diff --git a/src/lib/projects/authz.test.ts b/src/lib/projects/authz.test.ts
new file mode 100644
index 00000000..11681c59
--- /dev/null
+++ b/src/lib/projects/authz.test.ts
@@ -0,0 +1,270 @@
+import { describe, expect, it } from "vitest";
+import {
+  canManageProject,
+  canUpdateStage,
+  canUploadEvidence,
+  canViewProject,
+  isInternalEmail,
+  resolveProjectRole,
+  type AuthzUser,
+  type ProjectAuthzFacts,
+  type WorkOrderAuthzFacts,
+} from "./authz";
+
+const CLIENT_ORG = "11111111-1111-1111-1111-111111111111";
+const CONTRACTOR_ORG = "22222222-2222-2222-2222-222222222222";
+const OTHER_CONTRACTOR_ORG = "33333333-3333-3333-3333-333333333333";
+const UNRELATED_ORG = "44444444-4444-4444-4444-444444444444";
+
+function makeUser(overrides: Partial<AuthzUser> = {}): AuthzUser {
+  return {
+    id: 1n,
+    email: "someone@landlord.example",
+    organisationIds: [],
+    ...overrides,
+  };
+}
+
+function makeProject(overrides: Partial<ProjectAuthzFacts> = {}): ProjectAuthzFacts {
+  return {
+    id: 100n,
+    organisationId: CLIENT_ORG,
+    domnaAdminAccess: true,
+    contractorOrganisationIds: [CONTRACTOR_ORG, OTHER_CONTRACTOR_ORG],
+    ...overrides,
+  };
+}
+
+function makeWorkOrder(
+  overrides: Partial<WorkOrderAuthzFacts> = {},
+): WorkOrderAuthzFacts {
+  return {
+    id: 500n,
+    contractorOrganisationId: CONTRACTOR_ORG,
+    updateStagesPermission: false,
+    uploadDocumentsPermission: false,
+    ...overrides,
+  };
+}
+
+const internal = makeUser({ email: "dev@domna.homes" });
+const client = makeUser({ organisationIds: [CLIENT_ORG] });
+const contractor = makeUser({ organisationIds: [CONTRACTOR_ORG] });
+const stranger = makeUser({ organisationIds: [UNRELATED_ORG] });
+
+describe("isInternalEmail", () => {
+  it("recognises the Domna domain regardless of case or surrounding whitespace", () => {
+    expect(isInternalEmail("dev@domna.homes")).toBe(true);
+    expect(isInternalEmail("  Dev@Domna.Homes  ")).toBe(true);
+  });
+
+  it("does not treat a lookalike domain as internal", () => {
+    expect(isInternalEmail("attacker@evil-domna.homes")).toBe(false);
+    expect(isInternalEmail("attacker@domna.homes.evil.com")).toBe(false);
+    expect(isInternalEmail("someone@landlord.example")).toBe(false);
+  });
+});
+
+describe("resolveProjectRole", () => {
+  it("resolves a Domna user as internal when the project allows admin access", () => {
+    expect(resolveProjectRole(internal, makeProject())).toBe("internal");
+  });
+
+  it("resolves a member of the owning organisation as client", () => {
+    expect(resolveProjectRole(client, makeProject())).toBe("client");
+  });
+
+  it("resolves a member of an assigned contractor organisation as contractor", () => {
+    expect(resolveProjectRole(contractor, makeProject())).toBe("contractor");
+  });
+
+  it("resolves a member of any assigned contractor org, not just the first", () => {
+    const user = makeUser({ organisationIds: [OTHER_CONTRACTOR_ORG] });
+    expect(resolveProjectRole(user, makeProject())).toBe("contractor");
+  });
+
+  it("gives no role to a user with no relevant membership", () => {
+    expect(resolveProjectRole(stranger, makeProject())).toBeNull();
+  });
+
+  it("gives no role on a project with no contractors assigned yet", () => {
+    const project = makeProject({ contractorOrganisationIds: [] });
+    expect(resolveProjectRole(contractor, project)).toBeNull();
+  });
+
+  it("ranks client above contractor when one org is both owner and contractor", () => {
+    const project = makeProject({ contractorOrganisationIds: [CLIENT_ORG] });
