diff --git a/crates/ferese-layout/src/lib.rs b/crates/ferese-layout/src/lib.rs
index fa6ed9a..619b98e 100644
--- a/crates/ferese-layout/src/lib.rs
+++ b/crates/ferese-layout/src/lib.rs
@@ -2,6 +2,10 @@ use std::collections::{HashMap, HashSet};
 use std::error::Error;
 use std::fmt;
 
+mod workspace;
+
+pub use workspace::{Workspace, WorkspaceError, WorkspaceId, WorkspaceSet};
+
 const MIN_SPLIT_RATIO: f64 = 0.05;
 const MAX_SPLIT_RATIO: f64 = 0.95;
 
@@ -121,6 +125,10 @@ impl LayoutTree {
         self.windows.contains_key(&window)
     }
 
+    pub fn window_ids(&self) -> impl Iterator<Item = WindowId> + '_ {
+        self.windows.keys().copied()
+    }
+
     pub fn insert(
         &mut self,
         window: WindowId,
diff --git a/crates/ferese-layout/src/workspace.rs b/crates/ferese-layout/src/workspace.rs
new file mode 100644
index 0000000..cc9ed3b
--- /dev/null
+++ b/crates/ferese-layout/src/workspace.rs
@@ -0,0 +1,363 @@
+use std::collections::{HashMap, HashSet};
+use std::error::Error;
+use std::fmt;
+
+use crate::{Axis, LayoutError, LayoutTree, WindowId};
+
+#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
+pub struct WorkspaceId(pub u64);
+
+#[derive(Debug)]
+pub struct Workspace {
+    pub id: WorkspaceId,
+    pub name: String,
+    pub layout: LayoutTree,
+    pub last_focused: Option<WindowId>,
+}
+
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub enum WorkspaceError {
+    InvalidNumericName(u32),
+    UnknownWorkspace(WorkspaceId),
+    DuplicateWindow(WindowId),
+    InvalidState(&'static str),
+    Layout(LayoutError),
+}
+
+impl fmt::Display for WorkspaceError {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::InvalidNumericName(index) => {
+                write!(formatter, "invalid workspace number {index}")
+            }
+            Self::UnknownWorkspace(workspace) => {
+                write!(formatter, "unknown workspace {workspace:?}")
+            }
+            Self::DuplicateWindow(window) => write!(formatter, "duplicate window {window:?}"),
+            Self::InvalidState(reason) => write!(formatter, "invalid workspace state: {reason}"),
+            Self::Layout(error) => error.fmt(formatter),
+        }
+    }
+}
+
+impl Error for WorkspaceError {
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
+        match self {
+            Self::Layout(error) => Some(error),
+            _ => None,
+        }
+    }
+}
+
+impl From<LayoutError> for WorkspaceError {
+    fn from(error: LayoutError) -> Self {
+        Self::Layout(error)
+    }
+}
+
+#[derive(Debug)]
+pub struct WorkspaceSet {
+    active: WorkspaceId,
+    workspaces: HashMap<WorkspaceId, Workspace>,
+    names: HashMap<String, WorkspaceId>,
+    window_workspaces: HashMap<WindowId, WorkspaceId>,
+    next_id: u64,
+}
+
+impl Default for WorkspaceSet {
+    fn default() -> Self {
+        let active = WorkspaceId(1);
+        let workspace = Workspace {
+            id: active,
+            name: "1".to_owned(),
+            layout: LayoutTree::default(),
+            last_focused: None,
+        };
+        let workspaces = HashMap::from([(active, workspace)]);
+        let names = HashMap::from([("1".to_owned(), active)]);
+
+        Self {
+            active,
+            workspaces,
+            names,
+            window_workspaces: HashMap::new(),
+            next_id: 2,
+        }
+    }
+}
+
+impl WorkspaceSet {
+    pub fn active_id(&self) -> WorkspaceId {
+        self.active
+    }
+
+    pub fn active(&self) -> &Workspace {
+        self.workspaces
+            .get(&self.active)
+            .expect("active workspace always exists")
+    }
+
+    pub fn active_mut(&mut self) -> &mut Workspace {
+        self.workspaces
+            .get_mut(&self.active)
+            .expect("active workspace always exists")
+    }
+
+    pub fn workspace(&self, id: WorkspaceId) -> Option<&Workspace> {
+        self.workspaces.get(&id)
+    }
+
+    pub fn workspace_for_window(&self, window: WindowId) -> Option<WorkspaceId> {
+        self.window_workspaces.get(&window).copied()
