diff --git a/schemars/src/_private.rs b/schemars/src/_private.rs
index 86898567..1519ca2d 100644
--- a/schemars/src/_private.rs
+++ b/schemars/src/_private.rs
@@ -18,6 +18,8 @@ pub fn json_schema_for_flatten<T: ?Sized + JsonSchema>(
     }
 
     SemiRecursiveTransform(|schema: &mut Schema| {
+        // Always allow aditional/unevaluated properties, because the outer struct determines
+        // whether it denies unknown fields.
         if let Some(obj) = schema.as_object_mut() {
             if obj.get("additionalProperties").and_then(Value::as_bool) == Some(false) {
                 obj.remove("additionalProperties");
@@ -214,13 +216,6 @@ pub fn flatten(schema: &mut Schema, other: Schema) {
                             }
                         }
                     }
-                    "additionalProperties" | "unevaluatedProperties" => {
-                        // Even if an outer type has `deny_unknown_fields`, unknown fields
-                        // may be accepted by the flattened type
-                        if occupied.get() == &Value::Bool(false) {
-                            *occupied.into_mut() = value2;
-                        }
-                    }
                     "oneOf" | "anyOf" => {
                         // `OccupiedEntry` currently has no `.remove_entry()` method :(
                         let key = occupied.key().clone();
@@ -245,16 +240,49 @@ pub fn flatten(schema: &mut Schema, other: Schema) {
     match other.try_to_object() {
         Err(false) => {}
         Err(true) => {
-            schema
-                .ensure_object()
-                .insert("unevaluatedProperties".to_owned(), true.into());
+            if let Some(obj) = schema.as_object_mut() {
+                if !obj.contains_key("additionalProperties")
+                    && !obj.contains_key("unevaluatedProperties")
+                {
+                    let key = if contains_immediate_subschema(obj) {
+                        "unevaluatedProperties"
+                    } else {
+                        "additionalProperties"
+                    };
+                    obj.insert(key.to_owned(), true.into());
+                }
+            }
         }
-        Ok(obj2) => {
+        Ok(mut obj2) => {
             let obj1 = schema.ensure_object();
 
+            // For complex merges, replace `additionalProperties` with `unevaluatedProperties`
+            // which usually "works out better".
+            normalise_additional_unevaluated_properties(obj1, &obj2);
+            normalise_additional_unevaluated_properties(&mut obj2, obj1);
+
             for (key, value2) in obj2 {
                 flatten_property(obj1, key, value2);
             }
         }
     }
 }
+
+fn normalise_additional_unevaluated_properties(
+    schema_obj1: &mut Map<String, Value>,
+    schema_obj2: &Map<String, Value>,
+) {
+    if schema_obj1.contains_key("additionalProperties")
+        && (schema_obj2.contains_key("unevaluatedProperties")
+            || contains_immediate_subschema(schema_obj2))
+    {
+        let ap = schema_obj1.remove("additionalProperties");
+        schema_obj1.insert("unevaluatedProperties".to_owned(), ap.into());
+    }
+}
+
+fn contains_immediate_subschema(schema_obj: &Map<String, Value>) -> bool {
+    ["if", "then", "else", "allOf", "anyOf", "oneOf", "$ref"]
+        .into_iter()
+        .any(|k| schema_obj.contains_key(k))
+}
diff --git a/schemars/src/json_schema_impls/maps.rs b/schemars/src/json_schema_impls/maps.rs
index ff179d09..86d43c8b 100644
--- a/schemars/src/json_schema_impls/maps.rs
+++ b/schemars/src/json_schema_impls/maps.rs
@@ -21,7 +21,7 @@ macro_rules! map_impl {
             fn json_schema(generator: &mut SchemaGenerator) -> Schema {
                 json_schema!({
                     "type": "object",
-                    "unevaluatedProperties": generator.subschema_for::<V>(),
+                    "additionalProperties": generator.subschema_for::<V>(),
                 })
             }
         }
diff --git a/schemars/tests/enum_deny_unknown_fields.rs b/schemars/tests/enum_deny_unknown_fields.rs
index 2979981e..ef56d058 100644
--- a/schemars/tests/enum_deny_unknown_fields.rs
+++ b/schemars/tests/enum_deny_unknown_fields.rs
@@ -17,8 +17,8 @@ struct Struct {
     bar: bool,
 }
 
