diff --git a/src/app/components/LivePreview.module.css b/src/app/components/LivePreview.module.css
index f154807..b861919 100644
--- a/src/app/components/LivePreview.module.css
+++ b/src/app/components/LivePreview.module.css
@@ -18,6 +18,7 @@
 }
 
 .body {
+  position: relative; /* establishes stacking context for the busy overlay */
   flex: 1 1 auto;
   min-height: 0;
   overflow: auto;
@@ -88,3 +89,46 @@
   white-space: pre-wrap;
   word-break: break-word;
 }
+
+/*
+ * Busy overlay — non-blocking; floats over the chart body only (arch §10.2;
+ * spec §04/§10). position:absolute relative to .body's position:relative.
+ * pointer-events:none ensures the editor and other controls stay fully
+ * interactive while the overlay is visible.
+ *
+ * The spinner animation is a pure CSS transform-based rotation so it respects
+ * the global prefers-reduced-motion rule in base.css (animation-duration → 0.01ms).
+ */
+.busyOverlay {
+  position: absolute;
+  inset: 0;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: var(--space-3);
+  background: color-mix(in srgb, var(--bg) 75%, transparent);
+  pointer-events: none;
+}
+
+.busySpinner {
+  display: block;
+  width: var(--icon-lg);
+  height: var(--icon-lg);
+  border: 2px solid var(--border-strong);
+  border-top-color: var(--accent);
+  border-radius: 50%;
+  animation: busySpin 600ms linear infinite;
+}
+
+.busyLabel {
+  font-size: 12px;
+  color: var(--text-secondary);
+  line-height: 1.4;
+}
+
+@keyframes busySpin {
+  to {
+    transform: rotate(360deg);
+  }
+}
diff --git a/src/app/components/LivePreview.test.tsx b/src/app/components/LivePreview.test.tsx
new file mode 100644
index 0000000..71d8182
--- /dev/null
+++ b/src/app/components/LivePreview.test.tsx
@@ -0,0 +1,87 @@
+/**
+ * LivePreview — busy overlay guard (spec §04; arch §10.2).
+ *
+ * The render pipeline is integration-heavy (vega-embed, IndexedDB, Monaco); the
+ * busy OVERLAY itself is purely a function of `PreviewStore.busy`. These tests
+ * set that flag directly and assert the DOM result — no timing, no mocking of the
+ * async render path.
+ */
+
+import { afterEach, beforeEach, describe, expect, test, vi } from 'vitest';
+import { act } from 'react';
+import { createRoot, type Root } from 'react-dom/client';
+import { usePreviewStore } from '../stores/PreviewStore';
+import { useSnippetStore } from '../stores/SnippetStore';
+import { useDatasetStore } from '../stores/DatasetStore';
+import { LivePreview } from './LivePreview';
+
+// Heavy services and sub-components the LivePreview wires — not under test here.
+vi.mock('../services/chart-renderer', () => ({
+  renderSpec: () => Promise.resolve({ destroy() {}, resize() {} }),
+}));
+vi.mock('./SettingsPopover', () => ({
+  SettingsPopover: () => null,
+  SettingRow: () => null,
+  RangeControl: () => null,
+}));
+
+(globalThis as { IS_REACT_ACT_ENVIRONMENT?: boolean }).IS_REACT_ACT_ENVIRONMENT = true;
+
+let container: HTMLDivElement;
+let root: Root;
+
+beforeEach(() => {
+  usePreviewStore.setState({ error: null, busy: false });
+  useSnippetStore.getState().reset();
+  useDatasetStore.getState().reset();
+
+  container = document.createElement('div');
+  document.body.appendChild(container);
+  root = createRoot(container);
+  act(() => root.render(<LivePreview />));
+});
+
+afterEach(() => {
+  act(() => root.unmount());
+  container.remove();
+  usePreviewStore.setState({ error: null, busy: false });
+  vi.clearAllMocks();
+});
+
+describe('LivePreview busy overlay', () => {
+  test('does not render the busy overlay when busy=false', () => {
+    // The overlay element should not be in the DOM at all during normal operation.
+    expect(container.querySelector('[aria-hidden="true"]')).toBeNull();
+  });
+
+  test('renders the busy overlay when PreviewStore.busy=true', () => {
+    act(() => usePreviewStore.setState({ busy: true }));
+    const overlay = container.querySelector('[aria-hidden="true"]');
+    expect(overlay).not.toBeNull();
+  });
+
+  test('overlay carries a visible label for sighted users', () => {
+    act(() => usePreviewStore.setState({ busy: true }));
+    const label = container.querySelector('[aria-hidden="true"]')?.textContent;
+    expect(label).toMatch(/rendering/i);
+  });
+
+  test('the preview body carries aria-busy=true when busy', () => {
+    act(() => usePreviewStore.setState({ busy: true }));
+    // The body element has aria-busy when the store says busy.
+    const busyEl = container.querySelector('[aria-busy="true"]');
+    expect(busyEl).not.toBeNull();
+  });
+
+  test('aria-busy is absent when busy=false (no aria-busy="false" noise)', () => {
+    // aria-busy="false" is technically valid but needlessly verbose; we omit it.
+    expect(container.querySelector('[aria-busy]')).toBeNull();
+  });
+
+  test('overlay disappears when busy returns to false', () => {
+    act(() => usePreviewStore.setState({ busy: true }));
+    expect(container.querySelector('[aria-hidden="true"]')).not.toBeNull();
+    act(() => usePreviewStore.setState({ busy: false }));
+    expect(container.querySelector('[aria-hidden="true"]')).toBeNull();
+  });
+});
diff --git a/src/app/components/LivePreview.tsx b/src/app/components/LivePreview.tsx
index 3f2396c..1aa2367 100644
--- a/src/app/components/LivePreview.tsx
+++ b/src/app/components/LivePreview.tsx
@@ -110,11 +110,14 @@ export function LivePreview() {
   const lastLoadRef = useRef({ bufferEpoch: -1, editorView });
   const error = usePreviewStore((s) => s.error);
   const setError = usePreviewStore((s) => s.setError);
+  const busy = usePreviewStore((s) => s.busy);
+  const setBusy = usePreviewStore((s) => s.setBusy);
+
+  // Busy-indication timer ref: if a render exceeds ~1s we surface a non-blocking
+  // overlay (arch §10.2 NN/g: >1s owes a busy indication; <1s shows nothing to
+  // avoid flicker on typical fast renders).
+  const busyTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null);
 
