diff --git a/src/NJsonSchema.CodeGeneration.CSharp.Tests/AdditionalPropertiesTests.cs b/src/NJsonSchema.CodeGeneration.CSharp.Tests/AdditionalPropertiesTests.cs
index ec32c631b..6f5329547 100644
--- a/src/NJsonSchema.CodeGeneration.CSharp.Tests/AdditionalPropertiesTests.cs
+++ b/src/NJsonSchema.CodeGeneration.CSharp.Tests/AdditionalPropertiesTests.cs
@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using System.Text.RegularExpressions;
 using System.Threading.Tasks;
 using NJsonSchema.CodeGeneration.CSharp;
 using NJsonSchema.Generation;
@@ -43,6 +44,104 @@ public async Task When_additionalProperties_schema_is_set_for_object_then_specia
             Assert.Contains("public System.Collections.Generic.IDictionary<string, object> AdditionalProperties", code);
         }
 
+        [Fact]
+        public async Task When_using_SystemTextJson_additionalProperties_schema_is_set_for_object_then_special_property_is_rendered()
+        {
+            //// Arrange
+            var json =
+                @"{ 
+    ""properties"": {
+        ""Pet"": {
+            ""type"": ""object"",
+            ""properties"": {
+                ""id"": {
+                    ""type"": ""integer"",
+                    ""format"": ""int64""
+                },
+                ""category"": {
+                    ""type"": ""string""
+                }
+            },
+            ""additionalProperties"": {
+                ""type"": ""string""
+            }
+        }
+    }
+}";
+            var schema = await JsonSchema.FromJsonAsync(json);
+
+            //// Act
+            var generator = new CSharpGenerator(schema, new CSharpGeneratorSettings()
+            {
+                JsonLibrary = CSharpJsonLibrary.SystemTextJson
+            });
+            var code = generator.GenerateFile("Person");
+
+            //// Assert
+            Assert.Contains("[System.Text.Json.Serialization.JsonExtensionData]", code);
+            Assert.Contains("public System.Collections.Generic.IDictionary<string, object> AdditionalProperties", code);
+        }
+
+        [Fact]
+        public async Task When_using_SystemTextJson_additionalProperties_schema_is_set_for_object_then_special_property_is_rendered_only_for_base_class()
+        {
+            var json =
+                @"{ 
+    ""properties"": {
+        ""dog"": {
+            ""allOf"": [
+                {
+                    ""$ref"": ""#/components/Pet""
+                },
+                {
+                    ""description"": ""Dog""
+                }
+            ]
+        },
+        ""cat"": {
+            ""allOf"": [
+                {
+                    ""$ref"": ""#/components/Pet""
+                },
+                {
+                    ""description"": ""Cat""
+                }
+            ]
+        }
+    },
+    ""components"": {
+        ""Pet"": {
+            ""type"": ""object"",
+            ""description"": ""Pet"",
+            ""properties"": {
+                ""id"": {
+                    ""type"": ""integer"",
+                    ""format"": ""int64""
+                },
+                ""category"": {
+                    ""type"": ""string""
+                }
+            }
+        }
+    }
+}";
+
+            var schema = await JsonSchema.FromJsonAsync(json);
+
+            //// Act
+            var generator = new CSharpGenerator(schema, new CSharpGeneratorSettings()
+            {
+                JsonLibrary = CSharpJsonLibrary.SystemTextJson
+            });
+            var code = generator.GenerateFile("Person");
+
+            //// Assert
+            var matches = Regex.Matches(code, @"(\[System\.Text\.Json\.Serialization\.JsonExtensionData\])");
+            
+            // There are two matches, the Person class and the Pet class
+            Assert.Equal(2, matches.Count);
+        }
+
         public class Page
         {
         }
diff --git a/src/NJsonSchema.CodeGeneration.CSharp.Tests/CSharpJsonSerializerGeneratorTests.cs b/src/NJsonSchema.CodeGeneration.CSharp.Tests/CSharpJsonSerializerGeneratorTests.cs
index 1e7df06b9..41b204477 100644
--- a/src/NJsonSchema.CodeGeneration.CSharp.Tests/CSharpJsonSerializerGeneratorTests.cs
+++ b/src/NJsonSchema.CodeGeneration.CSharp.Tests/CSharpJsonSerializerGeneratorTests.cs
@@ -6,7 +6,7 @@ namespace NJsonSchema.CodeGeneration.CSharp.Tests
     public class CSharpJsonSerializerGeneratorTests
     {
         [Fact]
-        public void When_using_SytemTextJson_GenerateJsonSerializerParameterCode_generates_correctly()
+        public void When_using_SystemTextJson_GenerateJsonSerializerParameterCode_generates_correctly()
         {
             //// Arrange
             var additionalJsonConverters = new string[] { "AdditionalConverter1", "AdditionalConverter2" };
@@ -43,7 +43,7 @@ public void When_using_NewtonsoftJson_with_JsonConverters_GenerateJsonSerializer
         }
 
