Ecco:
codice:
<html> 
<head> 
<title></title> 
<script language="javascript"> 
<!-- 
function campo(opz){ 
div = document.getElementById('divTesto') 
if(opz=='app')
{ 
div.innerHTML='

<select name="menu2">
<option>1piano</option>
<option>2piano</option>
<option>3piano</option>
</select>' 
} 
else 
div.innerHTML='' 
} 
//--> 
</script> 

<body> 
<form name="myForm"> 
</form> 
<select name="menu1" onchange="campo(this.options[this.selectedIndex].value)">
<option value=0>Seleziona un'opzione</option>
<option value='app'>appartamento</option>
<option value='box'>box</option>
<option value='ter'>terreno</option>
</select>
<div id="divTesto"></div> 
</body> 
</form>
ciao!!!