+    expect(resolveProjectRole(client, project)).toBe("client");
+  });
+
+  describe("domna_admin_access = false", () => {
+    const locked = makeProject({ domnaAdminAccess: false });
+
+    it("denies a Domna user any role when they have no membership of their own", () => {
+      expect(resolveProjectRole(internal, locked)).toBeNull();
+    });
+
+    it("still resolves a Domna user as client when they belong to the owning org", () => {
+      const domnaClient = makeUser({
+        email: "dev@domna.homes",
+        organisationIds: [CLIENT_ORG],
+      });
+      expect(resolveProjectRole(domnaClient, locked)).toBe("client");
+    });
+
+    it("still resolves a Domna user as contractor when they belong to an assigned org", () => {
+      const domnaContractor = makeUser({
+        email: "dev@domna.homes",
+        organisationIds: [CONTRACTOR_ORG],
+      });
+      expect(resolveProjectRole(domnaContractor, locked)).toBe("contractor");
+    });
+
+    it("does not affect a non-Domna client", () => {
+      expect(resolveProjectRole(client, locked)).toBe("client");
+    });
+  });
+});
+
+describe("canViewProject", () => {
+  it("admits every resolved role", () => {
+    const project = makeProject();
+    expect(canViewProject(internal, project)).toBe(true);
+    expect(canViewProject(client, project)).toBe(true);
+    expect(canViewProject(contractor, project)).toBe(true);
+  });
+
+  it("refuses a user with no role", () => {
+    expect(canViewProject(stranger, makeProject())).toBe(false);
+  });
+
+  it("refuses a Domna user on a project with admin access switched off", () => {
+    expect(canViewProject(internal, makeProject({ domnaAdminAccess: false }))).toBe(
+      false,
+    );
+  });
+});
+
+describe("canManageProject", () => {
+  it("admits internal and client", () => {
+    const project = makeProject();
+    expect(canManageProject(internal, project)).toBe(true);
+    expect(canManageProject(client, project)).toBe(true);
+  });
+
+  it("refuses a contractor even though they can view the project", () => {
+    const project = makeProject();
+    expect(canViewProject(contractor, project)).toBe(true);
+    expect(canManageProject(contractor, project)).toBe(false);
+  });
+
+  it("refuses a user with no role", () => {
+    expect(canManageProject(stranger, makeProject())).toBe(false);
+  });
+
+  it("refuses a Domna user on a project with admin access switched off", () => {
+    expect(
+      canManageProject(internal, makeProject({ domnaAdminAccess: false })),
+    ).toBe(false);
+  });
+});
+
+describe("canUpdateStage", () => {
+  const project = makeProject();
+
+  it("admits internal and client regardless of the assignment's flags", () => {
+    const workOrder = makeWorkOrder({ updateStagesPermission: false });
+    expect(canUpdateStage(internal, project, workOrder)).toBe(true);
+    expect(canUpdateStage(client, project, workOrder)).toBe(true);
+  });
+
+  it("admits a contractor holding the assignment and the permission", () => {
+    const workOrder = makeWorkOrder({ updateStagesPermission: true });
+    expect(canUpdateStage(contractor, project, workOrder)).toBe(true);
+  });
+
+  it("refuses a contractor whose assignment lacks update_stages_permission", () => {
+    const workOrder = makeWorkOrder({ updateStagesPermission: false });
+    expect(canUpdateStage(contractor, project, workOrder)).toBe(false);
+  });
+
+  it("refuses a contractor from a different assigned org, even with the flag set", () => {