         [Fact]
-        public void When_using_SytemTextJson_with_JsonSerializerSettingsOrOptionsTransformationMethod_GenerateJsonSerializerParameterCode_generates_correctly()
+        public void When_using_SystemTextJson_with_JsonSerializerSettingsOrOptionsTransformationMethod_GenerateJsonSerializerParameterCode_generates_correctly()
         {
             //// Arrange
             var settings = new CSharpGeneratorSettings
@@ -82,7 +82,7 @@ public void When_using_NewtonsoftJson_with_HandleReferences_and_JsonConverters_a
         }
 
         [Fact]
-        public void When_using_SytemTextJson_with_JsonConverters_GenerateJsonConvertersArrayCode_generates_correctly()
+        public void When_using_SystemTextJson_with_JsonConverters_GenerateJsonConvertersArrayCode_generates_correctly()
         {
             //// Arrange
             var additionalJsonConverters = new string[] { "AdditionalConverter1", "AdditionalConverter2" };
diff --git a/src/NJsonSchema.CodeGeneration.CSharp.Tests/GeneralGeneratorTests.cs b/src/NJsonSchema.CodeGeneration.CSharp.Tests/GeneralGeneratorTests.cs
index cff1db46e..d1f938522 100644
--- a/src/NJsonSchema.CodeGeneration.CSharp.Tests/GeneralGeneratorTests.cs
+++ b/src/NJsonSchema.CodeGeneration.CSharp.Tests/GeneralGeneratorTests.cs
@@ -1830,7 +1830,7 @@ private static void AssertCompile(string code)
         }
 
         [Fact]
-        public async Task When_using_SytemTextJson_without_JsonConverters_generates_FromJson_and_ToJson_correctly()
+        public async Task When_using_SystemTextJson_without_JsonConverters_generates_FromJson_and_ToJson_correctly()
         {
             //// Arrange
             var expectedToJsonMethod =
@@ -1870,7 +1870,7 @@ public static Person FromJson(string data)
         }
 
         [Fact]
-        public async Task When_using_SytemTextJson_with_JsonConverters_generates_FromJson_and_ToJson_correctly()
+        public async Task When_using_SystemTextJson_with_JsonConverters_generates_FromJson_and_ToJson_correctly()
         {
             //// Arrange
             var expectedToJsonMethod =
diff --git a/src/NJsonSchema.CodeGeneration.CSharp/Models/ClassTemplateModel.cs b/src/NJsonSchema.CodeGeneration.CSharp/Models/ClassTemplateModel.cs
index af1e29cc4..4847682ae 100644
--- a/src/NJsonSchema.CodeGeneration.CSharp/Models/ClassTemplateModel.cs
+++ b/src/NJsonSchema.CodeGeneration.CSharp/Models/ClassTemplateModel.cs
@@ -62,14 +62,15 @@ public ClassTemplateModel(string typeName, CSharpGeneratorSettings settings,
         /// <summary>Gets a value indicating whether the C#8 nullable reference types are enabled for this file.</summary>
         public bool GenerateNullableReferenceTypes => _settings.GenerateNullableReferenceTypes;
 
-        /// <summary>Gets a value indicating whether an additional properties type is available.</summary>
+        /// <summary>Gets a value indicating whether an additional properties type is available and needed.</summary>
         public bool HasAdditionalPropertiesType =>
             !_schema.IsDictionary &&
             !_schema.ActualTypeSchema.IsDictionary &&
             !_schema.IsArray &&
             !_schema.ActualTypeSchema.IsArray &&
             (_schema.ActualTypeSchema.AllowAdditionalProperties ||
-             _schema.ActualTypeSchema.AdditionalPropertiesSchema != null);
+             _schema.ActualTypeSchema.AdditionalPropertiesSchema != null)
+            && BaseClass?.HasAdditionalPropertiesType != true; // if base class already has extension data array, we need to avoid it in the subclass
 
         /// <summary>Gets the type of the additional properties.</summary>
         public string AdditionalPropertiesType => HasAdditionalPropertiesType ? "object" : null; // TODO: Find a way to use typed dictionaries