gistfile1.txt

add_action('pre_get_posts', 'to_let_only_by_default');
function to_let_only_by_default($q) 
{
    if (is_admin())
        return;

	if (!$q->is_main_query())
		return;

	if (!$q->is_post_type_archive('property') && !$q->is_tax(get_object_taxonomies('property')))
		return;

	if (isset($_GET['shortlisted']))
		return;
	
	if (isset($_GET['department']) && $_GET['department'] == 'residential-lettings')
	{
	    $tax_query = $q->get('tax_query');

	    $tax_query[] = array(
		'taxonomy' => 'availability',
		'field' => 'term_id',
		'terms' => array(123), // CHANGE 123 FOR 'To Let' ID. THIS CAN BE FOUND UNDER 'Property Hive > settings > custom Fields > Availabilities'
		'operator' => 'IN'
	    );

	    $q->set('tax_query', $tax_query);
	}
}