+    canChangeThreadType &&
+    canCreateSubchannelsInParent
+  );
+}
+
 type PromoteSidebarType = {
   +onPromoteSidebar: () => void,
   +loading: LoadingStatus,
@@ -39,18 +58,8 @@
   const parentThreadInfo: ?ThreadInfo = useSelector(state =>
     parentThreadID ? threadInfoSelector(state)[parentThreadID] : null,
   );
-  const canChangeThreadType = threadHasPermission(
-    threadInfo,
-    threadPermissions.EDIT_PERMISSIONS,
-  );
-  const canCreateSubchannelsInParent = threadHasPermission(
-    parentThreadInfo,
-    threadPermissions.CREATE_SUBCHANNELS,
-  );
-  const canPromoteSidebar =
-    threadInfo.type === threadTypes.SIDEBAR &&
-    canChangeThreadType &&
-    canCreateSubchannelsInParent;
+
+  const canPromote = canPromoteSidebar(threadInfo, parentThreadInfo);
 
   const onClick = React.useCallback(() => {
     try {
@@ -73,12 +82,12 @@
     () => ({
       onPromoteSidebar: onClick,
       loading: loadingStatus,
-      canPromoteSidebar,
+      canPromoteSidebar: canPromote,
     }),
-    [onClick, loadingStatus, canPromoteSidebar],
+    [onClick, loadingStatus, canPromote],
   );
 
   return returnValues;
 }
 
-export { usePromoteSidebar };
+export { usePromoteSidebar, canPromoteSidebar };
diff --git a/web/modals/threads/notifications/notifications-modal.css b/web/modals/threads/notifications/notifications-modal.css
--- a/web/modals/threads/notifications/notifications-modal.css
+++ b/web/modals/threads/notifications/notifications-modal.css
@@ -3,8 +3,15 @@
   flex-direction: column;
   color: var(--fg);
   margin: 24px 32px;
-  row-gap: 40px;
+  row-gap: 20px;
   min-width: 343px;
+  width: min-content;
+}
+
+p.notice {
+  text-align: center;
+  font-size: var(--xs-font-12);
+  color: var(--notification-settings-option-color);
 }
 
 div.optionsContainer {
diff --git a/web/modals/threads/notifications/notifications-modal.react.js b/web/modals/threads/notifications/notifications-modal.react.js
--- a/web/modals/threads/notifications/notifications-modal.react.js
+++ b/web/modals/threads/notifications/notifications-modal.react.js
@@ -6,6 +6,7 @@
   updateSubscription,
   updateSubscriptionActionTypes,
 } from 'lib/actions/user-actions';
+import { canPromoteSidebar } from 'lib/hooks/promote-sidebar.react';
 import { threadInfoSelector } from 'lib/selectors/thread-selectors';
 import { threadTypes } from 'lib/types/thread-types';
 import {
@@ -91,6 +92,10 @@
   const { onClose, threadID } = props;
   const threadInfo = useSelector(state => threadInfoSelector(state)[threadID]);
   const { subscription } = threadInfo.currentUser;
+  const { parentThreadID } = threadInfo;
+  const parentThreadInfo = useSelector(state =>
+    parentThreadID ? threadInfoSelector(state)[parentThreadID] : null,
+  );
   const isSidebar = threadInfo.type === threadTypes.SIDEBAR;
 
   const initialThreadSetting = React.useMemo<NotificationSettings>(() => {
@@ -192,9 +197,45 @@
     ? 'Thread notifications'
     : 'Channel notifications';
 
-  return (
-    <Modal name={modalName} size="fit-content" onClose={onClose}>
-      <div className={css.container}>
+  let modalContent;
+  if (isSidebar && !parentThreadInfo?.currentUser.subscription.home) {
+    modalContent = (
+      <>
+        <p>
+          {"It's not possible to change the notif settings for a thread whose parent is in Background. " +
+            "That's because Comm's design always shows threads underneath their parent in the Inbox, " +
+            "which means that if a thread's parent is in Background, the thread must also be there."}
+        </p>
+        <p>
+          {canPromoteSidebar(threadInfo, parentThreadInfo)
+            ? 'If you want to change the notif settings for this thread, ' +
+              'you can either change the notif settings for the parent, ' +
+              'or you can promote the thread to a channel.'
+            : 'If you want to change the notif settings for this thread, ' +
+              "you'll have to change the notif settings for the parent."}
+        </p>
+      </>
+    );
+  } else {
+    const noticeText = isSidebar ? (
+      <>
+        <p className={css.notice}>
+          {"It's not possible to move this thread to Background. " +
+            "That's because Comm's design always shows threads underneath their parent in the Inbox, " +
+            "which means that if a thread's parent is in Focused, the thread must also be there."}
+        </p>
+        <p className={css.notice}>
+          {canPromoteSidebar(threadInfo, parentThreadInfo)
+            ? 'If you want to move this thread to Background, ' +
+              'you can either move the parent to Background, ' +
+              'or you can promote the thread to a channel.'
+            : 'If you want to move this thread to Background, ' +
+              "you'll have to move the parent to Background."}
+        </p>
+      </>
+    ) : null;
+    modalContent = (
+      <>
         <div className={css.optionsContainer}>
           {focusedItem}
           {focusedBadgeOnlyItem}
@@ -207,7 +248,14 @@
         >
           Save
         </Button>
-      </div>
+        {noticeText}
+      </>
+    );
+  }
+
+  return (
+    <Modal name={modalName} size="fit-content" onClose={onClose}>
+      <div className={css.container}>{modalContent}</div>
     </Modal>
   );
 }