[monolithed]

Можно проще:

<input type="checkbox" name="type_demand" id="input" />1
<div id="div"></div>

<style type="text/css">
div {background: red; display: none; width: 100px; height: 100px;}
</style>
<script type="text/javascript">
window.onload = function(){
     var div = document.getElementById('div');
     document.getElementById('input').onclick = function(){
        div.style.display = (div.style.display == '') ? 'block' : '';
   };
};
</script>