diff --git a/src/app/api/sites/[id]/route.test.ts b/src/app/api/sites/[id]/route.test.ts
index 11fff42..f0962c6 100644
--- a/src/app/api/sites/[id]/route.test.ts
+++ b/src/app/api/sites/[id]/route.test.ts
@@ -2,23 +2,33 @@ import "@/test-utils/named-admin-session";
 import { describe, it, expect, vi, beforeEach } from "vitest";
 
 const pbUpdate = vi.fn();
+const pbGetOne = vi.fn();
+const pbGetList = vi.fn();
 
 vi.mock("@/lib/pb", () => ({
   pbAdmin: async () => ({
+    filter: (expression: string, params: Record<string, string>) => expression.replace(/\{:(\w+)\}/g, (_, key: string) => JSON.stringify(params[key])),
     collection: (name: string) => {
-      if (name === "sites") return { update: pbUpdate };
+      if (name === "sites") return { update: pbUpdate, getOne: pbGetOne, getList: pbGetList };
       throw new Error(`unexpected collection: ${name}`);
     },
   }),
 }));
 
 import { PATCH } from "./route";
+import { getCurrentUser } from "@/lib/auth/session";
+import { namedAdmin } from "@/test-utils/named-admin-session";
 
 const SITE_ID = "site_abc";
 
 beforeEach(() => {
   pbUpdate.mockReset();
   pbUpdate.mockResolvedValue({ id: SITE_ID });
+  pbGetOne.mockReset();
+  pbGetOne.mockResolvedValue({ id: SITE_ID, project_code: "001" });
+  pbGetList.mockReset();
+  pbGetList.mockResolvedValue({ totalItems: 0, items: [] });
+  vi.mocked(getCurrentUser).mockResolvedValue(namedAdmin);
 });
 
 const patch = (body: unknown) =>
@@ -88,3 +98,37 @@ describe("PATCH /api/sites/[id] — coordinates", () => {
     expect(payload).not.toHaveProperty("longitude");
   });
 });
+
+describe("PATCH /api/sites/[id] — project code", () => {
+  it("lets an admin correct the project code, trimmed", async () => {
+    const res = await patch({ project_code: "  C26/27 " });
+    expect(res.status).toBe(200);
+    expect(pbUpdate).toHaveBeenCalledWith(SITE_ID, expect.objectContaining({ project_code: "C26/27" }));
+  });
+
+  it("refuses a project code another site already uses", async () => {
+    pbGetList.mockResolvedValue({ totalItems: 1, items: [{ id: "other" }] });
+    const res = await patch({ project_code: "C26/09" });
+    expect(res.status).toBe(409);
+    expect(pbUpdate).not.toHaveBeenCalled();
+  });
+
+  it("refuses an empty or over-long project code", async () => {
+    expect((await patch({ project_code: "  " })).status).toBe(400);
+    expect((await patch({ project_code: "x".repeat(65) })).status).toBe(400);
+    expect(pbUpdate).not.toHaveBeenCalled();
+  });
+
+  it("does not let a site manager change it", async () => {
+    vi.mocked(getCurrentUser).mockResolvedValue({ ...namedAdmin, role: "site_user", siteIds: [SITE_ID] });
+    const res = await patch({ project_code: "C26/27" });
+    expect(res.status).toBe(403);
+    expect(pbUpdate).not.toHaveBeenCalled();
+  });
+
+  it("writes nothing extra when the code is unchanged", async () => {
+    const res = await patch({ project_code: "001", name: "Nellmapius Ext 25" });
+    expect(res.status).toBe(200);
+    expect(pbUpdate.mock.calls[0][1]).not.toHaveProperty("project_code");
+  });
+});
diff --git a/src/app/api/sites/[id]/route.ts b/src/app/api/sites/[id]/route.ts
index cd7a311..a4ebfab 100644
--- a/src/app/api/sites/[id]/route.ts
+++ b/src/app/api/sites/[id]/route.ts
@@ -2,13 +2,15 @@ import { authorizeOperation } from "@/lib/auth/operation";
 import { NextResponse } from "next/server";
 import { pbAdmin } from "@/lib/pb";
 import { parseCoordinate, type CoordinateKind } from "@/lib/site-coordinates";
+import { requireApiUser } from "@/lib/auth/guard";
 
