diff --git a/ucalc/Data/Billing.cs b/ucalc/Data/Billing.cs
index bd9a0c9..d7604bb 100644
--- a/ucalc/Data/Billing.cs
+++ b/ucalc/Data/Billing.cs
@@ -2,6 +2,8 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.CompilerServices;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Converters;
 
 #pragma warning disable 659
 
@@ -43,9 +45,17 @@ namespace UCalc.Data
 
     public class Address
     {
+        [JsonProperty(PropertyName = "street"), JsonRequired, JsonConverter(typeof(JsonNullToEmptyStringConverter))]
         public string Street { get; set; }
+
+        [JsonProperty(PropertyName = "houseNumber"), JsonRequired,
+         JsonConverter(typeof(JsonNullToEmptyStringConverter))]
         public string HouseNumber { get; set; }
+
+        [JsonProperty(PropertyName = "city"), JsonRequired, JsonConverter(typeof(JsonNullToEmptyStringConverter))]
         public string City { get; set; }
+
+        [JsonProperty(PropertyName = "postCode"), JsonRequired, JsonConverter(typeof(JsonNullToEmptyStringConverter))]
         public string Postcode { get; set; }
 
         public Address()
@@ -83,8 +93,13 @@ namespace UCalc.Data
 
     public class BankAccount
     {
+        [JsonProperty(PropertyName = "iban"), JsonRequired, JsonConverter(typeof(JsonNullToEmptyStringConverter))]
         public string Iban { get; set; }
+
+        [JsonProperty(PropertyName = "bic"), JsonRequired, JsonConverter(typeof(JsonNullToEmptyStringConverter))]
         public string Bic { get; set; }
+
+        [JsonProperty(PropertyName = "bankName"), JsonRequired, JsonConverter(typeof(JsonNullToEmptyStringConverter))]
         public string BankName { get; set; }
 
         public BankAccount()
@@ -119,8 +134,13 @@ namespace UCalc.Data
 
     public class Flat
     {
-        public Guid Id { get; private set; }
+        [JsonProperty(PropertyName = "id"), JsonRequired]
+        public Guid Id { get; set; }
+
+        [JsonProperty(PropertyName = "name"), JsonRequired, JsonConverter(typeof(JsonNullToEmptyStringConverter))]
         public string Name { get; set; }
+
+        [JsonProperty(PropertyName = "size"), JsonRequired]
         public int Size { get; set; }
 
         public Flat()
@@ -161,16 +181,37 @@ namespace UCalc.Data
 
     public class Tenant
     {
-        public Guid Id { get; private set; }
+        [JsonProperty(PropertyName = "id"), JsonRequired]
+        public Guid Id { get; set; }
+
+        [JsonProperty(PropertyName = "salutation"), JsonRequired, JsonConverter(typeof(StringEnumConverter))]
         public Salutation Salutation { get; set; }
+
+        [JsonProperty(PropertyName = "name"), JsonRequired, JsonConverter(typeof(JsonNullToEmptyStringConverter))]
         public string Name { get; set; }
+
+        [JsonProperty(PropertyName = "personCount"), JsonRequired]
         public int PersonCount { get; set; }
-        public BankAccount BankAccount { get; private set; }
+
+        [JsonProperty(PropertyName = "bankAccount"), JsonRequired]
+        public BankAccount BankAccount { get; set; }
+
+        [JsonProperty(PropertyName = "entryDate")]
         public DateTime? EntryDate { get; set; }
+
+        [JsonProperty(PropertyName = "departureDate")]
         public DateTime? DepartureDate { get; set; }
-        public HashSet<Flat> RentedFlats { get; private set; }
+
+        [JsonProperty(PropertyName = "rentedFlats"), JsonRequired, JsonConverter(typeof(FlatSerializationConverter))]
+        public HashSet<Flat> RentedFlats { get; set; }
+
+        [JsonProperty(PropertyName = "paidRent"), JsonRequired]
         public decimal PaidRent { get; set; }
+
+        [JsonProperty(PropertyName = "customMessage1"), JsonConverter(typeof(JsonNullToEmptyStringConverter))]
         public string CustomMessage1 { get; set; }
+
+        [JsonProperty(PropertyName = "customMessage2"), JsonConverter(typeof(JsonNullToEmptyStringConverter))]
         public string CustomMessage2 { get; set; }
 
         public Tenant()
@@ -179,6 +220,8 @@ namespace UCalc.Data
             Name = "";
             BankAccount = new BankAccount();
             RentedFlats = new HashSet<Flat>();
+            CustomMessage1 = "";
+            CustomMessage2 = "";
         }
 