-  // TODO (council backfill, doc §10.2): a render that exceeds ~1s owes a
-  // non-blocking busy indication (overlay + aria-busy), gated by a threshold so
-  // sub-1s renders show nothing. Deferred — it pairs with the heavier M3 dataset
-  // renders the budget flags; today's inline-data renders are effectively instant.
   useEffect(() => {
     const node = hostRef.current;
     if (!node) return;
@@ -147,6 +150,25 @@ export function LivePreview() {
         }
 
         const mine = ++generationRef.current;
+
+        // Arm the busy indicator: if the render hasn't settled within ~1s, flip the
+        // flag. Sub-1s renders (the common case) never show the overlay — no flicker
+        // (arch §10.2; NN/g ≤1s = uninterrupted thought, >1s = noticeably waiting).
+        if (busyTimerRef.current !== null) clearTimeout(busyTimerRef.current);
+        busyTimerRef.current = setTimeout(() => {
+          // Only arm for the current generation; a superseded render doesn't own busy.
+          if (mine === generationRef.current) setBusy(true);
+        }, 1000);
+
+        /** Clear busy and the timer unconditionally — called on settle or error. */
+        const clearBusy = () => {
+          if (busyTimerRef.current !== null) {
+            clearTimeout(busyTimerRef.current);
+            busyTimerRef.current = null;
+          }
+          setBusy(false);
+        };
+
         try {
           const prepared = prepareSpecForRender(parsed, { fitMode, datasets });
           const config = chartConfigFor(uiTheme);
@@ -165,8 +187,10 @@ export function LivePreview() {
           }
           handleRef.current = handle;
           setError(null);
+          clearBusy();
         } catch (e) {
           if (mine === generationRef.current) {
+            clearBusy();
             // A missing dataset reference is not a JSON/spec problem, so it gets a
             // tailored, fixable message instead of the generic syntax hint (council:
             // GOV.UK error-message + NN/g #9 — name the problem, give the real fix).
@@ -187,7 +211,17 @@ export function LivePreview() {
     }, delay);
 
     return () => clearTimeout(timer);
-  }, [shownText, fitMode, uiTheme, datasets, setError, bufferEpoch, editorView, renderDebounce]);
+  }, [
+    shownText,
+    fitMode,
+    uiTheme,
+    datasets,
+    setError,
+    setBusy,
+    bufferEpoch,
+    editorView,
+    renderDebounce,
+  ]);
 
   // Re-fit the chart when its container resizes (e.g. a pane drag). Vega doesn't
   // observe the element, so we do: one observer on the stable host node for the
@@ -205,13 +239,15 @@ export function LivePreview() {
     return () => ro.disconnect();
   }, []);
 