 /**
  * PATCH /api/sites/[id]
  *
  * Partial update — only fields present in the body are written.
- * project_code is locked (changing it would break joins); use null on
- * job_start_date to clear it explicitly.
+ * project_code can be corrected by an admin only. Every other record links
+ * to the site by id, so a corrected code breaks nothing; it must stay unique.
+ * Use null on job_start_date to clear it explicitly.
  */
 export async function PATCH(
   request: Request,
@@ -24,14 +26,35 @@ export async function PATCH(
     return NextResponse.json({ error: "Body must be JSON" }, { status: 400 });
   }
 
+  const payload: Record<string, unknown> = {};
+  const pb = await pbAdmin();
+
   if ("project_code" in body) {
-    return NextResponse.json(
-      { error: "project_code cannot be changed once a site exists" },
-      { status: 400 },
-    );
+    const projectCode = typeof body.project_code === "string" ? body.project_code.trim() : "";
+    if (!projectCode) return NextResponse.json({ error: "project_code is required" }, { status: 400 });
+    if (projectCode.length > 64) {
+      return NextResponse.json({ error: "project_code must be 64 characters or fewer" }, { status: 400 });
+    }
+    let current: { project_code?: string };
+    try {
+      current = await pb.collection("sites").getOne(id, { fields: "project_code" });
+    } catch {
+      return NextResponse.json({ error: "Site not found" }, { status: 404 });
+    }
+    if (projectCode !== current.project_code) {
+      const admin = await requireApiUser("dashboard", { admin: true });
+      if (admin instanceof Response) return NextResponse.json({ error: "Only an admin can change a project code" }, { status: 403 });
+      const clash = await pb.collection("sites").getList(1, 1, {
+        filter: pb.filter("project_code = {:code} && id != {:id}", { code: projectCode, id }),
+        fields: "id",
+      });
+      if (clash.totalItems > 0) {
+        return NextResponse.json({ error: `project_code "${projectCode}" is already in use` }, { status: 409 });
+      }
+      payload.project_code = projectCode;
+    }
   }
 
-  const payload: Record<string, unknown> = {};
   for (const k of ["name", "duration"] as const) {
     if (typeof body[k] === "string") payload[k] = body[k];
   }
@@ -66,7 +89,6 @@ export async function PATCH(
     }
   }
 
-  const pb = await pbAdmin();
   try {
     const updated = await pb.collection("sites").update(id, payload);
     return NextResponse.json({ site: updated });
diff --git a/src/app/dashboard/sites/[id]/page.tsx b/src/app/dashboard/sites/[id]/page.tsx
index 07579c2..bc63912 100644
--- a/src/app/dashboard/sites/[id]/page.tsx
+++ b/src/app/dashboard/sites/[id]/page.tsx
@@ -28,6 +28,7 @@ import {
 import { Topbar } from "@/components/layout/sidebar";
 import { Button } from "@/components/ui/button";
 import { EditSiteButton } from "@/components/site-form-dialog";
+import { getCurrentUser } from "@/lib/auth/session";
 import { type SearchParams } from "@/lib/site-scope";
 import { getSiteScope } from "@/lib/site-scope-server";
 import {
@@ -63,6 +64,7 @@ export default async function SiteDetailPage({
   // to revisit five files.
   const scope = await getSiteScope(await searchParams, "sites");
   if (!scope.allowed.some((s) => s.id === id)) notFound();
+  const isAdmin = (await getCurrentUser())?.role === "admin";
 
   // Pull everything we need for the financial roll-up
   const [
@@ -347,6 +349,7 @@ export default async function SiteDetailPage({
                 latitude: site.latitude ?? null,
                 longitude: site.longitude ?? null,
               }}
+              canEditProjectCode={isAdmin}
             />
             {siteMap && (
               <Link href={`/dashboard/surveying/maps/${id}/map`}>
diff --git a/src/components/site-form-dialog.tsx b/src/components/site-form-dialog.tsx
index 42554df..64238ca 100644
--- a/src/components/site-form-dialog.tsx
+++ b/src/components/site-form-dialog.tsx
@@ -72,11 +72,12 @@ export function AddSiteButton({
   );
 }
 
