immobilienscout_plus_plus.user.js

// ==UserScript==
// @name        immobilienscout++
// @namespace   kch42
// @include     http://www.immobilienscout24.de/Suche/*
// @version     1
// @grant       none
// ==/UserScript==

/*
 * Immobilienscout24 Suche verbessern.
 * Im Moment wird die Warmmiete in den Suchergebnissen angezeigt.
 */

function mkcrit(title, value) {
  var nc = document.createElement("dl");
  nc.classList.add("criteria");
  var dt = document.createElement("dt");
  dt.classList.add("title");
  dt.textContent = title;
  var dd = document.createElement("dd");
  dd.classList.add("value");
  dd.textContent = value;
  dd.style.fontSize = "75%";
  nc.appendChild(dt);
  nc.appendChild(dd);
  return nc;
}

function procresult(result) {
  if(result.classList.contains("isplusplus_processed")) {
    return;
  }
  
  var url = result.querySelector("span.title a").getAttribute("href");
  var req = new XMLHttpRequest();
  req.addEventListener("load", function() {
    var doc = this.responseXML;
    var warmmiete = doc.querySelector(".is24qa-gesamtmiete");
    result.querySelector(".resultlist_criteria").appendChild(mkcrit("Warmmiete", warmmiete ? warmmiete.textContent : "?"));
    result.classList.add("isplusplus_processed")
  });
  req.open("GET", url);
  req.responseType = "document";
  req.send();
}

var results = document.querySelectorAll("#resultListItems > li.resultlist");
for (var i = 0; i < results.length; i++) {
  procresult(results[i]);
}

Updated this script to make it compatible with the current version of immobilienscout24 (as of 2020/11/13):
https://gist.github.com/yutamago/266e393047355cbcf48197da67badd14