You can use nested panes. The plugin supports unlimited nesting through childOptions. http://layout.jquery-dev.com/demos/nested_simple.html
I'd suggest naming the child panes uniquely.
<div id="mainContainer">
<iframe class="ui-layout-center" id="someiframecontent-right">Center Right content</iframe>
<div class="ui-layout-west">
<div class="ui-layout-childsouth">North West content </div>
<div class="ui-layout-childcenter">South west content </div>
</div>
</div>
<script type="text/javascript">
$('#mainContainer').layout({
west__minSize: 100,
west__size: 0.5,
west__maxSize: 0.9,
center__minSize: 100,
center__size: 0.5,
center__maxSize: 0.9,
livePaneResizing: true,
west__childOptions: {
center__paneSelector: '.ui-layout-childcenter',
center__minSize: 100,
center__size: 0.5,
center__maxSize: 0.9,
south__paneSelector: '.ui-layout-childsouth',
south__minSize: 100,
south__size: 0.5,
south__maxSize: 0.9,
livePaneResizing: true
}
});
</script>
Remember that center pane is always necessary (including in the child panes), as it's required to fill up the remaining spaces left over by other panes.