diff --git a/README.md b/README.md
index bf51709f..16086579 100644
--- a/README.md
+++ b/README.md
@@ -318,6 +318,19 @@ comma-separated options can be added to `--pbandk_out` before the colon and out
 
 *   To log debug logs during generation, `log=debug` can be set as well.
 
+*   **EXPERIMENTAL**: To generate code with not-null (required) message types, you can use a custom boolean option
+    by extending `google.protobuf.FieldOptions` to include a `bool` field such as `is_required` and then setting
+    `required_field_extension_number`. 
+ 
+    Note that this will invoke the default constructor for messages so if there is a circular dependency,
+    it is user's responsibility to omit it.
+
+    ```
+    protoc --pbandk_out=required_field_extension_number=50008:src/main/kotlin sample.proto
+    ```
+    
+    You can refer to `protoc-gen-pbandk/lib/src/jvmTest/resources/protos/custom_options.proto` as an example.
+
 Multiple options can be added to a single `--pbandk_out` argument by separating them with commas.
 
 In addition to running `protoc` manually, the
diff --git a/protoc-gen-pbandk/lib/src/commonMain/kotlin/pbandk/gen/CodeGenerator.kt b/protoc-gen-pbandk/lib/src/commonMain/kotlin/pbandk/gen/CodeGenerator.kt
index 31bc1acd..c33c2876 100644
--- a/protoc-gen-pbandk/lib/src/commonMain/kotlin/pbandk/gen/CodeGenerator.kt
+++ b/protoc-gen-pbandk/lib/src/commonMain/kotlin/pbandk/gen/CodeGenerator.kt
@@ -9,6 +9,7 @@ public open class CodeGenerator(
     public val params: Map<String, String>
 ) {
     protected val visibility: String = params["visibility"] ?: "public"
+    protected val requiredFieldNumber: Int? = params["required_field_extension_number"]?.toIntOrNull()
 
     protected val bld: StringBuilder = StringBuilder()
     protected var indent: String = ""
@@ -39,7 +40,7 @@ public open class CodeGenerator(
                 addDeprecatedAnnotation(field)
                 line(
                     "val ${field.extendeeKotlinType}.${field.kotlinFieldName}: ${
-                        field.kotlinValueType(true)
+                        field.kotlinValueType()
                     } "
                 ).indented {
                     line("get() = getExtension(${file.kotlinPackageName}.${field.kotlinFieldName})")
@@ -155,7 +156,7 @@ public open class CodeGenerator(
         if (field is File.Field.Numbered.Standard && field.required) {
             lineMid("val ${field.kotlinFieldName}: ${field.kotlinValueType(false)}")
         } else {
-            lineMid("val ${field.kotlinFieldName}: ${field.kotlinValueType(true)}")
+            lineMid("val ${field.kotlinFieldName}: ${field.kotlinValueType()}")
             lineMid(" = ${field.defaultValue}")
         }
         return this
@@ -507,7 +508,12 @@ public open class CodeGenerator(
     protected val File.Type.Message.mapEntryValueKotlinType: String?
         get() = if (!mapEntry) null else (fields[1] as File.Field.Numbered.Standard).kotlinValueType(true)
 
-    protected fun File.Field.Numbered.kotlinValueType(allowNulls: Boolean): String = when (this) {
+    protected val File.Field.Numbered.allowNulls: Boolean
+        get() =  requiredFieldNumber?.let {
+            // 1 means `required = true` so we can't allow nulls
+            this.options.unknownFields[it]?.values?.get(0)?.rawBytes?.array?.get(0)?.toInt() != 1
+        } ?: true
+    protected fun File.Field.Numbered.kotlinValueType(allowNulls: Boolean = this.allowNulls): String = when (this) {
         is File.Field.Numbered.Standard -> kotlinValueType(allowNulls)
         is File.Field.Numbered.Wrapper -> kotlinValueType(allowNulls)
     }
@@ -558,7 +564,7 @@ public open class CodeGenerator(
                 else "var $kotlinFieldName: pbandk.MessageMap.Builder<" +
                     "${mapEntry.mapEntryKeyKotlinType}, ${mapEntry.mapEntryValueKotlinType}>? = null"
             }
-            requiresExplicitTypeWithVal -> "var $kotlinFieldName: ${kotlinValueType(true)} = ${defaultValue()}"
+            requiresExplicitTypeWithVal -> "var $kotlinFieldName: ${kotlinValueType()} = ${defaultValue()}"
             else -> "var $kotlinFieldName = ${defaultValue()}"
         }
     protected val File.Field.Numbered.Standard.decodeWithVarDone: String
@@ -576,12 +582,12 @@ public open class CodeGenerator(
         else -> kotlinQualifiedTypeName
     }
 