         private bool Equals(Tenant other)
@@ -218,12 +261,24 @@ namespace UCalc.Data
 
     public class Landlord
     {
+        [JsonProperty(PropertyName = "salutation"), JsonRequired, JsonConverter(typeof(StringEnumConverter))]
         public Salutation Salutation { get; set; }
+
+        [JsonProperty(PropertyName = "name"), JsonRequired, JsonConverter(typeof(JsonNullToEmptyStringConverter))]
         public string Name { get; set; }
+
+        [JsonProperty(PropertyName = "mailAddress"), JsonRequired,
+         JsonConverter(typeof(JsonNullToEmptyStringConverter))]
         public string MailAddress { get; set; }
+
+        [JsonProperty(PropertyName = "phone"), JsonRequired, JsonConverter(typeof(JsonNullToEmptyStringConverter))]
         public string Phone { get; set; }
-        public Address Address { get; private set; }
-        public BankAccount BankAccount { get; private set; }
+
+        [JsonProperty(PropertyName = "address"), JsonRequired]
+        public Address Address { get; set; }
+
+        [JsonProperty(PropertyName = "bankAccount"), JsonRequired]
+        public BankAccount BankAccount { get; set; }
 
         public Landlord()
         {
@@ -263,8 +318,11 @@ namespace UCalc.Data
 
     public class House
     {
-        public Address Address { get; private set; }
-        public List<Flat> Flats { get; private set; }
+        [JsonProperty(PropertyName = "address"), JsonRequired]
+        public Address Address { get; set; }
+
+        [JsonProperty(PropertyName = "flats"), JsonRequired]
+        public List<Flat> Flats { get; set; }
 
         public House()
         {
@@ -296,9 +354,14 @@ namespace UCalc.Data
 
     public class CostEntryDetails
     {
+        [JsonProperty(PropertyName = "totalAmount"), JsonRequired]
         public decimal TotalAmount { get; set; }
+
+        [JsonProperty(PropertyName = "unitCount"), JsonRequired]
         public decimal UnitCount { get; set; }
-        public List<decimal> DiscountsInUnits { get; private set; }
+
+        [JsonProperty(PropertyName = "discountsInUnits"), JsonRequired]
+        public List<decimal> DiscountsInUnits { get; set; }
 
         public CostEntryDetails()
         {
@@ -331,9 +394,16 @@ namespace UCalc.Data
 
     public class CostEntry
     {
+        [JsonProperty(PropertyName = "startDate"), JsonRequired]
         public DateTime StartDate { get; set; }
+
+        [JsonProperty(PropertyName = "endDate"), JsonRequired]
         public DateTime EndDate { get; set; }
+
+        [JsonProperty(PropertyName = "amount"), JsonRequired]
         public decimal Amount { get; set; }
+
+        [JsonProperty(PropertyName = "details"), JsonRequired]
         public CostEntryDetails Details { get; set; }
 
         public CostEntry()
@@ -393,12 +463,25 @@ namespace UCalc.Data
 
     public class Cost
     {
+        [JsonProperty(PropertyName = "name"), JsonRequired, JsonConverter(typeof(JsonNullToEmptyStringConverter))]
         public string Name { get; set; }
+
+        [JsonProperty(PropertyName = "division"), JsonRequired, JsonConverter(typeof(StringEnumConverter))]
         public CostDivision Division { get; set; }
+
+        [JsonProperty(PropertyName = "affectsAll"), JsonRequired]
         public bool AffectsAll { get; set; }
+
+        [JsonProperty(PropertyName = "includeUnrented"), JsonRequired]
         public bool IncludeUnrented { get; set; }
-        public HashSet<Flat> AffectedFlats { get; private set; }
-        public List<CostEntry> Entries { get; private set; }
+
+        [JsonProperty(PropertyName = "affectedFlats"), JsonRequired, JsonConverter(typeof(FlatSerializationConverter))]
+        public HashSet<Flat> AffectedFlats { get; set; }
+
+        [JsonProperty(PropertyName = "entries"), JsonRequired]
+        public List<CostEntry> Entries { get; set; }
+
+        [JsonProperty(PropertyName = "displayInBill"), JsonRequired]
         public bool DisplayInBill { get; set; }
 