+    const other = makeUser({ organisationIds: [OTHER_CONTRACTOR_ORG] });
+    const workOrder = makeWorkOrder({
+      contractorOrganisationId: CONTRACTOR_ORG,
+      updateStagesPermission: true,
+    });
+    expect(resolveProjectRole(other, project)).toBe("contractor");
+    expect(canUpdateStage(other, project, workOrder)).toBe(false);
+  });
+
+  it("refuses a user with no role on the project", () => {
+    const workOrder = makeWorkOrder({ updateStagesPermission: true });
+    expect(canUpdateStage(stranger, project, workOrder)).toBe(false);
+  });
+
+  it("refuses a Domna user when admin access is off and they have no membership", () => {
+    const locked = makeProject({ domnaAdminAccess: false });
+    const workOrder = makeWorkOrder({ updateStagesPermission: true });
+    expect(canUpdateStage(internal, locked, workOrder)).toBe(false);
+  });
+
+  it("ignores upload_documents_permission", () => {
+    const workOrder = makeWorkOrder({
+      updateStagesPermission: false,
+      uploadDocumentsPermission: true,
+    });
+    expect(canUpdateStage(contractor, project, workOrder)).toBe(false);
+  });
+});
+
+describe("canUploadEvidence", () => {
+  const project = makeProject();
+
+  it("admits internal and client regardless of the assignment's flags", () => {
+    const workOrder = makeWorkOrder({ uploadDocumentsPermission: false });
+    expect(canUploadEvidence(internal, project, workOrder)).toBe(true);
+    expect(canUploadEvidence(client, project, workOrder)).toBe(true);
+  });
+
+  it("admits a contractor holding the assignment and the permission", () => {
+    const workOrder = makeWorkOrder({ uploadDocumentsPermission: true });
+    expect(canUploadEvidence(contractor, project, workOrder)).toBe(true);
+  });
+
+  it("refuses a contractor whose assignment lacks upload_documents_permission", () => {
+    const workOrder = makeWorkOrder({ uploadDocumentsPermission: false });
+    expect(canUploadEvidence(contractor, project, workOrder)).toBe(false);
+  });
+
+  it("refuses a contractor from a different assigned org, even with the flag set", () => {
+    const other = makeUser({ organisationIds: [OTHER_CONTRACTOR_ORG] });
+    const workOrder = makeWorkOrder({
+      contractorOrganisationId: CONTRACTOR_ORG,
+      uploadDocumentsPermission: true,
+    });
+    expect(canUploadEvidence(other, project, workOrder)).toBe(false);
+  });
+
+  it("refuses a user with no role on the project", () => {
+    const workOrder = makeWorkOrder({ uploadDocumentsPermission: true });
+    expect(canUploadEvidence(stranger, project, workOrder)).toBe(false);
+  });
+
+  it("ignores update_stages_permission", () => {
+    const workOrder = makeWorkOrder({
+      uploadDocumentsPermission: false,
+      updateStagesPermission: true,
+    });
+    expect(canUploadEvidence(contractor, project, workOrder)).toBe(false);
+  });
+
+  it("is independent of canUpdateStage — the two flags grant separately", () => {
+    const uploadOnly = makeWorkOrder({
+      updateStagesPermission: false,
+      uploadDocumentsPermission: true,
+    });
+    expect(canUpdateStage(contractor, project, uploadOnly)).toBe(false);
+    expect(canUploadEvidence(contractor, project, uploadOnly)).toBe(true);
+  });
+});
diff --git a/src/lib/projects/authz.ts b/src/lib/projects/authz.ts
new file mode 100644
index 00000000..a65b0588
--- /dev/null
+++ b/src/lib/projects/authz.ts
@@ -0,0 +1,180 @@
+/**
+ * Ara Projects authorization — the single source of permission truth for every
+ * Ara Projects route handler (issue #408). No route may re-implement any of
+ * this inline.
