diff --git a/src/lib/server/services/__tests__/tenant-admin-service.test.ts b/src/lib/server/services/__tests__/tenant-admin-service.test.ts
index 335c8a5..65cc89f 100644
--- a/src/lib/server/services/__tests__/tenant-admin-service.test.ts
+++ b/src/lib/server/services/__tests__/tenant-admin-service.test.ts
@@ -84,11 +84,17 @@ describe("TenantAdminService", () => {
         returning: vi.fn().mockResolvedValue([mockCreatedTenant]),
       };
 
+      const mockSelectBuilder = {
+        from: vi.fn().mockReturnThis(),
+        where: vi.fn().mockResolvedValue([]),
+      };
+
       const mockDeleteBuilder = {
         where: vi.fn().mockResolvedValue({ count: 1 }),
       };
 
       mockCentralDb.insert.mockReturnValue(mockInsertBuilder);
+      mockCentralDb.select.mockReturnValue(mockSelectBuilder);
       mockCentralDb.delete.mockReturnValue(mockDeleteBuilder);
       mockTenantConfig.create.mockResolvedValue(mockConfig);
       mockTenantMigrationService.createAndInitializeTenantDatabase.mockResolvedValue();
@@ -126,11 +132,17 @@ describe("TenantAdminService", () => {
         returning: vi.fn().mockResolvedValue([mockCreatedTenant]),
       };
 
+      const mockSelectBuilder = {
+        from: vi.fn().mockReturnThis(),
+        where: vi.fn().mockResolvedValue([]),
+      };
+
       const mockDeleteBuilder = {
         where: vi.fn().mockResolvedValue({ count: 1 }),
       };
 
       mockCentralDb.insert.mockReturnValue(mockInsertBuilder);
+      mockCentralDb.select.mockReturnValue(mockSelectBuilder);
       mockCentralDb.delete.mockReturnValue(mockDeleteBuilder);
       mockTenantMigrationService.createAndInitializeTenantDatabase.mockRejectedValue(
         new Error("Database initialization failed"),
diff --git a/src/lib/server/services/tenant-admin-service.ts b/src/lib/server/services/tenant-admin-service.ts
index bc7877a..861442b 100644
--- a/src/lib/server/services/tenant-admin-service.ts
+++ b/src/lib/server/services/tenant-admin-service.ts
@@ -8,7 +8,7 @@ import { env } from "$env/dynamic/private";
 import { eq } from "drizzle-orm";
 import logger from "$lib/logger";
 import z from "zod/v4";
-import { ValidationError, NotFoundError } from "../utils/errors";
+import { ValidationError, NotFoundError, ConflictError } from "../utils/errors";
 import { sendTenantAdminInviteEmail } from "../email/email-service";
 
 if (!env.DATABASE_URL) throw new Error("DATABASE_URL is not set");
@@ -68,6 +68,26 @@ export class TenantAdminService {
       shortName: request.shortName,
     });
 
+    // Check if tenant can be created without any duplications. Do not create tenant if:
+    // - short name already exists
+    // - invited tenant admin email already exists
+    const tenantExists = await centralDb
+      .select()
+      .from(centralSchema.tenant)
+      .where(eq(centralSchema.tenant.shortName, request.shortName));
+    if (tenantExists.length > 0) {
+      throw new ConflictError("Tenant with shortname already exists");
+    }
+    if (request.inviteAdmin) {
+      const adminExists = await centralDb
+        .select()
+        .from(centralSchema.user)
+        .where(eq(centralSchema.user.email, request.inviteAdmin));
+      if (adminExists.length > 0) {
+        throw new ConflictError("Tenant Admin E-Mail Address already exists");
+      }
+    }
+
     const configuration = TenantAdminService.getConfigDefaults();
 
     const urlParts = env.DATABASE_URL?.split("/") ?? [];
diff --git a/src/lib/server/utils/errors.ts b/src/lib/server/utils/errors.ts
index 2e9b24d..fc5d064 100644
--- a/src/lib/server/utils/errors.ts
+++ b/src/lib/server/utils/errors.ts
@@ -1,19 +1,51 @@
-export class AuthenticationError extends Error {
-  constructor(message: string) {
+import { json } from "@sveltejs/kit";
+
+export class BackendError extends Error {
+  constructor(
+    message: string,
+    public code: number,
+  ) {
     super(message);
+  }
+
+  public toJson = () => json({ message: this.message }, { status: this.code });
+}
+
+export class AuthenticationError extends BackendError {
+  constructor(
+    message: string,
+    public code = 403,
+  ) {
+    super(message, code);
     this.name = "AuthenticationError";
   }
 }
-export class ValidationError extends Error {
-  constructor(message: string) {
-    super(message);
+export class ValidationError extends BackendError {
+  constructor(
+    message: string,
+    public code = 422,
+  ) {
+    super(message, code);
     this.name = "ValidationError";
   }
 }
 
-export class NotFoundError extends Error {
-  constructor(message: string) {
-    super(message);
+export class NotFoundError extends BackendError {
+  constructor(
+    message: string,
+    public code = 404,
+  ) {
+    super(message, code);
     this.name = "ValidationError";
   }
 }
+
+export class ConflictError extends BackendError {
+  constructor(
+    message: string,
+    public code = 409,
+  ) {
+    super(message, code);
+    this.name = "ConflictError";
+  }
+}
diff --git a/src/routes/api/tenants/+server.ts b/src/routes/api/tenants/+server.ts
index 38a3d5d..22ff3e9 100644
--- a/src/routes/api/tenants/+server.ts
+++ b/src/routes/api/tenants/+server.ts
@@ -1,6 +1,6 @@
 import { json } from "@sveltejs/kit";
 import { TenantAdminService } from "$lib/server/services/tenant-admin-service";
-import { ValidationError } from "$lib/server/utils/errors";
+import { BackendError } from "$lib/server/utils/errors";
 import type { RequestHandler } from "@sveltejs/kit";
 import { registerOpenAPIRoute } from "$lib/server/openapi";
 import { db } from "$lib/server/db";
@@ -178,8 +178,8 @@ export const POST: RequestHandler = async ({ request }) => {
   } catch (error) {
     log.error("Tenant creation error:", JSON.stringify(error || "?"));
 
-    if (error instanceof ValidationError) {
-      return json({ error: error.message }, { status: 400 });
+    if (error instanceof BackendError) {
+      return error.toJson();
     }
 
     // Handle unique constraint violation (shortName already exists)
diff --git a/src/routes/api/tenants/tenants.test.ts b/src/routes/api/tenants/tenants.test.ts
index f52dcc7..8f1fbe9 100644
--- a/src/routes/api/tenants/tenants.test.ts
+++ b/src/routes/api/tenants/tenants.test.ts
@@ -35,15 +35,8 @@ vi.mock("$lib/logger", () => ({
   },
 }));
 
-// Mock ValidationError
-vi.mock("$lib/server/utils/errors", () => ({
-  ValidationError: class ValidationError extends Error {
-    constructor(message: string) {
-      super(message);
-      this.name = "ValidationError";
-    }
-  },
-}));
+// Don't mock errors - use real ones
+// vi.mock("$lib/server/utils/errors");
 
 describe("/api/tenants", () => {
   beforeEach(() => {
@@ -173,9 +166,9 @@ describe("/api/tenants", () => {
       const data = await response.json();
 
       expect(data).toEqual({
-        error: "Invalid tenant creation request",
+        message: "Invalid tenant creation request",
       });
-      expect(response.status).toBe(400);
+      expect(response.status).toBe(422);
     });
 
     it("should handle unique constraint violations", async () => {