-  // Finalize the live view on unmount, and clear the shared error so a stale
-  // message never outlives this pane.
+  // Finalize the live view on unmount, and clear the shared error + busy state so
+  // stale transient state never outlives this pane.
   useEffect(
     () => () => {
       handleRef.current?.destroy();
       handleRef.current = null;
+      if (busyTimerRef.current !== null) clearTimeout(busyTimerRef.current);
       usePreviewStore.getState().setError(null);
+      usePreviewStore.getState().setBusy(false);
     },
     [],
   );
@@ -222,7 +258,12 @@ export function LivePreview() {
         <FitControl />
         <PreviewSettings />
       </div>
-      <div className={styles.body}>
+      {/*
+       * aria-busy on the chart region tells AT the area is being updated (arch §10.2;
+       * spec §04). The overlay is a non-blocking sibling inside the relative-positioned
+       * body; it never covers the header or editor, so editing stays fully interactive.
+       */}
+      <div className={styles.body} aria-busy={busy || undefined}>
         {/* Frame is React-owned and carries the fit-sizing class; the inner host
             is owned by vega-embed (it brands it `.vega-embed` and mutates its
             classList at runtime), so its className stays static and React never
@@ -234,6 +275,18 @@ export function LivePreview() {
             editor pane's role="alert" (one producer, two subscribers; doc §10.1),
             so adding one here would double-announce it. */}
         {error !== null && <pre className={styles.error}>{error}</pre>}
+        {/*
+         * Busy overlay: non-blocking, overlays only the chart body, never the header
+         * or the editor (arch §10.2; spec §04/§10). Shown only after the ~1s threshold
+         * so sub-1s renders produce no flicker. The spinner animation is suppressed
+         * under prefers-reduced-motion (base.css *{animation-duration:0.01ms}).
+         */}
+        {busy && (
+          <div className={styles.busyOverlay} aria-hidden="true">
+            <span className={styles.busySpinner} />
+            <span className={styles.busyLabel}>Rendering…</span>
+          </div>
+        )}
       </div>
     </div>
   );
diff --git a/src/app/stores/PreviewStore.test.ts b/src/app/stores/PreviewStore.test.ts
new file mode 100644
index 0000000..ed5d361
--- /dev/null
+++ b/src/app/stores/PreviewStore.test.ts
@@ -0,0 +1,54 @@
+import { afterEach, describe, expect, it } from 'vitest';
+import { usePreviewStore } from './PreviewStore';
+
+const store = () => usePreviewStore.getState();
+
+afterEach(() => {
+  // Reset to a known clean state between tests so store leaks don't affect order.
+  store().setError(null);
+  store().setBusy(false);
+});
+
+describe('PreviewStore — error slice', () => {
+  it('starts with null error', () => {
+    expect(store().error).toBeNull();
+  });
+
+  it('setError stores the provided message', () => {
+    store().setError('Rendering error: something went wrong.');
+    expect(store().error).toBe('Rendering error: something went wrong.');
+  });
+
+  it('setError(null) clears the message', () => {
+    store().setError('an error');
+    store().setError(null);
+    expect(store().error).toBeNull();
+  });
+});
+
+describe('PreviewStore — busy slice', () => {
+  it('starts with busy=false', () => {
+    expect(store().busy).toBe(false);
+  });
+
+  it('setBusy(true) sets busy to true', () => {
+    store().setBusy(true);
+    expect(store().busy).toBe(true);
+  });
+
+  it('setBusy(false) clears busy', () => {
+    store().setBusy(true);
+    store().setBusy(false);
+    expect(store().busy).toBe(false);
+  });
+
+  it('busy and error are independent — setting one does not affect the other', () => {
+    store().setBusy(true);
+    store().setError('some error');
+    expect(store().busy).toBe(true);
+    expect(store().error).toBe('some error');
+
+    store().setBusy(false);
+    expect(store().error).toBe('some error'); // error unchanged by clearing busy
+  });
+});
diff --git a/src/app/stores/PreviewStore.ts b/src/app/stores/PreviewStore.ts
index 1288f5b..a983215 100644
--- a/src/app/stores/PreviewStore.ts
+++ b/src/app/stores/PreviewStore.ts
@@ -9,6 +9,11 @@
  * Live Preview is the single producer; it writes the current error here and both
  * panes subscribe. `null` means the current spec rendered cleanly (or is blank).
  *
+ * `busy` tracks whether a render is in flight long enough to warrant a visible
+ * indicator (arch §10.2: >~1s owes a non-blocking busy overlay). LivePreview arms
+ * a 1 s timer when a render starts and sets `busy = true` only if the render has
+ * not settled by then; it clears `busy` on settle or error regardless.
+ *
  * Kept as its own tiny store rather than folded into the SnippetStore: this is
  * transient render state, not durable domain data, and it must not be persisted.
  */
@@ -20,9 +25,19 @@ export interface PreviewState {
   error: string | null;
   /** Set (or clear) the current render error. */
   setError: (error: string | null) => void;
+  /**
+   * True while a render has been in flight for longer than the ~1s NN/g threshold
+   * (arch §10.2). The LivePreview overlay reads this to show a non-blocking busy
+   * indication; aria-busy on the preview region mirrors it.
+   */
+  busy: boolean;
+  /** Set or clear the busy flag. */
+  setBusy: (busy: boolean) => void;
 }
 
 export const usePreviewStore = create<PreviewState>((set) => ({
   error: null,
   setError: (error) => set({ error }),
+  busy: false,
+  setBusy: (busy) => set({ busy }),
 }));