-// Outer container should always have unevaluatedProperties: false
-// `Struct` variant should have unevaluatedProperties: false
+// Outer container should always have additionalProperties: false
+// `Struct` variant should have additionalProperties: false
 #[allow(dead_code)]
 #[derive(JsonSchema)]
 #[schemars(rename_all = "camelCase", deny_unknown_fields)]
@@ -42,7 +42,7 @@ fn enum_external_tag() -> TestResult {
     test_default_generated_schema::<External>("enum-external-duf")
 }
 
-// Only `Struct` variant should have unevaluatedProperties: false
+// Only `Struct` variant should have additionalProperties: false
 #[allow(dead_code)]
 #[derive(JsonSchema)]
 #[schemars(tag = "typeProperty", deny_unknown_fields)]
@@ -65,7 +65,7 @@ fn enum_internal_tag() -> TestResult {
     test_default_generated_schema::<Internal>("enum-internal-duf")
 }
 
-// Only `Struct` variant should have unevaluatedProperties: false
+// Only `Struct` variant should have additionalProperties: false
 #[allow(dead_code)]
 #[derive(JsonSchema)]
 #[schemars(untagged, deny_unknown_fields)]
@@ -88,7 +88,7 @@ fn enum_untagged() -> TestResult {
     test_default_generated_schema::<Untagged>("enum-untagged-duf")
 }
 