+    }
+
+    pub fn ensure_numeric(&mut self, index: u32) -> Result<WorkspaceId, WorkspaceError> {
+        if index == 0 {
+            return Err(WorkspaceError::InvalidNumericName(index));
+        }
+
+        let name = index.to_string();
+
+        if let Some(id) = self.names.get(&name) {
+            return Ok(*id);
+        }
+
+        let id = WorkspaceId(self.next_id);
+        self.next_id += 1;
+        self.names.insert(name.clone(), id);
+        self.workspaces.insert(
+            id,
+            Workspace {
+                id,
+                name,
+                layout: LayoutTree::default(),
+                last_focused: None,
+            },
+        );
+
+        debug_assert!(self.validate().is_ok());
+        Ok(id)
+    }
+
+    pub fn switch_to_numeric(&mut self, index: u32) -> Result<Option<WindowId>, WorkspaceError> {
+        self.active = self.ensure_numeric(index)?;
+
+        debug_assert!(self.validate().is_ok());
+        Ok(self.active().last_focused)
+    }
+
+    pub fn insert_window(
+        &mut self,
+        window: WindowId,
+        axis: Axis,
+        ratio: f64,
+    ) -> Result<(), WorkspaceError> {
+        if self.window_workspaces.contains_key(&window) {
+            return Err(WorkspaceError::DuplicateWindow(window));
+        }
+
+        let workspace = self.active_mut();
+        workspace
+            .layout
+            .insert(window, workspace.last_focused, axis, ratio)?;
+        workspace.last_focused = Some(window);
+        self.window_workspaces.insert(window, self.active);
+
+        debug_assert!(self.validate().is_ok());
+        Ok(())
+    }
+
+    pub fn remove_window(&mut self, window: WindowId) -> Result<(), WorkspaceError> {
+        let workspace_id = self
+            .window_workspaces
+            .remove(&window)
+            .ok_or(LayoutError::UnknownWindow(window))?;
+        let workspace = self
+            .workspaces
+            .get_mut(&workspace_id)
+            .ok_or(WorkspaceError::UnknownWorkspace(workspace_id))?;
+
+        workspace.layout.remove(window)?;
+
+        if workspace.last_focused == Some(window) {
+            workspace.last_focused = first_window(&workspace.layout);
+        }
+
+        debug_assert!(self.validate().is_ok());
+        Ok(())
+    }
+
+    pub fn move_window_to_numeric(
+        &mut self,
+        window: WindowId,
+        index: u32,
+        axis: Axis,
+        ratio: f64,
+    ) -> Result<WorkspaceId, WorkspaceError> {
+        let source_id = self
+            .workspace_for_window(window)
+            .ok_or(LayoutError::UnknownWindow(window))?;
+        let destination_id = self.ensure_numeric(index)?;
+
+        if source_id == destination_id {
+            return Ok(destination_id);
+        }
+
+        let source = self
+            .workspaces
+            .get_mut(&source_id)
+            .ok_or(WorkspaceError::UnknownWorkspace(source_id))?;
+        source.layout.remove(window)?;
+
+        if source.last_focused == Some(window) {
+            source.last_focused = first_window(&source.layout);
+        }
+
+        let destination = self
+            .workspaces
+            .get_mut(&destination_id)
+            .ok_or(WorkspaceError::UnknownWorkspace(destination_id))?;
+        destination
+            .layout
+            .insert(window, destination.last_focused, axis, ratio)?;
+        destination.last_focused = Some(window);
+        self.window_workspaces.insert(window, destination_id);
+
+        debug_assert!(self.validate().is_ok());
+        Ok(destination_id)
+    }
+
+    pub fn validate(&self) -> Result<(), WorkspaceError> {
+        if !self.workspaces.contains_key(&self.active) {
+            return Err(WorkspaceError::InvalidState("active workspace is missing"));
+        }
+        if self.names.len() != self.workspaces.len() {
+            return Err(WorkspaceError::InvalidState(
+                "workspace name index is inconsistent",
+            ));
+        }
+
+        let mut seen_windows = HashSet::new();
+