+ *
+ * This module is deliberately **pure**: it imports no database client, so the
+ * guards unit-test without mocking a connection pool. The DB-backed resolvers
+ * that produce its inputs live next door in `./authzQueries` — a route handler
+ * loads facts there, then decides here.
+ *
+ *   const [user, project] = await Promise.all([
+ *     loadAuthzUser(userId),
+ *     loadProjectAuthzFacts(projectId),
+ *   ]);
+ *   if (!canViewProject(user, project)) return forbidden();
+ *
+ * Scoping note: Ara Projects are **organisation**-scoped. The existing
+ * `team_portfolio_permissions` machinery is portfolio-scoped and plays no part
+ * here — do not reach for it. A user's organisations come from
+ * `team_members → team → org_id` (see CONTEXT.md, "Ara Projects").
+ */
+
+/** The email domain that identifies a Domna (internal) user. */
+export const INTERNAL_EMAIL_DOMAIN = "@domna.homes";
+
+/**
+ * A user's standing on one particular Project.
+ *
+ * - `internal`   — a Domna user, on a project that opts into admin access.
+ * - `client`     — a member of the organisation that owns the project.
+ * - `contractor` — a member of an organisation assigned to one of the
+ *                  project's workstreams.
+ *
+ * Absence of a role (`null`) means no access at all, not read-only access.
+ */
+export type ProjectRole = "internal" | "client" | "contractor";
+
+/** The requesting user, with their organisation memberships already resolved. */
+export interface AuthzUser {
+  id: bigint;
+  email: string;
+  /** `organisation.id` values (uuid) reached via team_members → team → org_id. */
+  organisationIds: string[];
+}
+
+/** The permission-relevant facts about one Project. */
+export interface ProjectAuthzFacts {
+  id: bigint;
+  /** `project.organisation_id` — the client organisation that owns the project. */
+  organisationId: string;
+  /** `project.domna_admin_access` — see `resolveProjectRole`. */
+  domnaAdminAccess: boolean;
+  /**
+   * Every organisation assigned as contractor to any of this project's
+   * workstreams, via `project_workstream_contractor`.
+   */
+  contractorOrganisationIds: string[];
+}
+
+/**
+ * The permission-relevant facts about one Work order, flattened from its
+ * `project_workstream_contractor` assignment. The permission flags belong to
+ * the *assignment*, not to the organisation globally — the same contractor org
+ * may hold different permissions on two workstreams of the same project.
+ */
+export interface WorkOrderAuthzFacts {
+  id: bigint;
+  /** `project_workstream_contractor.organisation_id` for this work order. */
+  contractorOrganisationId: string;
+  updateStagesPermission: boolean;
+  uploadDocumentsPermission: boolean;
+}
+
+/** True when the email sits on the Domna domain (case- and whitespace-tolerant). */
+export function isInternalEmail(email: string): boolean {
+  return email.trim().toLowerCase().endsWith(INTERNAL_EMAIL_DOMAIN);
+}
+
+/**
+ * Resolve what a user is to a project, most-privileged first.
+ *
+ * `project.domna_admin_access` gates the `internal` role only: when it is
+ * false, a Domna user gets no elevated standing on that project, but still
+ * falls through to whatever their organisation memberships earn them — a Domna
+ * user who is genuinely a member of the client org remains a `client`.
+ *
+ * Client outranks contractor: an organisation that both owns the project and
+ * delivers one of its workstreams is treated as the client.
+ */
+export function resolveProjectRole(
+  user: AuthzUser,
+  project: ProjectAuthzFacts,
+): ProjectRole | null {
+  if (project.domnaAdminAccess && isInternalEmail(user.email)) return "internal";
+
+  const memberships = new Set(user.organisationIds);
+  if (memberships.has(project.organisationId)) return "client";
+  if (project.contractorOrganisationIds.some((orgId) => memberships.has(orgId))) {
+    return "contractor";
+  }
+  return null;
+}
+
+/** Any resolved role may read the project. */
+export function canViewProject(
+  user: AuthzUser,
+  project: ProjectAuthzFacts,
+): boolean {
+  return resolveProjectRole(user, project) !== null;
+}
+
+/**
+ * Administering the project — editing it, configuring workstreams, assigning
+ * contractors. Internal and client only; a contractor never manages.