-    protected fun File.Field.Numbered.Standard.defaultValue(allowNulls: Boolean = true): String = when {
+    protected fun File.Field.Numbered.Standard.defaultValue(allowNulls: Boolean = this.allowNulls): String = when {
         map -> "emptyMap()"
         repeated -> "emptyList()"
         allowNulls && hasPresence -> "null"
         type == File.Field.Type.ENUM -> "$kotlinQualifiedTypeName.fromValue(0)"
-        else -> type.defaultValue
+        else -> type.defaultValue(allowNulls, kotlinQualifiedTypeName)
     }
     protected val File.Field.Numbered.Standard.requiresExplicitTypeWithVal: Boolean
         get() = repeated || hasPresence || type.requiresExplicitTypeWithVal
@@ -648,8 +654,7 @@ public open class CodeGenerator(
             File.Field.Type.UINT32 -> "Int"
             File.Field.Type.UINT64 -> "Long"
         }
-    protected val File.Field.Type.defaultValue: String
-        get() = when (this) {
+    protected fun File.Field.Type.defaultValue(allowNulls: Boolean, kotlinQualifiedTypeName: String): String = when (this) {
             File.Field.Type.BOOL -> "false"
             File.Field.Type.BYTES -> "pbandk.ByteArr.empty"
             File.Field.Type.DOUBLE -> "0.0"
@@ -659,7 +664,7 @@ public open class CodeGenerator(
             File.Field.Type.FIXED64, File.Field.Type.INT64, File.Field.Type.SFIXED64,
             File.Field.Type.SINT64, File.Field.Type.UINT64 -> "0L"
             File.Field.Type.FLOAT -> "0.0F"
-            File.Field.Type.MESSAGE -> "null"
+            File.Field.Type.MESSAGE  -> if (allowNulls) "null" else  "$kotlinQualifiedTypeName()"
             File.Field.Type.STRING -> "\"\""
         }
     protected val File.Field.Type.requiresExplicitTypeWithVal: Boolean
diff --git a/protoc-gen-pbandk/lib/src/jvmTest/kotlin/CodeGeneratorTest.kt b/protoc-gen-pbandk/lib/src/jvmTest/kotlin/CodeGeneratorTest.kt
index e950f8b4..004bd3fa 100644
--- a/protoc-gen-pbandk/lib/src/jvmTest/kotlin/CodeGeneratorTest.kt
+++ b/protoc-gen-pbandk/lib/src/jvmTest/kotlin/CodeGeneratorTest.kt
@@ -46,6 +46,29 @@ class CodeGeneratorTest {
         assertTrue(deprecatedField.hasAnnotation<Deprecated>())
     }
 
