$result = $modx->db->query( 'SELECT nid, field_house_kavels_number, field_house_kavels_price, field_house_kavels_surface, field_house_kavels_livingsurface, field_house_kavels_content, field_house_kavels_sold FROM `content_field_house_kavels` WHERE nid = ' . $node . ' ORDER BY field_house_kavels_number ASC' );
This question has been answered by appeltje. See the first response.
ORDER BY CONVERT(field_house_kavels_number, UNSIGNED) ASC