-// Outer container and `Struct` variant should have unevaluatedProperties: false
+// Outer container and `Struct` variant should have additionalProperties: false
 #[allow(dead_code)]
 #[derive(JsonSchema)]
 #[schemars(tag = "t", content = "c", deny_unknown_fields)]
diff --git a/schemars/tests/expected/enum-adjacent-tagged-duf.json b/schemars/tests/expected/enum-adjacent-tagged-duf.json
index 6f3950cc..dfb3bb8b 100644
--- a/schemars/tests/expected/enum-adjacent-tagged-duf.json
+++ b/schemars/tests/expected/enum-adjacent-tagged-duf.json
@@ -15,7 +15,7 @@
       "required": [
         "t"
       ],
-      "unevaluatedProperties": false
+      "additionalProperties": false
     },
     {
       "type": "object",
@@ -28,7 +28,7 @@
         },
         "c": {
           "type": "object",
-          "unevaluatedProperties": {
+          "additionalProperties": {
             "type": "string"
           }
         }
@@ -37,7 +37,7 @@
         "t",
         "c"
       ],
-      "unevaluatedProperties": false
+      "additionalProperties": false
     },
     {
       "type": "object",
@@ -56,7 +56,7 @@
         "t",
         "c"
       ],
-      "unevaluatedProperties": false
+      "additionalProperties": false
     },
     {
       "type": "object",
@@ -75,7 +75,7 @@
         "t",
         "c"
       ],
-      "unevaluatedProperties": false
+      "additionalProperties": false
     },
     {
       "type": "object",
@@ -97,7 +97,7 @@
               "type": "boolean"
             }
           },
-          "unevaluatedProperties": false,
+          "additionalProperties": false,
           "required": [
             "foo",
             "bar"
@@ -108,7 +108,7 @@
         "t",
         "c"
       ],
-      "unevaluatedProperties": false
+      "additionalProperties": false
     },
     {
       "type": "object",
@@ -138,7 +138,7 @@
         "t",
         "c"
       ],
-      "unevaluatedProperties": false
+      "additionalProperties": false
     },
     {
       "type": "object",
@@ -153,7 +153,7 @@
       "required": [
         "t"
       ],
-      "unevaluatedProperties": false
+      "additionalProperties": false
     },
     {
       "type": "object",
@@ -173,7 +173,7 @@
         "t",
         "c"
       ],
-      "unevaluatedProperties": false
+      "additionalProperties": false
     }
   ],
   "$defs": {
diff --git a/schemars/tests/expected/enum-adjacent-tagged.json b/schemars/tests/expected/enum-adjacent-tagged.json
index be374ae9..c631ae54 100644
--- a/schemars/tests/expected/enum-adjacent-tagged.json
+++ b/schemars/tests/expected/enum-adjacent-tagged.json
@@ -27,7 +27,7 @@
         },
         "c": {
           "type": "object",
-          "unevaluatedProperties": {
+          "additionalProperties": {
             "type": "string"
           }
         }
diff --git a/schemars/tests/expected/enum-external-duf.json b/schemars/tests/expected/enum-external-duf.json
index 8d123023..76be5b3f 100644
--- a/schemars/tests/expected/enum-external-duf.json
+++ b/schemars/tests/expected/enum-external-duf.json
@@ -14,7 +14,7 @@
       "properties": {
         "stringMap": {
           "type": "object",
-          "unevaluatedProperties": {
+          "additionalProperties": {
             "type": "string"
           }
         }
@@ -62,7 +62,7 @@
               "type": "boolean"
             }
           },
-          "unevaluatedProperties": false,
+          "additionalProperties": false,
           "required": [
             "foo",
             "bar"
diff --git a/schemars/tests/expected/enum-external.json b/schemars/tests/expected/enum-external.json
index 08ffad5b..3c660fb9 100644
--- a/schemars/tests/expected/enum-external.json
+++ b/schemars/tests/expected/enum-external.json
@@ -14,7 +14,7 @@
       "properties": {
         "stringMap": {
           "type": "object",
-          "unevaluatedProperties": {
+          "additionalProperties": {
             "type": "string"
           }
         }
diff --git a/schemars/tests/expected/enum-internal-duf.json b/schemars/tests/expected/enum-internal-duf.json
index 4056f903..73e4743b 100644
--- a/schemars/tests/expected/enum-internal-duf.json
+++ b/schemars/tests/expected/enum-internal-duf.json
@@ -23,7 +23,7 @@
           "const": "StringMap"
         }
       },
-      "unevaluatedProperties": {
+      "additionalProperties": {
         "type": "string"
       },
       "required": [
@@ -79,7 +79,7 @@
           "const": "Struct"
         }
       },
-      "unevaluatedProperties": false,
+      "additionalProperties": false,
       "required": [
         "typeProperty",
         "foo",
diff --git a/schemars/tests/expected/enum-internal.json b/schemars/tests/expected/enum-internal.json
index 436202ad..2fd9770f 100644
--- a/schemars/tests/expected/enum-internal.json
+++ b/schemars/tests/expected/enum-internal.json
@@ -22,7 +22,7 @@
           "const": "StringMap"
         }
       },
-      "unevaluatedProperties": {
+      "additionalProperties": {
         "type": "string"
       },
       "required": [
diff --git a/schemars/tests/expected/enum-untagged-duf.json b/schemars/tests/expected/enum-untagged-duf.json
index 65a20fc1..58bdbe16 100644