+    @Test
+    fun testCustomOptionsWithExtensionNumber() {
+        val result = compileProto("custom_options.proto", "required_field_extension_number=50008")
+
+        assertEquals(ExitCode.OK, result.exitCode, result.messages)
+        val fooClazz = result.classLoader.loadClass("foobar.RequestType").kotlin
+
+        val requiredField = fooClazz.declaredMemberProperties.single { it.name == "myRequiredField" }
+        assertFalse(requiredField.returnType.isMarkedNullable)
+    }
+
+
+    @Test
+    fun testCustomOptionsWithoutExtensionNumber() {
+        val result = compileProto("custom_options.proto")
+
+        assertEquals(ExitCode.OK, result.exitCode, result.messages)
+        val fooClazz = result.classLoader.loadClass("foobar.RequestType").kotlin
+
+        val requiredField = fooClazz.declaredMemberProperties.single { it.name == "myRequiredField" }
+        assertTrue(requiredField.returnType.isMarkedNullable)
+    }
+
     @Test
     fun testOneOf_SameNameField() {
         val result = compileProto("oneof_same_name.proto")
diff --git a/protoc-gen-pbandk/lib/src/jvmTest/resources/protos/custom_options.proto b/protoc-gen-pbandk/lib/src/jvmTest/resources/protos/custom_options.proto
new file mode 100644
index 00000000..a4d8bdba
--- /dev/null
+++ b/protoc-gen-pbandk/lib/src/jvmTest/resources/protos/custom_options.proto
@@ -0,0 +1,73 @@
+// Examples taken from https://developers.google.com/protocol-buffers/docs/proto#customoptions
+syntax = "proto3";
+package foobar;
+
+import "google/protobuf/descriptor.proto";
+
+extend google.protobuf.FileOptions {
+  optional string my_file_option = 50000;
+}
+extend google.protobuf.MessageOptions {
+  optional int32 my_message_option = 50001;
+}
+extend google.protobuf.FieldOptions {
+  optional float my_field_option = 50002;
+  optional bool is_required = 50008;
+}
+extend google.protobuf.OneofOptions {
+  optional int64 my_oneof_option = 50003;
+}
+extend google.protobuf.EnumOptions {
+  optional bool my_enum_option = 50004;
+}
+extend google.protobuf.EnumValueOptions {
+  optional uint32 my_enum_value_option = 50005;
+}
+extend google.protobuf.ServiceOptions {
+  optional MyEnum my_service_option = 50006;
+}
+extend google.protobuf.MethodOptions {
+  optional MyMessage my_method_option = 50007;
+}
+
+option (my_file_option) = "Hello world!";
+
+message MyMessage {
+  option (my_message_option) = 1234;
+
+  optional int32 foo = 1 [(my_field_option) = 4.5];
+  optional string bar = 2;
+  oneof qux {
+    option (my_oneof_option) = 42;
+
+    string quux = 3;
+  }
+}
+
+enum MyEnum {
+  option (my_enum_option) = true;
+
+  FOO = 0 [(my_enum_value_option) = 321];
+  BAR = 1;
+}
+
+message RequestType {
+  MyMessage my_not_required_field = 1 [(is_required) = false];
+  MyMessage my_required_field = 2 [(is_required) = true];
+  repeated MyMessage my_not_required_fields = 3 [(is_required) = false];
+  repeated MyMessage my_required_fields = 4 [(is_required) = true];
+  map<int32, MyMessage> my_not_required_map = 5 [(is_required) = false];
+  map<int32, MyMessage> my_required_map = 6 [(is_required) = true];
+}
+message ResponseType {}
+
+service MyService {
+  option (my_service_option) = FOO;
+
+  rpc MyMethod(RequestType) returns(ResponseType) {
+    // Note:  my_method_option has type MyMessage.  We can set each field
+    //   within it using a separate "option" line.
+    option (my_method_option).foo = 567;
+    option (my_method_option).bar = "Some string";
+  }
+}
diff --git a/protoc-gen-pbandk/lib/src/jvmTest/resources/protos/options.proto b/protoc-gen-pbandk/lib/src/jvmTest/resources/protos/options.proto
index 9c4b418a..284fbd15 100644
--- a/protoc-gen-pbandk/lib/src/jvmTest/resources/protos/options.proto
+++ b/protoc-gen-pbandk/lib/src/jvmTest/resources/protos/options.proto
@@ -3,4 +3,4 @@ package foobar;
 
 message Foo {
     int32 deprecated_field = 1 [deprecated = true];
-}
\ No newline at end of file
+}
diff --git a/protoc-gen-pbandk/lib/src/jvmTest/resources/protos/simple.proto b/protoc-gen-pbandk/lib/src/jvmTest/resources/protos/simple.proto
index 9930cfd6..85c65ff9 100644
--- a/protoc-gen-pbandk/lib/src/jvmTest/resources/protos/simple.proto
+++ b/protoc-gen-pbandk/lib/src/jvmTest/resources/protos/simple.proto
@@ -7,4 +7,4 @@ message Message1 {
 
 message Message2 {
     string str_val = 1;
-}
\ No newline at end of file
+}