diff --git a/components/ideas/idea-detail-layout.tsx b/components/ideas/idea-detail-layout.tsx
index 7257314..d46b126 100644
--- a/components/ideas/idea-detail-layout.tsx
+++ b/components/ideas/idea-detail-layout.tsx
@@ -22,6 +22,7 @@ import { isIdeaEditable } from '@/lib/idea-status'
 import type { IdeaDto } from '@/lib/idea-dto'
 import { updateIdeaAction, archiveIdeaAction } from '@/actions/ideas'
 import { IdeaRowActions } from '@/components/ideas/idea-row-actions'
+import { IdeaMdEditor } from '@/components/ideas/idea-md-editor'
 
 const API_TO_DB: Record<IdeaStatusApi, Parameters<typeof getIdeaStatusBadge>[0]> = {
   draft: 'DRAFT',
@@ -215,7 +216,10 @@ export function IdeaDetailLayout({
         <MdSection
           kind="grill"
           markdown={grill_md}
-          editable={false /* T-511 enables this in GRILLED|PLAN_READY */}
+          // M12 grill-keuze 12: grill_md editable in GRILLED + PLAN_READY.
+          editable={
+            !isDemo && (idea.status === 'grilled' || idea.status === 'plan_ready')
+          }
           ideaId={idea.id}
         />
       )}
@@ -223,7 +227,8 @@ export function IdeaDetailLayout({
         <MdSection
           kind="plan"
           markdown={plan_md}
-          editable={false /* T-511 enables in PLAN_READY */}
+          // M12 grill-keuze 12: plan_md editable alleen in PLAN_READY.
+          editable={!isDemo && idea.status === 'plan_ready'}
           ideaId={idea.id}
         />
       )}
@@ -350,20 +355,52 @@ interface MdProps {
   ideaId: string
 }
 
-function MdSection({ kind, markdown }: MdProps) {
-  if (!markdown) {
+function MdSection({ kind, markdown, editable, ideaId }: MdProps) {
+  const [editing, setEditing] = useState(false)
+
+  if (editing) {
     return (
-      <p className="text-sm text-muted-foreground py-8 text-center italic">
-        {kind === 'grill'
-          ? 'Nog geen grill-resultaat. Klik "Grill" in de header om te starten.'
-          : 'Nog geen plan. Voltooi eerst de grill-fase en klik dan "Plan".'}
-      </p>
+      <IdeaMdEditor
+        ideaId={ideaId}
+        kind={kind}
+        initialValue={markdown ?? ''}
+        onCancel={() => setEditing(false)}
+      />
     )
   }
+
+  if (!markdown) {
+    return (
+      <div className="space-y-3 py-6">
+        <p className="text-sm text-muted-foreground text-center italic">
+          {kind === 'grill'
+            ? 'Nog geen grill-resultaat. Klik "Grill" in de header om te starten.'
+            : 'Nog geen plan. Voltooi eerst de grill-fase en klik dan "Plan".'}
+        </p>
+        {editable && (
+          <div className="flex justify-center">
+            <Button size="sm" variant="outline" onClick={() => setEditing(true)}>
+              Schrijf zelf
+            </Button>
+          </div>
+        )}
+      </div>
+    )
+  }
+
   return (
-    <pre className="rounded-md border border-input bg-surface-container p-4 text-sm whitespace-pre-wrap font-mono leading-relaxed overflow-x-auto">
-      {markdown}
-    </pre>
+    <div className="space-y-3">
+      {editable && (
+        <div className="flex justify-end">
+          <Button size="sm" variant="outline" onClick={() => setEditing(true)}>
+            Bewerk
+          </Button>
+        </div>
+      )}
+      <pre className="rounded-md border border-input bg-surface-container p-4 text-sm whitespace-pre-wrap font-mono leading-relaxed overflow-x-auto">
+        {markdown}
+      </pre>
+    </div>
   )
 }
 
