diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 86c0da8..50de854 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -63,6 +63,7 @@ wdk_add_driver(kf-test WINVER NTDDI_WIN10 STL
     AutoSpinLockTest.cpp
     EResourceSharedLockTest.cpp
     RecursiveAutoSpinLockTest.cpp
+    UStringBuilderTest.cpp
     USimpleStringTest.cpp
 )
 
diff --git a/test/UStringBuilderTest.cpp b/test/UStringBuilderTest.cpp
new file mode 100644
index 0000000..0d9d991
--- /dev/null
+++ b/test/UStringBuilderTest.cpp
@@ -0,0 +1,170 @@
+#include "pch.h"
+#include <kf/UStringBuilder.h>
+#include <kf/UString.h>
+
+using namespace kf;
+
+SCENARIO("UStringBuilder")
+{
+    GIVEN("A default constructed builder")
+    {
+        UStringBuilder<PagedPool> sb;
+
+        THEN("It holds an empty string")
+        {
+            REQUIRE(sb.string().charLength() == 0);
+            REQUIRE(sb.string().maxCharLength() == 0);
+        }
+
+        WHEN("reserve(0) is called")
+        {
+            NTSTATUS status = sb.reserve(0);
+
+            THEN("It succeeds and remains empty")
+            {
+                REQUIRE_NT_SUCCESS(status);
+                REQUIRE(sb.string().charLength() == 0);
+                REQUIRE(sb.string().maxCharLength() == 0);
+            }
+        }
+
+        WHEN("reserve(10) is called")
+        {
+            NTSTATUS status = sb.reserve(10);
+
+            THEN("Capacity grows and length stays 0")
+            {
+                REQUIRE_NT_SUCCESS(status);
+                REQUIRE(sb.string().charLength() == 0);
+                REQUIRE(sb.string().maxCharLength() == 10);
+            }
+        }
+    }
+
+    GIVEN("An empty builder")
+    {
+        UStringBuilder<PagedPool> sb;
+
+        WHEN("A single PCWSTR is appended")
+        {
+            NTSTATUS status = sb.append(L"abc");
+
+            THEN("The resulting string equals 'abc'")
+            {
+                REQUIRE_NT_SUCCESS(status);
+                REQUIRE(sb.string().equals(L"abc"));
+            }
+        }
+
+        WHEN("A single USimpleString is appended")
+        {
+            NTSTATUS status = sb.append(USimpleString(L"def"));
+            THEN("The resulting string equals 'def'")
+            {
+                REQUIRE_NT_SUCCESS(status);
+                REQUIRE(sb.string().equals(L"def"));
+            }
+        }
+
+        WHEN("A single UNICODE_STRING is appended")
+        {
+            UNICODE_STRING us = RTL_CONSTANT_STRING(L"ghi");
+            NTSTATUS status = sb.append(us);
+            THEN("The resulting string equals 'ghi'")
+            {
+                REQUIRE_NT_SUCCESS(status);
+                REQUIRE(sb.string().equals(L"ghi"));
+            }
+        }
+
+        WHEN("A single UString is appended")
+        {
+            UString<PagedPool> us;
+            REQUIRE_NT_SUCCESS(us.init(L"xyz"));
+
+            NTSTATUS status = sb.append(us);
+            THEN("The resulting string equals 'xyz'")
+            {
+                REQUIRE_NT_SUCCESS(status);
+                REQUIRE(sb.string().equals(L"xyz"));
+            }
+        }
+
+        WHEN("A single span<const wchar_t> is appended")
+        {
+            const wchar_t data[] = { L'm', L'n', L'o' };
+            NTSTATUS status = sb.append(span{ data });
+
+            THEN("The resulting string equals 'mno'")
+            {
+                REQUIRE_NT_SUCCESS(status);
+                REQUIRE(sb.string().equals(L"mno"));
+            }
+        }
+
+        WHEN("A single span<const std::byte> is appended")
+        {
+            // UTF-16LE bytes for "uvw": u=0x0075, v=0x0076, w=0x0077
+            const std::byte bytes[] = 
+            {
+                std::byte{0x75}, std::byte{0x00},
+                std::byte{0x76}, std::byte{0x00},
+                std::byte{0x77}, std::byte{0x00}
+            };
+            NTSTATUS status = sb.append(span { bytes });
+
+            THEN("The resulting string equals 'uvw'")
+            {
+                REQUIRE_NT_SUCCESS(status);
+                REQUIRE(sb.string().equals(L"uvw"));
+            }
+        }
+
+        WHEN("USimpleString and PCWSTR are appended in sequence")
+        {
+            NTSTATUS status1 = sb.append(L"ab");
+            NTSTATUS status2 = sb.append(USimpleString(L"cd"));
+            NTSTATUS status3 = sb.append(L"");
+
+            THEN("They concatenate to 'abcd'")
+            {
+                REQUIRE_NT_SUCCESS(status1);
+                REQUIRE_NT_SUCCESS(status2);
+                REQUIRE_NT_SUCCESS(status3);
+                REQUIRE(sb.string().equals(L"abcd"));
+            }
+        }
+
+        WHEN("Variadic append with multiple arguments is used")
+        {
+            NTSTATUS status = sb.append(USimpleString(L"ab"), USimpleString(L"cd"), L"ef");
+
+            THEN("All parts are appended in order")
+            {
+                REQUIRE_NT_SUCCESS(status);
+                REQUIRE(sb.string().equals(L"abcdef"));
+            }
+        }
+    }
+
+    GIVEN("A builder with content 'Hello'")
+    {
+        UStringBuilder<PagedPool> sb;
+        REQUIRE_NT_SUCCESS(sb.append(L"Hello"));
+
+        WHEN("reserve is called with a smaller size than current length")
+        {
+            // Note: current charLength is 5, reserve(3) will reallocate to 3 and truncate
+            NTSTATUS status = sb.reserve(3);
+
+            THEN("It truncates the current content")
+            {
+                REQUIRE_NT_SUCCESS(status);
+                REQUIRE(sb.string().charLength() == 3);
+                REQUIRE(sb.string().maxCharLength() == 3);
+                REQUIRE(sb.string().equals(L"Hel"));
+            }
+        }
+    }
+}
+
diff --git a/test/pch.h b/test/pch.h
index 5e51399..80e5a60 100644
--- a/test/pch.h
+++ b/test/pch.h
@@ -4,6 +4,7 @@
 #include <algorithm>
 #include <ranges>
 #include <array>
+#include <span>
 #include <kmtest/kmtest.h>
 
 // TODO: move this default implementation to kf