[Serializable]
    [XmlInclude(typeof(BuildingImpl))]   // см. сюда
    public abstract class Building
    {
        public double Cost { get; set; }
        public ushort CadasterNumber { get; set; }
        public byte CountFloors { get; set; }
        public string Type { get; set; }
        abstract public double AverageCost(double payment);
    }
    
    public class BuildingImpl : Building
    {
        public override double AverageCost(double payment)
        {
            return default(double);
        }
    }


    class Program
    {
        static void Main(string[] args)
        {
            List<Building> list = new List<Building>(new[] { new BuildingImpl() { CadasterNumber = 1, Cost = 11, CountFloors = 5, Type = "house" },  new BuildingImpl() { CadasterNumber = 2, Cost = 22, CountFloors = 222, Type = "skygrabber" }, });

            XmlSerializer ser = new XmlSerializer(typeof(List<Building>));
            using (var ms = new MemoryStream())
            {
                ser.Serialize(ms, list);
            }

            Console.ReadLine();
        }
    }