         public Cost()
@@ -439,12 +522,23 @@ namespace UCalc.Data
 
     public class Billing
     {
+        [JsonProperty(PropertyName = "startDate"), JsonRequired]
         public DateTime StartDate { get; set; }
+
+        [JsonProperty(PropertyName = "endDate"), JsonRequired]
         public DateTime EndDate { get; set; }
-        public Landlord Landlord { get; private set; }
-        public House House { get; private set; }
-        public List<Tenant> Tenants { get; private set; }
-        public List<Cost> Costs { get; private set; }
+
+        [JsonProperty(PropertyName = "landlord"), JsonRequired]
+        public Landlord Landlord { get; set; }
+
+        [JsonProperty(PropertyName = "house"), JsonRequired]
+        public House House { get; set; }
+
+        [JsonProperty(PropertyName = "tenants"), JsonRequired]
+        public List<Tenant> Tenants { get; set; }
+
+        [JsonProperty(PropertyName = "costs"), JsonRequired]
+        public List<Cost> Costs { get; set; }
 
         public Billing()
         {
diff --git a/ucalc/Data/BillingLoader.cs b/ucalc/Data/BillingLoader.cs
index 3bf9595..f1b18d2 100644
--- a/ucalc/Data/BillingLoader.cs
+++ b/ucalc/Data/BillingLoader.cs
@@ -1,18 +1,129 @@
 using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.IO;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
 
 namespace UCalc.Data
 {
+    internal class FlatSerializationConverter : JsonConverter
+    {
+        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        {
+            var set = (HashSet<Flat>) value;
+
+            writer.WriteStartArray();
+
+            foreach (var flat in set)
+            {
+                writer.WriteValue(flat.Id.ToString());
+            }
+
+            writer.WriteEndArray();
+        }
+
+        public override object ReadJson(JsonReader reader, Type objectType, object existingValue,
+            JsonSerializer serializer)
+        {
+            if (reader.TokenType != JsonToken.StartArray)
+            {
+                throw new JsonException("Expected [");
+            }
+
+            while (reader.Read() && reader.TokenType == JsonToken.String)
+            {
+            }
+
+            if (reader.TokenType != JsonToken.EndArray)
+            {
+                throw new JsonException("Expected ]");
+            }
+
+            return new HashSet<Flat>();
+        }
+
+        public override bool CanConvert(Type objectType)
+        {
+            throw new InvalidOperationException();
+        }
+    }
+
+    internal class JsonNullToEmptyStringConverter : JsonConverter
+    {
+        public override bool CanConvert(Type objectType)
+        {
+            return true;
+        }
+
+        public override object ReadJson(JsonReader reader, Type objectType, object existingValue,
+            JsonSerializer serializer)
+        {
+            return existingValue ?? string.Empty;
+        }
+
+        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        {
+            writer.WriteValue(value ?? string.Empty);
+        }
+    }
+
     public class BillingLoader
     {
+        [SuppressMessage("ReSharper", "PossibleNullReferenceException")]
         public Billing Load(string path)
         {
-            // TODO
-            return new Billing();
+            // TODO: Support older format
+
+            var content = File.ReadAllText(path);
+            try
+            {
+                var billing = JsonConvert.DeserializeObject<Billing>(content);
+
+                var idToFlat = new Dictionary<string, Flat>();
+                foreach (var flat in billing.House.Flats)
+                {
+                    idToFlat.Add(flat.Id.ToString(), flat);
+                }
+
+                var root = JObject.Parse(content);
+                var i = 0;
+                foreach (var tenantItem in root["tenants"])
+                {
+                    foreach (var rentedFlatId in tenantItem["rentedFlats"])
+                    {
+                        billing.Tenants[i].RentedFlats.Add(idToFlat[rentedFlatId.Value<string>()]);
+                    }
+
+                    ++i;
+                }
+
+                i = 0;
+                foreach (var tenantItem in root["costs"])
+                {
+                    foreach (var rentedFlatId in tenantItem["affectedFlats"])
+                    {
+                        billing.Costs[i].AffectedFlats.Add(idToFlat[rentedFlatId.Value<string>()]);
+                    }
+
+                    ++i;
+                }
+
+                return billing;
+            }
+            catch (JsonException e)
+            {
+                throw new IOException($"Could not load JSON with error:\n{e.Message}", e);
+            }
+            catch (InvalidCastException e)
+            {
+                throw new IOException($"Could not load JSON with error:\n{e.Message}", e);
+            }
         }
 
