diff --git a/build/image/image.gni b/build/image/image.gni
index 016546c..8004a8b 100644
--- a/build/image/image.gni
+++ b/build/image/image.gni
@@ -211,6 +211,31 @@
   }
 }
 
+template("fdt_overlay") {
+  action(target_name) {
+    forward_variables_from(invoker,
+                           [
+                             "testonly",
+                             "deps",
+                           ])
+    script = "//build/image/dtc.py"
+
+    sources = [
+      invoker.base,
+      invoker.overlay,
+    ]
+    outputs = [
+      invoker.output,
+    ]
+    args = [
+      "overlay",
+      rebase_path(outputs[0]),
+      rebase_path(sources[0]),
+      rebase_path(sources[1]),
+    ]
+  }
+}
+
 template("incbin") {
   source_set(target_name) {
     forward_variables_from(invoker,
@@ -255,10 +280,41 @@
   manifest_target = "${target_name}__manifest"
   manifest_file = "${base_out_dir}/manifest.dtb"
 
-  # Generate manifest.dtbo
-  device_tree(manifest_target) {
-    source = invoker.manifest
-    output = manifest_file
+  # Check if the invoker specified an overlay for the manifest.
+  if (defined(invoker.manifest_overlay) && invoker.manifest_overlay != "") {
+    manifest_base_target = "${target_name}__manifest_base"
+    manifest_base_file = "${base_out_dir}/manifest_base.dtb"
+    manifest_overlay_target = "${target_name}__manifest_overlay"
+    manifest_overlay_file = "${base_out_dir}/manifest_overlay.dtbo"
+
+    # Compile the base manifest.
+    device_tree(manifest_base_target) {
+      source = invoker.manifest
+      output = manifest_base_file
+    }
+
+    # Compile the manifest overlay.
+    device_tree(manifest_overlay_target) {
+      source = invoker.manifest_overlay
+      output = manifest_overlay_file
+    }
+
+    # Merge the base manifest and the overlay, producing the final manifest.
+    fdt_overlay(manifest_target) {
+      base = manifest_base_file
+      overlay = manifest_overlay_file
+      output = manifest_file
+      deps = [
+        ":$manifest_base_target",
+        ":$manifest_overlay_target",
+      ]
+    }
+  } else {
+    # No overlay specified. Compile the manifest to DTB.
+    device_tree(manifest_target) {
+      source = invoker.manifest
+      output = manifest_file
+    }
   }
 
   action(target_name) {
diff --git a/test/hftest/BUILD.gn b/test/hftest/BUILD.gn
index b11a50b..054a319 100644
--- a/test/hftest/BUILD.gn
+++ b/test/hftest/BUILD.gn
@@ -13,11 +13,7 @@
 # limitations under the License.
 
 import("//build/toolchain/platform.gni")
-
-declare_args() {
-  hftest_ctrl = ":ctrl_fdt"
-  hftest_device = ":device_psci"
-}
+import("//test/hftest/args.gni")
 
 config("hftest_config") {
   include_dirs = [ "//test/inc" ]
diff --git a/test/hftest/args.gni b/test/hftest/args.gni
new file mode 100644
index 0000000..89bf3a6
--- /dev/null
+++ b/test/hftest/args.gni
@@ -0,0 +1,19 @@
+# Copyright 2020 The Hafnium Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+declare_args() {
+  hftest_ctrl = ":ctrl_fdt"
+  hftest_device = ":device_psci"
+  hftest_manifest_overlay = ""
+}
diff --git a/test/linux/BUILD.gn b/test/linux/BUILD.gn
index 0be5630..0807b2f 100644
--- a/test/linux/BUILD.gn
+++ b/test/linux/BUILD.gn
@@ -14,6 +14,7 @@
 
 import("//build/image/image.gni")
 import("//build/toolchain/platform.gni")
+import("//test/hftest/args.gni")
 
 executable("test_binary") {
   include_dirs = [
@@ -58,6 +59,7 @@
   testonly = true
 
   manifest = "manifest.dts"
+  manifest_overlay = hftest_manifest_overlay
   primary_name = "vmlinuz"
   primary_vm = "//third_party/linux:linux__prebuilt"
   primary_initrd = ":linux_test_initrd"
diff --git a/test/vmapi/arch/aarch64/BUILD.gn b/test/vmapi/arch/aarch64/BUILD.gn
index 8441677..c4ebc61 100644
--- a/test/vmapi/arch/aarch64/BUILD.gn
+++ b/test/vmapi/arch/aarch64/BUILD.gn
@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import("//build/image/image.gni")
+import("//test/hftest/args.gni")
 
 group("arch") {
   testonly = true
@@ -43,6 +44,7 @@
   testonly = true
 
   manifest = "manifest.dts"
+  manifest_overlay = hftest_manifest_overlay
   primary_name = "aarch64_test"
   primary_vm = ":aarch64_test_vm"
 }
diff --git a/test/vmapi/arch/aarch64/gicv3/BUILD.gn b/test/vmapi/arch/aarch64/gicv3/BUILD.gn
index 57ceb99..490f65b 100644
--- a/test/vmapi/arch/aarch64/gicv3/BUILD.gn
+++ b/test/vmapi/arch/aarch64/gicv3/BUILD.gn
@@ -14,6 +14,7 @@
 
 import("//build/image/image.gni")
 import("//build/toolchain/platform.gni")
+import("//test/hftest/args.gni")
 
 config("config") {
   include_dirs = [ "inc" ]
@@ -43,6 +44,7 @@
   testonly = true
 
   manifest = "manifest.dts"
+  manifest_overlay = hftest_manifest_overlay
   primary_name = "gicv3_test"
   primary_vm = ":gicv3_test_vm"
   secondary_vms = [ [
diff --git a/test/vmapi/primary_only/BUILD.gn b/test/vmapi/primary_only/BUILD.gn
index cdb35b9..bb084de 100644
--- a/test/vmapi/primary_only/BUILD.gn
+++ b/test/vmapi/primary_only/BUILD.gn
@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import("//build/image/image.gni")
+import("//test/hftest/args.gni")
 
 # Tests with no secondary VMs.
 vm_kernel("primary_only_test_vm") {
@@ -31,6 +32,7 @@
 initrd("primary_only_test") {
   testonly = true
   manifest = "manifest.dts"
+  manifest_overlay = hftest_manifest_overlay
   primary_name = "primary_only_test"
   primary_vm = ":primary_only_test_vm"
 }
diff --git a/test/vmapi/primary_with_secondaries/BUILD.gn b/test/vmapi/primary_with_secondaries/BUILD.gn
index ee3ab92..b0c3885 100644
--- a/test/vmapi/primary_with_secondaries/BUILD.gn
+++ b/test/vmapi/primary_with_secondaries/BUILD.gn
@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import("//build/image/image.gni")
+import("//test/hftest/args.gni")
 
 config("config") {
   include_dirs = [ "inc" ]
@@ -54,6 +55,7 @@
   testonly = true
 
   manifest = "manifest.dts"
+  manifest_overlay = hftest_manifest_overlay
 
   primary_name = "primary_with_secondaries_test"
   primary_vm = ":primary_with_secondaries_test_vm"