diff --git a/src/base.rs b/src/base.rs
index b2299773..01731df6 100644
--- a/src/base.rs
+++ b/src/base.rs
@@ -194,8 +194,8 @@ pub enum MessageAction {
 pub enum SpaceAction {
     /// Add a room or update metadata.
     SetChild {
-        /// The room ID, alias, or a user whose DM room should be added to the space.
-        child: String,
+        /// The room that should be added to the space.
+        child: OwnedRoomOrAliasId,
         /// The order parameter to use when sorting children in the space.
         order: Option<String>,
         /// Whether the room is suggested.
diff --git a/src/commands.rs b/src/commands.rs
index 0047c481..5c7bf381 100644
--- a/src/commands.rs
+++ b/src/commands.rs
@@ -930,10 +930,13 @@ fn iamb_space(desc: CommandDescription, ctx: &mut ProgContext) -> ProgResult {
                 }
             }
 
-            let Some(child) = raw_child else {
+            let child = raw_child.ok_or_else(|| {
                 let msg = "Must specify a room to add";
-                return Err(CommandError::Error(msg.into()));
-            };
+                CommandError::Error(msg.into())
+            })?;
+            let child = OwnedRoomOrAliasId::from_str(&child).map_err(|e| {
+                CommandError::Error(format!("{child:?} is not a valid room identifier: {e}"))
+            })?;
 
             SpaceAction::SetChild { child, order, suggested }.into()
         },
@@ -1539,7 +1542,7 @@ mod tests {
         let cmd = "space child set !roomid:example.org";
         let res = cmds.input_cmd(cmd, ctx.clone()).unwrap();
         let act = SpaceAction::SetChild {
-            child: "!roomid:example.org".to_owned(),
+            child: owned_room_id!("!roomid:example.org").into(),
             order: None,
             suggested: false,
         };
@@ -1548,7 +1551,7 @@ mod tests {
         let cmd = "space child set ++order=abcd ++suggested !roomid:example.org";
         let res = cmds.input_cmd(cmd, ctx.clone()).unwrap();
         let act = SpaceAction::SetChild {
-            child: "!roomid:example.org".to_owned(),
+            child: owned_room_id!("!roomid:example.org").into(),
             order: Some("abcd".into()),
             suggested: true,
         };
diff --git a/src/completions.rs b/src/completions.rs
index 3547c82d..2bd910e6 100644
--- a/src/completions.rs
+++ b/src/completions.rs
@@ -295,6 +295,16 @@ fn complete_matrix_names(input: &str, store: &ChatStore) -> Vec<String> {
     store.rooms.complete(input).into_iter().map(|i| i.to_string()).collect()
 }
 
+/// Tab completion for known room aliases and ids.
+fn complete_room_alias_or_id(input: &str, store: &ChatStore) -> Vec<String> {
+    let list = store.names.complete(input);
+    if !list.is_empty() {
+        return list;
+    }
+
+    store.rooms.complete(input).into_iter().map(|i| i.to_string()).collect()
+}
+
 /// Tab completion for open verification requests
 fn complete_verification(input: &str, store: &ChatStore) -> Vec<String> {
     store.verifications.complete(input)
@@ -500,13 +510,13 @@ fn complete_iamb_space(args: Vec<String>, store: &ChatStore) -> Vec<String> {
         if arg.is_empty() {
             let mut opts = complete_options(args.as_slice(), &options);
             if !has_room {
-                opts.extend(complete_matrix_names(arg, store));
+                opts.extend(complete_room_alias_or_id(arg, store));
             }
             opts
         } else if arg.starts_with('+') {
             complete_options(args.as_slice(), &options)
         } else if !has_room {
-            complete_matrix_names(arg, store)
+            complete_room_alias_or_id(arg, store)
         } else {
             vec![]
         }
diff --git a/src/windows/room/space.rs b/src/windows/room/space.rs
index 6a652fc8..e3d8476d 100644
--- a/src/windows/room/space.rs
+++ b/src/windows/room/space.rs
@@ -105,9 +105,26 @@ impl SpaceState {
                     return Err(IambError::InsufficientPermission.into());
                 }
 
-                let child_id = store.application.worker.join_room(child)?;
+                let (child_id, via) = match OwnedRoomId::try_from(child) {
+                    Ok(room_id) => {
+                        // assume the new child is reachable the same way as the parent
+                        let via = self.room.route().await.map_err(IambError::from)?;
+
+                        (room_id, via)
+                    },
+                    Err(alias) => {
+                        let resp = store
+                            .application
+                            .worker
+                            .client
+                            .resolve_room_alias(&alias)
+                            .await
+                            .map_err(IambError::from)?;
+
+                        (resp.room_id, resp.servers)
+                    },
+                };
 
-                let via = self.room.route().await.map_err(IambError::from)?;
                 let mut ev = SpaceChildEventContent::new(via);
                 ev.order = order
                     .as_deref()