+        for (id, workspace) in &self.workspaces {
+            if workspace.id != *id || self.names.get(&workspace.name) != Some(id) {
+                return Err(WorkspaceError::InvalidState(
+                    "workspace index is inconsistent",
+                ));
+            }
+
+            workspace.layout.validate()?;
+
+            for window in workspace.layout.window_ids() {
+                if !seen_windows.insert(window) || self.window_workspaces.get(&window) != Some(id) {
+                    return Err(WorkspaceError::InvalidState(
+                        "window ownership is inconsistent",
+                    ));
+                }
+            }
+
+            if workspace
+                .last_focused
+                .is_some_and(|window| !workspace.layout.contains(window))
+            {
+                return Err(WorkspaceError::InvalidState(
+                    "workspace focus is inconsistent",
+                ));
+            }
+        }
+
+        if seen_windows.len() != self.window_workspaces.len() {
+            return Err(WorkspaceError::InvalidState(
+                "window index retains stale entries",
+            ));
+        }
+
+        Ok(())
+    }
+}
+
+fn first_window(layout: &LayoutTree) -> Option<WindowId> {
+    layout.window_ids().min_by_key(|window| window.0)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn numeric_workspaces_are_created_lazily_with_stable_ids() {
+        let mut workspaces = WorkspaceSet::default();
+        let first = workspaces.active_id();
+        let ninth = workspaces.ensure_numeric(9).unwrap();
+
+        assert_eq!(workspaces.ensure_numeric(1), Ok(first));
+        assert_eq!(workspaces.ensure_numeric(9), Ok(ninth));
+        assert_ne!(first, ninth);
+        assert!(workspaces.validate().is_ok());
+    }
+
+    #[test]
+    fn switching_restores_workspace_focus() {
+        let mut workspaces = WorkspaceSet::default();
+        workspaces
+            .insert_window(WindowId(1), Axis::Horizontal, 0.5)
+            .unwrap();
+
+        assert_eq!(workspaces.switch_to_numeric(2).unwrap(), None);
+        workspaces
+            .insert_window(WindowId(2), Axis::Horizontal, 0.5)
+            .unwrap();
+
+        assert_eq!(workspaces.switch_to_numeric(1).unwrap(), Some(WindowId(1)));
+        assert_eq!(workspaces.switch_to_numeric(2).unwrap(), Some(WindowId(2)));
+        assert!(workspaces.validate().is_ok());
+    }
+
+    #[test]
+    fn moving_a_window_updates_membership_without_switching() {
+        let mut workspaces = WorkspaceSet::default();
+        workspaces
+            .insert_window(WindowId(1), Axis::Horizontal, 0.5)
+            .unwrap();
+        workspaces
+            .insert_window(WindowId(2), Axis::Horizontal, 0.5)
+            .unwrap();
+        let first = workspaces.active_id();
+        let second = workspaces
+            .move_window_to_numeric(WindowId(2), 2, Axis::Horizontal, 0.5)
+            .unwrap();
+
+        assert_eq!(workspaces.active_id(), first);
+        assert_eq!(workspaces.workspace_for_window(WindowId(2)), Some(second));
+        assert!(
+            !workspaces
+                .workspace(first)
+                .unwrap()
+                .layout
+                .contains(WindowId(2))
+        );
+        assert!(
+            workspaces
+                .workspace(second)
+                .unwrap()
+                .layout
+                .contains(WindowId(2))
+        );
+        assert!(workspaces.validate().is_ok());
+    }
+
+    #[test]
+    fn removing_focus_chooses_a_deterministic_remaining_window() {
+        let mut workspaces = WorkspaceSet::default();
+        workspaces
+            .insert_window(WindowId(2), Axis::Horizontal, 0.5)
+            .unwrap();
+        workspaces
+            .insert_window(WindowId(1), Axis::Horizontal, 0.5)
+            .unwrap();
+        workspaces.remove_window(WindowId(1)).unwrap();
+
+        assert_eq!(workspaces.active().last_focused, Some(WindowId(2)));
+        assert!(workspaces.validate().is_ok());
+    }
+}