diff --git a/src/element.rs b/src/element.rs
index 2b149a8..1c04c98 100644
--- a/src/element.rs
+++ b/src/element.rs
@@ -58,7 +58,8 @@ pub struct Element {
     // namespace: String,
     // hashmap of explicit namespace declarations on the element itself only
     // possibly not needed as can be calculated at write time depending on context and qualified namespace, and for reading, element validity and namespaces are kept track of by the reader.
-    pub namespace_declarations: HashSet<NamespaceDeclaration>,
+    // change this to custom namespace declarations only, so you can override the definition of namespaces if you wish
+    pub namespace_declaration_overrides: HashSet<NamespaceDeclaration>,
     // attributes can be in a different namespace than the element. how to make sure they are valid?
     // maybe include the namespace instead of or with the prefix
     // you can calculate the prefix from the namespaced name and the current writer context
diff --git a/src/error.rs b/src/error.rs
index eda527e..85b5d70 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -1,6 +1,9 @@
 use std::{num::ParseIntError, str::Utf8Error};
 
-use crate::element::{Content, Name, NamespaceDeclaration};
+use crate::{
+    element::{Content, Name, NamespaceDeclaration},
+    Element,
+};
 
 #[derive(Debug)]
 pub enum Error {
diff --git a/src/reader.rs b/src/reader.rs
index ee8d491..e6bb57c 100644
--- a/src/reader.rs
+++ b/src/reader.rs
@@ -436,7 +436,7 @@ impl<R> Reader<R> {
 
         return Ok(Element {
             name: element_name,
-            namespace_declarations: element_namespace_declarations,
+            namespace_declaration_overrides: element_namespace_declarations,
             attributes,
             content: Vec::new(),
         });
@@ -657,7 +657,7 @@ impl<R> Reader<R> {
 
         return Ok(Element {
             name: element_name,
-            namespace_declarations: element_namespace_declarations,
+            namespace_declaration_overrides: element_namespace_declarations,
             attributes,
             content,
         });
diff --git a/src/writer.rs b/src/writer.rs
index e319fdc..8b45869 100644
--- a/src/writer.rs
+++ b/src/writer.rs
@@ -93,20 +93,38 @@ impl<W: AsyncWrite + Unpin + Send> Writer<W> {
     }
 
     pub async fn write_empty(&mut self, element: &Element) -> Result<()> {
-        let namespace_declarations_stack: Vec<_> = self
+        let mut namespace_declarations_stack: Vec<_> = self
             .namespace_declarations
             .iter()
             .flatten()
-            .chain(&element.namespace_declarations)
+            .chain(&element.namespace_declaration_overrides)
             .collect();
 
+        let mut namespace_declarations = element.namespace_declaration_overrides.clone();
+
+        let default_namespace_declaration;
         let prefix;
         if let Some(namespace) = &element.name.namespace {
-            let name_namespace_declaration = namespace_declarations_stack
+            if let Some(name_namespace_declaration) = namespace_declarations_stack
                 .iter()
                 .rfind(|namespace_declaration| namespace_declaration.namespace == *namespace)
-                .ok_or(Error::UndeclaredNamespace(namespace.clone()))?;
-            prefix = name_namespace_declaration.prefix.as_ref();
+            {
+                prefix = name_namespace_declaration.prefix.as_ref();
+            } else {
+                default_namespace_declaration = NamespaceDeclaration {
+                    prefix: None,
+                    namespace: namespace.clone(),
+                };
+                if namespace_declarations.insert(default_namespace_declaration.clone()) {
+                    namespace_declarations_stack.push(&default_namespace_declaration);
+                    prefix = None
+                } else {
+                    return Err(Error::DuplicateNameSpaceDeclaration(NamespaceDeclaration {
+                        prefix: None,
+                        namespace: namespace.clone(),
+                    }));
+                }
+            }
         } else {
             prefix = None
         }
@@ -125,7 +143,7 @@ impl<W: AsyncWrite + Unpin + Send> Writer<W> {
 
         let mut attributes = Vec::new();
 
-        for namespace_declaration in &element.namespace_declarations {
+        for namespace_declaration in namespace_declarations.iter() {
             let ns_name = namespace_declaration
                 .prefix
                 .as_ref()
@@ -180,20 +198,38 @@ impl<W: AsyncWrite + Unpin + Send> Writer<W> {
     }
 
     pub async fn write_element_start(&mut self, element: &Element) -> Result<()> {
-        let namespace_declarations_stack: Vec<_> = self
+        let mut namespace_declarations_stack: Vec<_> = self
             .namespace_declarations
             .iter()
             .flatten()
-            .chain(&element.namespace_declarations)
+            .chain(&element.namespace_declaration_overrides)
             .collect();
 
+        let mut namespace_declarations = element.namespace_declaration_overrides.clone();
+
+        let default_namespace_declaration;
         let prefix;
         if let Some(namespace) = &element.name.namespace {
-            let name_namespace_declaration = namespace_declarations_stack
+            if let Some(name_namespace_declaration) = namespace_declarations_stack
                 .iter()
                 .rfind(|namespace_declaration| namespace_declaration.namespace == *namespace)
-                .ok_or(Error::UndeclaredNamespace(namespace.clone()))?;
-            prefix = name_namespace_declaration.prefix.as_ref();
+            {
+                prefix = name_namespace_declaration.prefix.as_ref();
+            } else {
+                default_namespace_declaration = NamespaceDeclaration {
+                    prefix: None,
+                    namespace: namespace.clone(),
+                };
+                if namespace_declarations.insert(default_namespace_declaration.clone()) {
+                    namespace_declarations_stack.push(&default_namespace_declaration);
+                    prefix = None
+                } else {
+                    return Err(Error::DuplicateNameSpaceDeclaration(NamespaceDeclaration {
+                        prefix: None,
+                        namespace: namespace.clone(),
+                    }));
+                }
+            }
         } else {
             prefix = None
         }
@@ -212,7 +248,7 @@ impl<W: AsyncWrite + Unpin + Send> Writer<W> {
 
         let mut attributes = Vec::new();
 
-        for namespace_declaration in &element.namespace_declarations {
+        for namespace_declaration in namespace_declarations.iter() {
             let ns_name = namespace_declaration
                 .prefix
                 .as_ref()
@@ -265,7 +301,7 @@ impl<W: AsyncWrite + Unpin + Send> Writer<W> {
 
         self.depth.push(element.name.clone());
         self.namespace_declarations
-            .push(element.namespace_declarations.clone());
+            .push(namespace_declarations.clone());
         Ok(())
     }