diff --git a/frontend/src/components/PlanOutlineNav.jsx b/frontend/src/components/PlanOutlineNav.jsx
index 761d48b..c61bd84 100644
--- a/frontend/src/components/PlanOutlineNav.jsx
+++ b/frontend/src/components/PlanOutlineNav.jsx
@@ -1,18 +1,84 @@
 import { NavLink, useLocation } from 'react-router-dom'
-import { useEffect, useState } from 'react'
+import { useEffect, useMemo, useState } from 'react'
+import { listInitiativeProjects } from '../api/projects.js'
 import { listInitiativeActions } from '../api/initiatives.js'
 import { listInitiativeBacklog } from '../api/backlog.js'
 import { useProgramScope } from '../context/ProgramScopeContext.jsx'
+import { buildProjectsByParent } from '../utils/projectTree.js'
+import { projectPath } from '../utils/routes.js'
 import {
   PLAN_OUTLINE_NODES,
   resolvePlanOutlineActiveKey,
+  resolvePlanOutlineProjectId,
 } from '../plan/planOutlineNodes.js'
 
+function PlanOutlineProjectTree({ projects, parentId, activeProjectId, depth = 0 }) {
+  const byParent = useMemo(() => buildProjectsByParent(projects), [projects])
+  const nodes = byParent.get(parentId || '') || []
+
+  if (nodes.length === 0) {
+    return null
+  }
+
+  return (
+    <ul className="analysis-split__nav-sublist">
+      {nodes.map((project) => (
+        <li key={project.id}>
+          <NavLink
+            to={projectPath(project.id)}
+            className={({ isActive }) =>
+              'analysis-split__nav-link analysis-split__nav-link--nested' +
+              (isActive || activeProjectId === project.id
+                ? ' analysis-split__nav-link--active'
+                : '')
+            }
+          >
+            {project.title}
+          </NavLink>
+          <PlanOutlineProjectTree
+            projects={projects}
+            parentId={project.id}
+            activeProjectId={activeProjectId}
+            depth={depth + 1}
+          />
+        </li>
+      ))}
+    </ul>
+  )
+}
+
 export function PlanOutlineNav() {
   const location = useLocation()
   const { initiativeId, initiativeTitle, hrefWithScope } = useProgramScope()
   const activeKey = resolvePlanOutlineActiveKey(location.pathname)
+  const activeProjectId = resolvePlanOutlineProjectId(location.pathname)
   const [counts, setCounts] = useState({ inbox: null, work: null })
+  const [projects, setProjects] = useState([])
+  const [structureOpen, setStructureOpen] = useState(
+    activeKey === 'structure' || Boolean(activeProjectId),
+  )
+
+  useEffect(() => {
+    setStructureOpen(activeKey === 'structure' || Boolean(activeProjectId))
+  }, [activeKey, activeProjectId])
+
+  useEffect(() => {
+    if (!initiativeId) {
+      setProjects([])
+      return undefined
+    }
+    let cancelled = false
+    listInitiativeProjects(initiativeId)
+      .then((items) => {
+        if (!cancelled) setProjects(Array.isArray(items) ? items : [])
+      })
+      .catch(() => {
+        if (!cancelled) setProjects([])
+      })
+    return () => {
+      cancelled = true
+    }
+  }, [initiativeId])
 
   useEffect(() => {
     if (!initiativeId) {
@@ -44,6 +110,9 @@ export function PlanOutlineNav() {
     if (node.key === 'work' && counts.work != null) {
       return `${node.label} (${counts.work})`
     }
+    if (node.key === 'structure' && projects.length > 0) {
+      return `${node.label} (${projects.length})`
+    }
     return node.label
   }
 
@@ -66,6 +135,8 @@ export function PlanOutlineNav() {
         {PLAN_OUTLINE_NODES.map((node) => {
           const needsInitiative = node.requiresInitiative && !initiativeId
           const isDisabled = node.disabled || needsInitiative
+          const isStructure = node.key === 'structure'
+          const hasProjectTree = isStructure && projects.length > 0 && initiativeId
 
           if (isDisabled) {
             return (
@@ -86,16 +157,38 @@ export function PlanOutlineNav() {
 