+ */
+export function canManageProject(
+  user: AuthzUser,
+  project: ProjectAuthzFacts,
+): boolean {
+  const role = resolveProjectRole(user, project);
+  return role === "internal" || role === "client";
+}
+
+/**
+ * Move a Work order along its stage ladder.
+ *
+ * Internal and client may always do so. A contractor may only when the work
+ * order is issued to an organisation they belong to *and* that assignment
+ * carries `update_stages_permission` — belonging to some *other* assigned org
+ * on the same project is not enough.
+ */
+export function canUpdateStage(
+  user: AuthzUser,
+  project: ProjectAuthzFacts,
+  workOrder: WorkOrderAuthzFacts,
+): boolean {
+  return allowsWorkOrderAction(
+    user,
+    project,
+    workOrder,
+    workOrder.updateStagesPermission,
+  );
+}
+
+/**
+ * Upload Evidence against a Work order. Same shape as `canUpdateStage`, gated
+ * on the assignment's `upload_documents_permission`.
+ */
+export function canUploadEvidence(
+  user: AuthzUser,
+  project: ProjectAuthzFacts,
+  workOrder: WorkOrderAuthzFacts,
+): boolean {
+  return allowsWorkOrderAction(
+    user,
+    project,
+    workOrder,
+    workOrder.uploadDocumentsPermission,
+  );
+}
+
+/**
+ * The shared shape of the two work-order guards: privileged roles pass
+ * unconditionally, a contractor needs both the assignment and its flag, and
+ * anyone without a role is refused.
+ */
+function allowsWorkOrderAction(
+  user: AuthzUser,
+  project: ProjectAuthzFacts,
+  workOrder: WorkOrderAuthzFacts,
+  permissionFlag: boolean,
+): boolean {
+  const role = resolveProjectRole(user, project);
+  if (role === "internal" || role === "client") return true;
+  if (role !== "contractor") return false;
+  return (
+    user.organisationIds.includes(workOrder.contractorOrganisationId) &&
+    permissionFlag
+  );
+}
diff --git a/src/lib/projects/authzQueries.ts b/src/lib/projects/authzQueries.ts
new file mode 100644
index 00000000..fea89557
--- /dev/null
+++ b/src/lib/projects/authzQueries.ts
@@ -0,0 +1,148 @@
+/**
+ * DB-backed fact loaders feeding the pure guards in `./authz` (issue #408).
+ *
+ * Kept separate from the guards so those stay unit-testable without a database.
+ * Route handlers load facts here, then decide with `authz`.
+ *
+ * Every loader is a single round trip; none probes per-row inside a list.
+ */
+import { db } from "@/app/db/db";
+import { and, eq, inArray } from "drizzle-orm";
+import { team, teamMembers } from "@/app/db/schema/team";
+import { user } from "@/app/db/schema/users";
+import {
+  project,
+  projectWorkstream,
+  projectWorkstreamContractor,
+  workOrder,
+} from "@/app/db/schema/projects/projects";
+import type {
+  AuthzUser,
+  ProjectAuthzFacts,
+  WorkOrderAuthzFacts,
+} from "./authz";
+
+/**
+ * The organisations a user belongs to, via `team_members → team → org_id`.
+ *
+ * This is the *only* membership path Ara Projects recognises. Each
+ * organisation has a default team, so contractor and client users alike arrive
+ * through the existing team structures. Deduplicated: a user on two teams of
+ * one organisation yields that organisation once.
