diff --git a/.changeset/existing-project-extend-framing.md b/.changeset/existing-project-extend-framing.md
new file mode 100644
index 0000000..19829e0
--- /dev/null
+++ b/.changeset/existing-project-extend-framing.md
@@ -0,0 +1,11 @@
+---
+'@gemstack/framework': minor
+---
+
+Extend an existing project instead of rebuilding it from scratch
+
+Pointed at a workspace that already has source, the build step now frames the wrapped agent to work *within* the existing codebase (read it, follow its conventions, add what was asked) rather than scaffold a fresh app. Greenfield runs (an empty workspace) are unchanged, and detection is gated on a real driver, so `--fake` stays deterministic. Combined with the live preset detection already wired from the real workspace, running in an existing project now detects its real stack and extends it.
+
+New exports: `extendPrompt`, `isWorkspaceEmpty`.
+
+Part of #110. Closes #185.
diff --git a/packages/framework/README.md b/packages/framework/README.md
index 2a52b08..99676d0 100644
--- a/packages/framework/README.md
+++ b/packages/framework/README.md
@@ -41,6 +41,12 @@ small structured JSON decision the agent returns; build and improve are prompts;
 the production-grade checklist gates on the `{ blockers }` verdict the agent ends
 its output with.
 
+**From-scratch or existing project.** Point `--cwd` at an empty directory and the
+agent scaffolds the whole app from scratch. Point it at a project that already has
+source and the framework detects the real stack (from its `package.json` + marker
+files) and frames the agent to **extend** the codebase — read it, follow its
+conventions, add what was asked — instead of rebuilding it from scratch.
+
 ## Library API
 
 ```ts
diff --git a/packages/framework/src/index.ts b/packages/framework/src/index.ts
index 5559ae2..ebdc250 100644
--- a/packages/framework/src/index.ts
+++ b/packages/framework/src/index.ts
@@ -43,8 +43,10 @@ export {
   parseArchitectPlan,
   architectPrompt,
   buildPrompt,
+  extendPrompt,
   improvePrompt,
   PRODUCTION_GRADE_PROMPT,
+  isWorkspaceEmpty,
   type DriverStepOptions,
 } from './steps.js'
 export {
diff --git a/packages/framework/src/steps.test.ts b/packages/framework/src/steps.test.ts
index 098d6a1..bce629d 100644
--- a/packages/framework/src/steps.test.ts
+++ b/packages/framework/src/steps.test.ts
@@ -12,6 +12,7 @@ import {
   driverBuild,
   driverChecklist,
   driverImprove,
+  extendPrompt,
   isWorkspaceEmpty,
   parseArchitectPlan,
 } from './steps.js'
@@ -200,6 +201,53 @@ test('driverBuild does not re-prompt when the build produced files', async () =>
   }
 })
 
+test('driverBuild extends an existing project instead of rebuilding it (#185)', async () => {
+  const cwd = makeWorkspace({ 'package.json': '{}', 'src/index.ts': 'export {}' })
+  try {
+    const session = await new FakeDriver({ turns: [{ text: 'added the feature' }] }).start({ cwd })
+    const events: SupervisorEvent[] = []
+    await driverBuild(session, { verifyWorkspace: true })({
+      plan: PLAN,
+      scope: 'full',
+      intent: 'add a search box',
+      onEvent: e => events.push(e),
+    })
+    // Existing codebase: extend framing, and no from-scratch scaffolding language.