         public void Store(string path, Billing billing)
         {
-            throw new NotImplementedException();
+            File.WriteAllText(path, JsonConvert.SerializeObject(billing, Formatting.Indented));
         }
     }
 }
\ No newline at end of file
diff --git a/ucalc/MainWindow.xaml.cs b/ucalc/MainWindow.xaml.cs
index 9a3163a..b88c5e0 100644
--- a/ucalc/MainWindow.xaml.cs
+++ b/ucalc/MainWindow.xaml.cs
@@ -1,5 +1,4 @@
-using System;
-using System.IO;
+using System.IO;
 using System.Windows;
 using System.Windows.Controls;
 using System.Windows.Controls.Primitives;
@@ -78,9 +77,10 @@ namespace UCalc
                 new BillingWindow(path, billing).Show();
                 Hide();
             }
-            catch (IOException)
+            catch (IOException e)
             {
-                throw new NotImplementedException();
+                MessageBox.Show($"Beim Laden der Datei \"{path}\" ist ein Fehler aufgetreten!\n\nDetails: {e.Message}",
+                    "Fehler!", MessageBoxButton.OK, MessageBoxImage.Error);
             }
         }
     }
diff --git a/ucalc/Models/FlatsProperty.cs b/ucalc/Models/FlatsProperty.cs
index d3d39e8..6216d22 100644
--- a/ucalc/Models/FlatsProperty.cs
+++ b/ucalc/Models/FlatsProperty.cs
@@ -51,7 +51,10 @@ namespace UCalc.Models
                 tenant.RentedFlats.Remove(flat);
             }
 
-            // TODO: Revalidate all flats + revalidate tenant rent lists + costs flat lists
+            foreach (var cost in Model.Root.Costs)
+            {
+                cost.AffectedFlats.Remove(flat);
+            }
         }
     }
 }
\ No newline at end of file
diff --git a/ucalc/Models/Model.cs b/ucalc/Models/Model.cs
index 0349be4..371ad03 100644
--- a/ucalc/Models/Model.cs
+++ b/ucalc/Models/Model.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.ComponentModel;
+using System.Linq;
 using System.Runtime.CompilerServices;
 using UCalc.Annotations;
 using UCalc.Data;
@@ -48,7 +49,7 @@ namespace UCalc.Models
                     return;
                 }
 
-                if (_validated.Add(property))
+                if (_validated.Add(property) && !_model._loading)
                 {
                     property.Validate();
                 }
@@ -106,6 +107,18 @@ namespace UCalc.Models
                             return -1;
                         }
 
+                        var d1 = prop1.DepthInTree();
+                        var d2 = prop2.DepthInTree();
+                        if (d1 < d2)
+                        {
+                            return 1;
+                        }
+
+                        if (d1 > d2)
+                        {
+                            return -1;
+                        }
+
                         return 0;
                     });
 
@@ -127,6 +140,7 @@ namespace UCalc.Models
         }
 
         private readonly Validator _validator;
+        private readonly bool _loading;
         public DateTime StartDate { get; }
         public DateTime EndDate { get; }
         public BillingProperty Root { get; }
@@ -137,8 +151,12 @@ namespace UCalc.Models
             StartDate = billing.StartDate;
             EndDate = billing.EndDate;
 
-            using var validator = BeginValidation();
+            _loading = true;
             Root = new BillingProperty(this, null, billing);
+            _loading = false;
+
+            using var validator = BeginValidation();
+            validator.Validate(Root);
         }
 