+ */
+export async function getUserOrganisations(userId: bigint): Promise<string[]> {
+  const rows = await db
+    .selectDistinct({ orgId: team.orgId })
+    .from(teamMembers)
+    .innerJoin(team, eq(teamMembers.teamId, team.id))
+    .where(eq(teamMembers.userId, userId));
+
+  return rows.map((r) => r.orgId);
+}
+
+/**
+ * Load a user plus their organisation memberships, ready to hand to a guard.
+ * Returns null when the user does not exist.
+ */
+export async function loadAuthzUser(userId: bigint): Promise<AuthzUser | null> {
+  const [[found], organisationIds] = await Promise.all([
+    db
+      .select({ id: user.id, email: user.email })
+      .from(user)
+      .where(eq(user.id, userId))
+      .limit(1),
+    getUserOrganisations(userId),
+  ]);
+
+  if (!found) return null;
+  return { id: found.id, email: found.email, organisationIds };
+}
+
+/**
+ * Load one project's permission-relevant facts, including every organisation
+ * assigned as contractor to any of its workstreams. Returns null when the
+ * project does not exist — callers should treat that as a 404, not a 403.
+ */
+export async function loadProjectAuthzFacts(
+  projectId: bigint,
+): Promise<ProjectAuthzFacts | null> {
+  const [[found], contractorRows] = await Promise.all([
+    db
+      .select({
+        id: project.id,
+        organisationId: project.organisationId,
+        domnaAdminAccess: project.domnaAdminAccess,
+      })
+      .from(project)
+      .where(eq(project.id, projectId))
+      .limit(1),
+    db
+      .selectDistinct({ organisationId: projectWorkstreamContractor.organisationId })
+      .from(projectWorkstreamContractor)
+      .innerJoin(
+        projectWorkstream,
+        eq(projectWorkstreamContractor.projectWorkstreamId, projectWorkstream.id),
+      )
+      .where(eq(projectWorkstream.projectId, projectId)),
+  ]);
+
+  if (!found) return null;
+  return {
+    ...found,
+    contractorOrganisationIds: contractorRows.map((r) => r.organisationId),
+  };
+}
+
+/**
+ * Load one work order's contractor assignment facts. Returns null when the
+ * work order does not exist.
+ *
+ * `projectId` narrows the lookup to the project named in the request path, so
+ * a work-order id belonging to another project cannot be authorized against
+ * the facts of the project the caller happens to have access to.
+ */
+export async function loadWorkOrderAuthzFacts(
+  workOrderId: bigint,
+  projectId: bigint,
+): Promise<WorkOrderAuthzFacts | null> {
+  const [found] = await db
+    .select({
+      id: workOrder.id,
+      contractorOrganisationId: projectWorkstreamContractor.organisationId,
+      updateStagesPermission: projectWorkstreamContractor.updateStagesPermission,
+      uploadDocumentsPermission:
+        projectWorkstreamContractor.uploadDocumentsPermission,
+    })
+    .from(workOrder)
+    .innerJoin(
+      projectWorkstreamContractor,
+      eq(workOrder.projectWorkstreamContractorId, projectWorkstreamContractor.id),
+    )
+    .innerJoin(
+      projectWorkstream,
+      eq(projectWorkstreamContractor.projectWorkstreamId, projectWorkstream.id),
+    )
+    .where(
+      and(eq(workOrder.id, workOrderId), eq(projectWorkstream.projectId, projectId)),
+    )
+    .limit(1);
+
+  return found ?? null;
+}
+
+/**
+ * The projects a set of organisations own, for list endpoints that must not
+ * leak projects the caller cannot see. Contractor visibility is resolved
+ * separately (a contractor sees a project through its workstream assignments),
+ * so this covers the client side only.
+ */
+export async function listProjectIdsForOrganisations(
+  organisationIds: string[],
+): Promise<bigint[]> {
+  if (organisationIds.length === 0) return [];
+  const rows = await db
+    .select({ id: project.id })
+    .from(project)
+    .where(inArray(project.organisationId, organisationIds));
+  return rows.map((r) => r.id);
+}