If statements in aspx files

Since you have two controls on the page it will render them both. The if-check you create, only determines whether it’s included in the output. The easiest way to prevent this is to change your code like this:

<div>
    <units:MyUserControl runat="server" SomeSetting="<%= Something %>" />
</div>

EDIT: Answer to edit in the original post:

<div>
    <% if(Something) { %>
        <div id="someUniqueMarkup">
            This markup should not be output if Something==true.

            <asp:placeholder id="phItemInDiv" runat="server" />
        </div>
    <% }
    else { %>
        <asp:placeholder id="phItemOutsideDiv" runat="server" />
    <% } %>
</div>



MyUserControl ctrl = (MyUserControl)LoadControl("/pathtousercontrol.ascx")
if (something){    
    phItemInDiv.Controls.Add(ctrl);
}
else{
    phItemOutsideDiv.Controls.Add(ctrl);
}

This way you will only have the user control emitted (and loaded) if Something is true