-export function EditSiteButton({ site }: { site: SiteFormInitial }) {
+export function EditSiteButton({ site, canEditProjectCode = false }: { site: SiteFormInitial; canEditProjectCode?: boolean }) {
   return (
     <SiteFormDialog
       mode="edit"
       initial={site}
+      canEditProjectCode={canEditProjectCode}
       trigger={
         <Button
           type="button"
@@ -98,17 +99,22 @@ function SiteFormDialog({
   initial,
   templates,
   trigger,
+  canEditProjectCode = false,
 }: {
   mode: Mode;
   initial?: SiteFormInitial;
   templates?: SiteTemplateOption[];
   trigger: React.ReactNode;
+  /** Admins can unlock a mistyped code by double-clicking it. Editing only. */
+  canEditProjectCode?: boolean;
 }) {
   const router = useRouter();
   const [open, setOpen] = React.useState(false);
   const [submitting, setSubmitting] = React.useState(false);
   const [error, setError] = React.useState<string | null>(null);
   const [templateId, setTemplateId] = React.useState<string>("blank");
+  const [codeUnlocked, setCodeUnlocked] = React.useState(false);
+  const codeEditable = mode === "create" || codeUnlocked;
 
   // Form state — strings everywhere so inputs stay editable; we coerce on submit.
   const [form, setForm] = React.useState(() => initialValues(initial));
@@ -154,11 +160,11 @@ function SiteFormDialog({
     setError(null);
 
     const projectCode = form.project_code.trim();
-    if (mode === "create" && !projectCode) {
+    if (codeEditable && !projectCode) {
       setError("Project code is required");
       return;
     }
-    if (mode === "create" && projectCode.length > 64) {
+    if (codeEditable && projectCode.length > 64) {
       setError("Project code must be 64 characters or fewer");
       return;
     }
@@ -193,6 +199,7 @@ function SiteFormDialog({
           method: "PATCH",
           headers: { "content-type": "application/json" },
           body: JSON.stringify({
+            ...(codeUnlocked && projectCode !== initial?.project_code ? { project_code: projectCode } : {}),
             name: form.name.trim(),
             duration: form.duration.trim() || null,
             job_start_date: form.job_start_date || null,
@@ -225,7 +232,9 @@ function SiteFormDialog({
   const sub =
     mode === "create"
       ? "Pick a template to clone the working-hours defaults, or start blank."
-      : "Update site details. Project code is permanent once a site exists.";
+      : canEditProjectCode
+        ? "Update site details. Double-click the project code to correct it."
+        : "Update site details. Only an admin can change the project code.";
 
   return (
     <Dialog
@@ -236,6 +245,7 @@ function SiteFormDialog({
           setError(null);
           setForm(initialValues(initial));
           setTemplateId("blank");
+          setCodeUnlocked(false);
         }
       }}
     >
@@ -276,14 +286,28 @@ function SiteFormDialog({
           <div className="grid grid-cols-1 gap-3 sm:grid-cols-2">
             <Field
               label="Project code"
-              hint={mode === "edit" ? "Locked once set" : "Required · max 64 chars"}
+              hint={
+                codeEditable
+                  ? "Required · max 64 chars"
+                  : canEditProjectCode
+                    ? "Double-click to change"
+                    : "Admins only"
+              }
             >
+              {/* readOnly, not disabled: a disabled input never receives the double-click. */}
               <Input
                 value={form.project_code}
                 onChange={(e) => setField("project_code", e.target.value)}
-                disabled={mode === "edit"}
+                readOnly={!codeEditable}
+                aria-readonly={!codeEditable}
+                onDoubleClick={(e) => {
+                  if (codeEditable || !canEditProjectCode) return;
+                  setCodeUnlocked(true);
+                  e.currentTarget.select();
+                }}
+                title={!codeEditable && canEditProjectCode ? "Double-click to change" : undefined}
                 placeholder="e.g. P-217"
-                className="font-mono"
+                className={cn("font-mono", !codeEditable && "cursor-default bg-muted text-muted-foreground")}
                 maxLength={64}
               />
             </Field>