+    assert.equal(session.prompts.length, 1)
+    assert.match(session.prompts[0]!, /existing codebase|do NOT re-scaffold/i)
+    assert.doesNotMatch(session.prompts[0]!, /scaffold the whole project|workspace may be empty/i)
+    const plan = events.find(e => e.type === 'plan')
+    assert.ok(plan?.type === 'plan')
+    assert.match(plan.subtasks[0]!.description, /existing codebase/i)
+  } finally {
+    rmSync(cwd, { recursive: true, force: true })
+  }
+})
+
+test('driverBuild uses greenfield framing for an empty workspace (#185)', async () => {
+  const cwd = makeWorkspace() // empty: a from-scratch build
+  try {
+    const session = await new FakeDriver({ turns: [{ text: 'scaffolded it' }] }).start({ cwd })
+    await driverBuild(session, { verifyWorkspace: true })({
+      plan: PLAN,
+      scope: 'full',
+      intent: 'a blog',
+      onEvent: () => {},
+    })
+    assert.match(session.prompts[0]!, /Build this app end to end/i)
+    assert.doesNotMatch(session.prompts[0]!, /existing codebase/i)
+  } finally {
+    rmSync(cwd, { recursive: true, force: true })
+  }
+})
+
+test('extendPrompt names the intent and the detected stack, and forbids a rebuild', () => {
+  const prompt = extendPrompt({ stack: 'Next.js', narration: 'n', decisions: [] }, 'add a settings page')
+  assert.match(prompt, /add a settings page/)
+  assert.match(prompt, /Next\.js/)
+  assert.match(prompt, /do NOT re-scaffold|do not.*swap its stack/i)
+})
+
 test('driverImprove scaffolds from scratch when the workspace is empty, else fixes blockers (#182)', async () => {
   const emptyCwd = makeWorkspace()
   const builtCwd = makeWorkspace({ 'src/app.ts': 'export {}' })
diff --git a/packages/framework/src/steps.ts b/packages/framework/src/steps.ts
index 7c172b7..97a90fe 100644
--- a/packages/framework/src/steps.ts
+++ b/packages/framework/src/steps.ts
@@ -59,6 +59,26 @@ export function buildPrompt(plan: ArchitectPlan, intent: string): string {
   ].join('\n')
 }
 
+/**
+ * Framing for a run against an *existing* codebase: extend it, do not rebuild it.
+ * The greenfield {@link buildPrompt} tells the agent the workspace may be empty
+ * and to scaffold from scratch, which is the wrong instruction when the user
+ * pointed the framework at a project that already exists (#185). Chosen when the
+ * workspace already holds source at build time.
+ */
+export function extendPrompt(plan: ArchitectPlan, intent: string): string {
+  return [
+    `Work within the existing codebase in this workspace to deliver: ${intent}`,
+    `Detected stack: ${plan.stack}`,
+    plan.narration,
+    'This project already exists — do NOT re-scaffold or rebuild it, and do not',
+    'replace its structure or swap its stack. Read the existing code first, follow',
+    'its conventions, and make the smallest coherent set of changes that adds what',
+    'is asked; new files and dependencies are fine when the feature needs them.',
+    'When done, summarize what you changed in one short paragraph.',
+  ].join('\n')
+}
+
 /**
  * A hard "the app does not exist yet — create it from scratch" directive. Used
  * when the workspace is empty at build or improve time, where the normal
@@ -182,17 +202,30 @@ export function driverBuild(
     verifyWorkspace?: boolean
   } & DriverStepOptions = {},
 ): (ctx: BuildContext) => Promise<SupervisorRun> {
-  const compose = opts.prompt ?? buildPrompt
+  const composeOverride = opts.prompt
   const promptOpts = {
     ...(opts.system ? { system: opts.system } : {}),
   }
   return async ctx => {
     const signalOpt = ctx.signal ? { signal: ctx.signal } : {}
-    const subtask: PlannedSubtask = { id: 'build-1', description: `Build with the wrapped agent` }
+    // An existing project (a non-empty workspace at build time) is *extended*, not
+    // rebuilt from scratch (#185). Gated on verifyWorkspace so the fake driver
+    // (which writes nothing, so its workspace always reads empty) always takes the
+    // greenfield path and stays deterministic. A caller-supplied prompt wins.
+    const existing = opts.verifyWorkspace === true && !isWorkspaceEmpty(session.cwd)
+    const firstPrompt = composeOverride
+      ? composeOverride(ctx.plan, ctx.intent)
+      : existing
+        ? extendPrompt(ctx.plan, ctx.intent)
+        : buildPrompt(ctx.plan, ctx.intent)
+    const subtask: PlannedSubtask = {
+      id: 'build-1',
+      description: existing ? 'Extend the existing codebase' : 'Build with the wrapped agent',
+    }
     ctx.onEvent({ type: 'plan', task: ctx.intent, subtasks: [subtask] })
     ctx.onEvent({ type: 'dispatch-start', subtask })
 
-    let turn = await session.prompt(compose(ctx.plan, ctx.intent), { ...promptOpts, ...signalOpt })
+    let turn = await session.prompt(firstPrompt, { ...promptOpts, ...signalOpt })
     const results: SubtaskResult[] = [{ subtask, text: turn.text, ok: true, usage: ZERO_USAGE }]
     ctx.onEvent({ type: 'dispatch-result', result: results[0]! })