diff --git a/components/ideas/idea-md-editor.tsx b/components/ideas/idea-md-editor.tsx
new file mode 100644
index 0000000..1d7c46e
--- /dev/null
+++ b/components/ideas/idea-md-editor.tsx
@@ -0,0 +1,167 @@
+'use client'
+
+// IdeaMdEditor — bewerk grill_md of plan_md.
+//
+// - kind='grill': geen yaml-validatie (vrije markdown).
+// - kind='plan' : preflight via parsePlanMd (server-side action herhaalt
+//   validation, dit is alleen UX om eerder te falen).
+//
+// Save → updateGrillMdAction / updatePlanMdAction. Cmd/Ctrl+S triggert save.
+// LocalStorage-backed draft per idea+kind, restore bij heropening.
+
+import { useEffect, useMemo, useState, useTransition } from 'react'
+import { useRouter } from 'next/navigation'
+import { Save, X } from 'lucide-react'
+import { toast } from 'sonner'
+
+import { Button } from '@/components/ui/button'
+import { Textarea } from '@/components/ui/textarea'
+import { parsePlanMd, type PlanParseError } from '@/lib/idea-plan-parser'
+import { updateGrillMdAction, updatePlanMdAction } from '@/actions/ideas'
+
+type Kind = 'grill' | 'plan'
+
+interface Props {
+  ideaId: string
+  kind: Kind
+  initialValue: string
+  onCancel: () => void
+}
+
+// Lazily compute the seed: read draft from localStorage on first render, fall
+// back to initialValue. Avoids setState-in-useEffect for hydration.
+function readSeed(draftKey: string, initialValue: string): {
+  value: string
+  restored: boolean
+} {
+  if (typeof window === 'undefined') return { value: initialValue, restored: false }
+  const draft = window.localStorage.getItem(draftKey)
+  if (draft && draft !== initialValue) return { value: draft, restored: true }
+  return { value: initialValue, restored: false }
+}
+
+export function IdeaMdEditor({ ideaId, kind, initialValue, onCancel }: Props) {
+  const router = useRouter()
+  const draftKey = `idea-md-draft-${ideaId}-${kind}`
+  const [seed] = useState(() => readSeed(draftKey, initialValue))
+  const [value, setValue] = useState(seed.value)
+  const [submitErrors, setSubmitErrors] = useState<PlanParseError[]>([])
+  const [submitting, startSubmit] = useTransition()
+
+  // Eenmalige toast voor restore — de seed is al toegepast bij mount.
+  useEffect(() => {
+    if (seed.restored) {
+      toast.info('Niet-opgeslagen wijziging hersteld uit lokale draft.')
+    }
+  }, [seed.restored])
+
+  // Auto-save naar localStorage on change.
+  useEffect(() => {
+    if (typeof window === 'undefined') return
+    if (value === initialValue) {
+      window.localStorage.removeItem(draftKey)
+    } else {
+      window.localStorage.setItem(draftKey, value)
+    }
+  }, [value, initialValue, draftKey])
+
+  // Live yaml-validatie als afgeleide state — geen useEffect nodig.
+  const validationErrors = useMemo<PlanParseError[]>(() => {
+    if (kind !== 'plan') return []
+    if (value === '' || value === initialValue) return []
+    const r = parsePlanMd(value)
+    return r.ok ? [] : r.errors
+  }, [value, initialValue, kind])
+
+  // Combine: validation errors voor live feedback, submitErrors voor server-side details.
+  const errors = submitErrors.length > 0 ? submitErrors : validationErrors
+
+  function save() {
+    if (errors.length > 0 && kind === 'plan') {
+      toast.error('Frontmatter heeft fouten — fix die eerst.')
+      return
+    }
+    setSubmitErrors([])
+    startSubmit(async () => {
+      const r =
+        kind === 'grill'
+          ? await updateGrillMdAction(ideaId, value)
+          : await updatePlanMdAction(ideaId, value)
+      if ('error' in r) {
+        toast.error(r.error)
+        if ('details' in r && Array.isArray(r.details)) {
+          setSubmitErrors(r.details as PlanParseError[])
+        }
+        return
+      }
+      toast.success('Opgeslagen')
+      window.localStorage.removeItem(draftKey)
+      router.refresh()
+      onCancel()
+    })
+  }
+
+  // Cmd/Ctrl+S → save
+  function onKeyDown(e: React.KeyboardEvent<HTMLTextAreaElement>) {
+    if ((e.metaKey || e.ctrlKey) && e.key.toLowerCase() === 's') {
+      e.preventDefault()
+      save()
+    }
+  }
+
+  const dirty = value !== initialValue
+
+  return (
+    <div className="space-y-3">
+      {errors.length > 0 && (
+        <div className="rounded-md border border-status-blocked/30 bg-status-blocked/10 p-3 space-y-1">
+          <p className="text-xs font-medium text-status-blocked">
+            {kind === 'plan' ? 'YAML-frontmatter fouten' : 'Validatiefouten'}
+          </p>
+          <ul className="text-xs text-status-blocked space-y-0.5">
+            {errors.map((err, i) => (
+              <li key={i}>
+                {err.line ? `Regel ${err.line}: ` : ''}
+                {err.message}
+              </li>
+            ))}
+          </ul>
+        </div>
+      )}
+
+      <Textarea
+        value={value}
+        onChange={(e) => setValue(e.target.value)}
+        onKeyDown={onKeyDown}
+        rows={24}
+        className="font-mono text-sm leading-relaxed"
+        placeholder={
+          kind === 'grill'
+            ? '# Idee — ...\n## Scope\n...'
+            : '---\npbi:\n  title: ...\n  priority: 2\nstories:\n  - title: ...\n---\n\n# Overwegingen\n...'
+        }
+        disabled={submitting}
+      />
+
+      <div className="flex items-center justify-between">
+        <p className="text-xs text-muted-foreground">
+          {dirty ? 'Niet-opgeslagen wijzigingen — Cmd/Ctrl+S om op te slaan' : 'Geen wijzigingen'}
+        </p>
+        <div className="flex gap-2">
+          <Button variant="outline" size="sm" onClick={onCancel} disabled={submitting}>
+            <X className="size-3.5 mr-1" />
+            Annuleer
+          </Button>
+          <Button
+            size="sm"
+            onClick={save}
+            disabled={!dirty || submitting || (errors.length > 0 && kind === 'plan')}
+          >
+            <Save className="size-3.5 mr-1" />
+            Opslaan
+          </Button>
+        </div>
+      </div>
+    </div>
+  )
+}