--- a/schemars/tests/expected/enum-untagged-duf.json
+++ b/schemars/tests/expected/enum-untagged-duf.json
@@ -7,7 +7,7 @@
     },
     {
       "type": "object",
-      "unevaluatedProperties": {
+      "additionalProperties": {
         "type": "string"
       }
     },
@@ -28,7 +28,7 @@
           "type": "boolean"
         }
       },
-      "unevaluatedProperties": false,
+      "additionalProperties": false,
       "required": [
         "foo",
         "bar"
diff --git a/schemars/tests/expected/enum-untagged.json b/schemars/tests/expected/enum-untagged.json
index 87e3f2e0..643cd20c 100644
--- a/schemars/tests/expected/enum-untagged.json
+++ b/schemars/tests/expected/enum-untagged.json
@@ -7,7 +7,7 @@
     },
     {
       "type": "object",
-      "unevaluatedProperties": {
+      "additionalProperties": {
         "type": "string"
       }
     },
diff --git a/schemars/tests/expected/flattened_value.json b/schemars/tests/expected/flattened_value.json
index 1b53cdbc..fe79d426 100644
--- a/schemars/tests/expected/flattened_value.json
+++ b/schemars/tests/expected/flattened_value.json
@@ -10,5 +10,5 @@
   "required": [
     "flag"
   ],
-  "unevaluatedProperties": true
+  "additionalProperties": true
 }
\ No newline at end of file
diff --git a/schemars/tests/expected/indexmap.json b/schemars/tests/expected/indexmap.json
index f0369228..8ba90a85 100644
--- a/schemars/tests/expected/indexmap.json
+++ b/schemars/tests/expected/indexmap.json
@@ -5,7 +5,7 @@
   "properties": {
     "map": {
       "type": "object",
-      "unevaluatedProperties": {
+      "additionalProperties": {
         "type": "boolean"
       }
     },
diff --git a/schemars/tests/expected/remote_derive_generic.json b/schemars/tests/expected/remote_derive_generic.json
index 135a7ff4..bef4d6ac 100644
--- a/schemars/tests/expected/remote_derive_generic.json
+++ b/schemars/tests/expected/remote_derive_generic.json
@@ -14,7 +14,7 @@
     },
     "fake_map": {
       "type": "object",
-      "unevaluatedProperties": {
+      "additionalProperties": {
         "type": "array",
         "uniqueItems": true,
         "items": {
diff --git a/schemars/tests/expected/schema_settings-2019_09.json b/schemars/tests/expected/schema_settings-2019_09.json
index 25ab2f86..bc99f155 100644
--- a/schemars/tests/expected/schema_settings-2019_09.json
+++ b/schemars/tests/expected/schema_settings-2019_09.json
@@ -13,7 +13,7 @@
     },
     "values": {
       "type": "object",
-      "unevaluatedProperties": true
+      "additionalProperties": true
     },
     "value": true,
     "inner": {
diff --git a/schemars/tests/expected/schema_settings-2020_12.json b/schemars/tests/expected/schema_settings-2020_12.json
index 68818f00..79640586 100644
--- a/schemars/tests/expected/schema_settings-2020_12.json
+++ b/schemars/tests/expected/schema_settings-2020_12.json
@@ -13,7 +13,7 @@
     },
     "values": {
       "type": "object",
-      "unevaluatedProperties": true
+      "additionalProperties": true
     },
     "value": true,
     "inner": {
diff --git a/schemars/tests/expected/struct-normal-additional-properties.json b/schemars/tests/expected/struct-normal-additional-properties.json
index 30f0062e..2c55fb7d 100644
--- a/schemars/tests/expected/struct-normal-additional-properties.json
+++ b/schemars/tests/expected/struct-normal-additional-properties.json
@@ -17,7 +17,7 @@
       ]
     }
   },
-  "unevaluatedProperties": false,
+  "additionalProperties": false,
   "required": [
     "foo",
     "bar"
diff --git a/schemars/tests/expected/test_flattened_struct_deny_unknown_fields.json b/schemars/tests/expected/test_flattened_struct_deny_unknown_fields.json
index 419576e7..1ac94b54 100644
--- a/schemars/tests/expected/test_flattened_struct_deny_unknown_fields.json
+++ b/schemars/tests/expected/test_flattened_struct_deny_unknown_fields.json
@@ -42,7 +42,7 @@
           "type": "boolean"
         }
       },
-      "unevaluatedProperties": false,
+      "additionalProperties": false,
       "required": [
         "middle_field",
         "inner_field"
diff --git a/schemars/tests/expected/validate.json b/schemars/tests/expected/validate.json
index 568f2bd2..57c7f521 100644
--- a/schemars/tests/expected/validate.json
+++ b/schemars/tests/expected/validate.json
@@ -68,7 +68,7 @@
     },
     "map_contains": {
       "type": "object",
-      "unevaluatedProperties": {
+      "additionalProperties": {
         "type": "null"
       },
       "required": [
diff --git a/schemars/tests/expected/validate_schemars_attrs.json b/schemars/tests/expected/validate_schemars_attrs.json
index 0122e43f..5e7d31d7 100644
--- a/schemars/tests/expected/validate_schemars_attrs.json
+++ b/schemars/tests/expected/validate_schemars_attrs.json
@@ -68,7 +68,7 @@
     },
     "map_contains": {
       "type": "object",
-      "unevaluatedProperties": {
+      "additionalProperties": {
         "type": "null"
       },
       "required": [
diff --git a/schemars/tests/flatten.rs b/schemars/tests/flatten.rs
index ffd8d1fa..a05a5c0b 100644
--- a/schemars/tests/flatten.rs
+++ b/schemars/tests/flatten.rs
@@ -76,24 +76,6 @@ struct FlattenMap {
     value: BTreeMap<String, Value>,
 }
 
