diff --git a/schemafy_lib/src/generator.rs b/schemafy_lib/src/generator.rs
index 74f1f64..8adbc80 100644
--- a/schemafy_lib/src/generator.rs
+++ b/schemafy_lib/src/generator.rs
@@ -1,9 +1,14 @@
-use crate::Expander;
+use crate::{Expander, ExpanderOptions};
 use std::{
     io,
     path::{Path, PathBuf},
 };
 
+#[derive(Debug, PartialEq)]
+pub struct GeneratorOptions {
+    pub deny_unknown_fields: bool,
+}
+
 /// A configurable builder for generating Rust types from a JSON
 /// schema.
 ///
@@ -23,6 +28,8 @@ pub struct Generator<'a, 'b> {
     pub schemafy_path: &'a str,
     /// The JSON schema file to read
     pub input_file: &'b Path,
+
+    pub generator_options: &'a GeneratorOptions,
 }
 
 impl<'a, 'b> Generator<'a, 'b> {
@@ -50,7 +57,15 @@ impl<'a, 'b> Generator<'a, 'b> {
                 err
             )
         });
-        let mut expander = Expander::new(self.root_name.as_deref(), self.schemafy_path, &schema);
+        let expander_options = ExpanderOptions {
+            deny_unknown_fields: self.generator_options.deny_unknown_fields,
+        };
+        let mut expander = Expander::new(
+            self.root_name.as_deref(),
+            self.schemafy_path,
+            &schema,
+            &expander_options,
+        );
         expander.expand(&schema)
     }
 
@@ -79,6 +94,9 @@ impl<'a, 'b> Default for GeneratorBuilder<'a, 'b> {
                 root_name: None,
                 schemafy_path: "::schemafy_core::",
                 input_file: Path::new("schema.json"),
+                generator_options: &GeneratorOptions {
+                    deny_unknown_fields: false,
+                },
             },
         }
     }
@@ -101,6 +119,10 @@ impl<'a, 'b> GeneratorBuilder<'a, 'b> {
         self.inner.schemafy_path = schemafy_path;
         self
     }
+    pub fn with_options(mut self, generator_options: &'a GeneratorOptions) -> Self {
+        self.inner.generator_options = generator_options;
+        self
+    }
     pub fn build(self) -> Generator<'a, 'b> {
         self.inner
     }
diff --git a/schemafy_lib/src/lib.rs b/schemafy_lib/src/lib.rs
index 0343b0f..a0519db 100644
--- a/schemafy_lib/src/lib.rs
+++ b/schemafy_lib/src/lib.rs
@@ -69,7 +69,7 @@ use uriparse::{Fragment, URI};
 
 pub use schema::{Schema, SimpleTypes};
 
-pub use generator::{Generator, GeneratorBuilder};
+pub use generator::{Generator, GeneratorBuilder, GeneratorOptions};
 
 use proc_macro2::{Span, TokenStream};
 
@@ -310,10 +310,23 @@ impl<'a, 'r> FieldExpander<'a, 'r> {
     }
 }
 
+pub struct ExpanderOptions {
+    deny_unknown_fields: bool,
+}
+
+impl Default for ExpanderOptions {
+    fn default() -> Self {
+        ExpanderOptions {
+            deny_unknown_fields: true,
+        }
+    }
+}
+
 pub struct Expander<'r> {
     root_name: Option<&'r str>,
     schemafy_path: &'r str,
     root: &'r Schema,
+    options: &'r ExpanderOptions,
     current_type: String,
     current_field: String,
     types: Vec<(String, TokenStream)>,
@@ -343,11 +356,13 @@ impl<'r> Expander<'r> {
         root_name: Option<&'r str>,
         schemafy_path: &'r str,
         root: &'r Schema,
+        options: &'r ExpanderOptions,
     ) -> Expander<'r> {
         Expander {
             root_name,
             root,
             schemafy_path,
+            options,
             current_field: "".into(),
             current_type: "".into(),
             types: Vec::new(),
@@ -610,6 +625,7 @@ impl<'r> Expander<'r> {
         let type_decl = if is_struct {
             let serde_deny_unknown = if schema.additional_properties == Some(Value::Bool(false))
                 && schema.pattern_properties.is_empty()
+                && self.options.deny_unknown_fields
             {
                 Some(quote! { #[serde(deny_unknown_fields)] })
             } else {
@@ -796,7 +812,13 @@ mod tests {
     fn test_expander_type_ref() {
         let json = std::fs::read_to_string("src/schema.json").expect("Read schema JSON file");
         let schema = serde_json::from_str(&json).unwrap_or_else(|err| panic!("{}", err));
-        let expander = Expander::new(Some("SchemaName"), "::schemafy_core::", &schema);
+        let expander_options = ExpanderOptions::default();
+        let expander = Expander::new(
+            Some("SchemaName"),
+            "::schemafy_core::",
+            &schema,
+            &expander_options,
+        );
 
         assert_eq!(expander.type_ref("normalField"), "NormalField");
         assert_eq!(expander.type_ref("#"), "SchemaName");
@@ -837,7 +859,8 @@ mod tests {
         let json = std::fs::read_to_string("tests/multiple-property-types.json")
             .expect("Read schema JSON file");
         let schema = serde_json::from_str(&json).unwrap();
-        let mut expander = Expander::new(Some("Root"), "UNUSED", &schema);
+        let expander_options = ExpanderOptions::default();
+        let mut expander = Expander::new(Some("Root"), "UNUSED", &schema, &expander_options);
         expander.expand(&schema);
 
         // check that the type names for embedded objects only include their
diff --git a/src/main.rs b/src/main.rs
index 386ad62..a178a22 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -3,7 +3,7 @@ use std::path::Path;
 use std::process::{Command, Stdio};
 
 use anyhow::{anyhow, bail, Context, Result};
-use schemafy_lib::Generator;
+use schemafy_lib::{Generator, GeneratorOptions};
 use structopt::StructOpt;
 use tempfile::NamedTempFile;
 
@@ -18,16 +18,22 @@ struct Opts {
     output: Option<String>,
     /// JSON schema file
     schema_path: String,
+    /// Flag to allow new fields from the server.
+    #[structopt(long)]
+    allow_unknown_fields: bool,
 }
 
 pub fn main() -> Result<()> {
     let opts = Opts::from_args();
-
+    let generator_options = GeneratorOptions {
+        deny_unknown_fields: !opts.allow_unknown_fields,
+    };
     // generate the Rust code
     let mut generated_file = NamedTempFile::new()?;
     Generator::builder()
         .with_root_name_str(&opts.root)
         .with_input_file(&opts.schema_path)
+        .with_options(&generator_options)
         .build()
         .generate_to_file(
             &generated_file