           return (
             <li key={node.key}>
-              <NavLink
-                to={hrefWithScope(node.to)}
-                end
-                className={({ isActive }) =>
-                  'analysis-split__nav-link' +
-                  (isActive ? ' analysis-split__nav-link--active' : '')
-                }
-              >
-                {nodeLabel(node)}
-              </NavLink>
+              <div className="analysis-split__nav-row">
+                {hasProjectTree && (
+                  <button
+                    type="button"
+                    className="analysis-split__nav-toggle"
+                    aria-expanded={structureOpen}
+                    aria-label={structureOpen ? 'Struktur einklappen' : 'Struktur ausklappen'}
+                    onClick={() => setStructureOpen((open) => !open)}
+                  >
+                    {structureOpen ? '▾' : '▸'}
+                  </button>
+                )}
+                <NavLink
+                  to={hrefWithScope(node.to)}
+                  end
+                  className={({ isActive }) =>
+                    'analysis-split__nav-link' +
+                    (isActive || (isStructure && activeProjectId)
+                      ? ' analysis-split__nav-link--active'
+                      : '')
+                  }
+                >
+                  {nodeLabel(node)}
+                </NavLink>
+              </div>
+              {hasProjectTree && structureOpen && (
+                <PlanOutlineProjectTree
+                  projects={projects}
+                  parentId=""
+                  activeProjectId={activeProjectId}
+                />
+              )}
             </li>
           )
         })}
diff --git a/frontend/src/plan/planOutlineNodes.js b/frontend/src/plan/planOutlineNodes.js
index 0ac24a1..c9d1a9b 100644
--- a/frontend/src/plan/planOutlineNodes.js
+++ b/frontend/src/plan/planOutlineNodes.js
@@ -20,8 +20,20 @@ export const PLAN_OUTLINE_NODES = [
  */
 export function resolvePlanOutlineActiveKey(pathname) {
   const path = pathname.replace(/\/$/, '') || pathname
+  if (path.startsWith('/projects/')) {
+    return 'structure'
+  }
   const match = PLAN_OUTLINE_NODES.find(
     (node) => path === node.to || path.startsWith(`${node.to}/`),
   )
   return match?.key ?? null
 }
+
+/**
+ * @param {string} pathname
+ * @returns {string | null}
+ */
+export function resolvePlanOutlineProjectId(pathname) {
+  const match = (pathname || '').match(/^\/projects\/([^/?]+)/)
+  return match?.[1] ?? null
+}
diff --git a/frontend/src/plan/planOutlineNodes.test.js b/frontend/src/plan/planOutlineNodes.test.js
index c484093..51132fc 100644
--- a/frontend/src/plan/planOutlineNodes.test.js
+++ b/frontend/src/plan/planOutlineNodes.test.js
@@ -19,5 +19,6 @@ describe('planOutlineNodes', () => {
     expect(resolvePlanOutlineActiveKey('/plan/inbox')).toBe('inbox')
     expect(resolvePlanOutlineActiveKey('/plan/work')).toBe('work')
     expect(resolvePlanOutlineActiveKey('/plan/portfolio')).toBe(null)
+    expect(resolvePlanOutlineActiveKey('/projects/abc-123')).toBe('structure')
   })
 })
diff --git a/frontend/src/styles/program-chrome.css b/frontend/src/styles/program-chrome.css
index b0dfea4..47cdabe 100644
--- a/frontend/src/styles/program-chrome.css
+++ b/frontend/src/styles/program-chrome.css
@@ -306,6 +306,48 @@
   cursor: not-allowed;
 }
 
+.analysis-split__nav-row {
+  display: flex;
+  align-items: stretch;
+  gap: 2px;
+}
+
+.analysis-split__nav-row .analysis-split__nav-link {
+  flex: 1;
+  min-width: 0;
+}
+
+.analysis-split__nav-toggle {
+  flex: 0 0 auto;
+  width: 28px;
+  border: none;
+  border-radius: 6px;
+  background: transparent;
+  color: var(--jk-text-muted);
+  cursor: pointer;
+  font-size: 11px;
+  line-height: 1;
+}
+
+.analysis-split__nav-toggle:hover {
+  background: var(--jk-primary-soft);
+  color: var(--jk-text);
+}
+
+.analysis-split__nav-sublist {
+  list-style: none;
+  margin: 2px 0 4px;
+  padding: 0 0 0 12px;
+  display: grid;
+  gap: 2px;
+  border-left: 1px solid var(--jk-border);
+}
+
+.analysis-split__nav-link--nested {
+  font-size: 12px;
+  font-weight: 500;
+}
+
 @media (max-width: 900px) {
   .analysis-split {
     grid-template-columns: 1fr;