-#[allow(dead_code)]
-#[derive(JsonSchema)]
-#[schemars(rename = "FlattenValue", deny_unknown_fields)]
-struct FlattenValueDenyUnknownFields {
-    flag: bool,
-    #[serde(flatten)]
-    value: Value,
-}
-
-#[allow(dead_code)]
-#[derive(JsonSchema)]
-#[schemars(rename = "FlattenValue", deny_unknown_fields)]
-struct FlattenMapDenyUnknownFields {
-    flag: bool,
-    #[serde(flatten)]
-    value: BTreeMap<String, Value>,
-}
-
 #[test]
 fn test_flattened_value() -> TestResult {
     test_default_generated_schema::<FlattenValue>("flattened_value")
@@ -105,18 +87,6 @@ fn test_flattened_map() -> TestResult {
     test_default_generated_schema::<FlattenMap>("flattened_value")
 }
 
-#[test]
-fn test_flattened_value_deny_unknown_fields() -> TestResult {
-    // intentionally using the same file as test_flattened_value, as the schema should be identical
-    test_default_generated_schema::<FlattenValueDenyUnknownFields>("flattened_value")
-}
-
-#[test]
-fn test_flattened_map_deny_unknown_fields() -> TestResult {
-    // intentionally using the same file as test_flattened_value, as the schema should be identical
-    test_default_generated_schema::<FlattenMapDenyUnknownFields>("flattened_value")
-}
-
 #[derive(JsonSchema)]
 pub struct OuterAllowUnknownFields {
     pub outer_field: bool,
diff --git a/schemars/tests/struct_additional_properties.rs b/schemars/tests/struct_additional_properties.rs
index 93a4f290..7fd64f33 100644
--- a/schemars/tests/struct_additional_properties.rs
+++ b/schemars/tests/struct_additional_properties.rs
@@ -2,8 +2,6 @@ mod util;
 use schemars::JsonSchema;
 use util::*;
 
-// TODO rename file and test
-
 #[allow(dead_code)]
 #[derive(JsonSchema)]
 #[serde(deny_unknown_fields)]
diff --git a/schemars_derive/src/schema_exprs.rs b/schemars_derive/src/schema_exprs.rs
index 6906b9f0..5b1eb977 100644
--- a/schemars_derive/src/schema_exprs.rs
+++ b/schemars_derive/src/schema_exprs.rs
@@ -303,9 +303,9 @@ fn expr_for_adjacent_tagged_enum<'a>(
                 })
             };
 
-            let set_unevaluated_properties = if deny_unknown_fields {
+            let set_additional_properties = if deny_unknown_fields {
                 quote! {
-                    "unevaluatedProperties": false,
+                    "additionalProperties": false,
                 }
             } else {
                 TokenStream::new()
@@ -324,7 +324,7 @@ fn expr_for_adjacent_tagged_enum<'a>(
                     ],
                     // As we're creating a "wrapper" object, we can honor the
                     // disposition of deny_unknown_fields.
-                    #set_unevaluated_properties
+                    #set_additional_properties
                 })
             };
 
@@ -498,9 +498,9 @@ fn expr_for_struct(
             })
         .collect();
 
-    let set_unevaluated_properties = if deny_unknown_fields {
+    let set_additional_properties = if deny_unknown_fields {
         quote! {
-            "unevaluatedProperties": false,
+            "additionalProperties": false,
         }
     } else {
         TokenStream::new()
@@ -510,7 +510,7 @@ fn expr_for_struct(
         #set_container_default
         let mut schema = schemars::json_schema!({
             "type": "object",
-            #set_unevaluated_properties
+            #set_additional_properties
         });
         #(#properties)*
         schema