         public Validator BeginValidation()
@@ -149,7 +167,83 @@ namespace UCalc.Models
 
         public Billing Dump()
         {
-            throw new NotImplementedException();
+            var billing = new Billing
+            {
+                StartDate = StartDate,
+                EndDate = EndDate,
+                Landlord =
+                {
+                    Salutation = (Salutation) Root.Landlord.Salutation.Value,
+                    Name = Root.Landlord.Name.Value,
+                    MailAddress = Root.Landlord.MailAddress.Value,
+                    Phone = Root.Landlord.Phone.Value,
+                    Address =
+                    {
+                        Street = Root.Landlord.Address.Street.Value,
+                        HouseNumber = Root.Landlord.Address.HouseNumber.Value,
+                        City = Root.Landlord.Address.City.Value,
+                        Postcode = Root.Landlord.Address.Postcode.Value
+                    },
+                    BankAccount =
+                    {
+                        Iban = Root.Landlord.BankAccount.Iban.Value,
+                        Bic = Root.Landlord.BankAccount.Bic.Value,
+                        BankName = Root.Landlord.BankAccount.BankName.Value
+                    }
+                },
+                House =
+                {
+                    Address =
+                    {
+                        Street = Root.House.Address.Street.Value,
+                        HouseNumber = Root.House.Address.HouseNumber.Value,
+                        City = Root.House.Address.City.Value,
+                        Postcode = Root.House.Address.Postcode.Value
+                    },
+                    Flats = Root.House.Flats.Select(flat => new Flat
+                        {Name = flat.Name.Value, Size = int.TryParse(flat.Size.Value, out var n) ? n : 0}).ToList()
+                }
+            };
+
+            var flatPropertyToFlat = new Dictionary<FlatProperty, Flat>();
+            for (var i = 0; i < Root.House.Flats.Count; ++i)
+            {
+                flatPropertyToFlat.Add(Root.House.Flats[i], billing.House.Flats[i]);
+            }
+
+            billing.Tenants = Root.Tenants.Select(tenant => new Tenant
+            {
+                Salutation = (Salutation) tenant.Salutation.Value,
+                Name = tenant.Name.Value,
+                PersonCount = int.TryParse(tenant.PersonCount.Value, out var n) ? n : 0,
+                BankAccount =
+                {
+                    Iban = tenant.BankAccount.Iban.Value,
+                    Bic = tenant.BankAccount.Bic.Value,
+                    BankName = tenant.BankAccount.BankName.Value
+                },
+                EntryDate = tenant.EntryDate.Value,
+                DepartureDate = tenant.DepartureDate.Value,
+                RentedFlats =
+                    new HashSet<Flat>(tenant.RentedFlats.Select(rentedFlat => flatPropertyToFlat[rentedFlat])),
+                PaidRent = decimal.TryParse(tenant.PaidRent.Value, out var d) ? d : 0,
+                CustomMessage1 = tenant.CustomMessage1.Value,
+                CustomMessage2 = tenant.CustomMessage2.Value
+            }).ToList();
+
+            billing.Costs = Root.Costs.Select(cost => new Cost
+            {
+                Name = cost.Name.Value,
+                Division = (CostDivision) cost.Division.Value,
+                AffectsAll = cost.AffectsAll.Value,
+                IncludeUnrented = cost.IncludeUnrented.Value,
+                AffectedFlats =
+                    new HashSet<Flat>(cost.AffectedFlats.Select(rentedFlat => flatPropertyToFlat[rentedFlat])),
+                // TODO: Entries = {},
+                DisplayInBill = cost.DisplayInBill.Value
+            }).ToList();
+
+            return billing;
         }
 
         public void ResetModified()
diff --git a/ucalc/Models/Properties.cs b/ucalc/Models/Properties.cs
index 837434c..89106e4 100644
--- a/ucalc/Models/Properties.cs
+++ b/ucalc/Models/Properties.cs
@@ -67,11 +67,25 @@ namespace UCalc.Models
 
             return false;
         }
+
+        public int DepthInTree()
+        {
+            var i = 0;
+            var property = Parent;
+
+            while (property != null)
+            {
+                ++i;
+                property = property.Parent;
+            }
+
+            return i;
+        }
     }
 
     public abstract class ValueProperty<T> : Property
     {
-        protected string Name;
+        protected readonly string Name;
         private T _value;
         private readonly List<string> _errors;
 
diff --git a/ucalc/Models/TenantProperty.cs b/ucalc/Models/TenantProperty.cs
index 224c71a..1dbedcb 100644
--- a/ucalc/Models/TenantProperty.cs
+++ b/ucalc/Models/TenantProperty.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Collections;
 using System.Collections.Generic;
 using UCalc.Controls;
 using UCalc.Data;
@@ -48,7 +49,7 @@ namespace UCalc.Models
         }
     }
 
-    public class RentedFlatsProperty : Property
+    public class RentedFlatsProperty : Property, IReadOnlyCollection<FlatProperty>
     {
         private const string NoFlatsError = "Gemietete Wohnungen: Es wurde keine Wohnung zugewiesen.";
         private readonly HashSet<FlatProperty> _flatProperties;
@@ -65,7 +66,8 @@ namespace UCalc.Models
                 _flatProperties.Add(flatToProperty?[flat] ?? throw new InvalidOperationException());
             }
 
-            Validate();
+            using var validator = Model.BeginValidation();
+            validator.Validate(this);
         }
 
         public override IReadOnlyList<string> Errors => _errors;
@@ -175,6 +177,18 @@ namespace UCalc.Models
 
             return thisStart.Intersects(thisEnd, otherStart, otherEnd);
         }
+
+        public IEnumerator<FlatProperty> GetEnumerator()
+        {
+            return _flatProperties.GetEnumerator();
+        }
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return GetEnumerator();
+        }
+
+        public int Count => _flatProperties.Count;
     }
 
     public class TenantProperty : NestedProperty
diff --git a/ucalc/NewWindow.xaml b/ucalc/NewWindow.xaml
index 72a3f5f..05ac936 100644
--- a/ucalc/NewWindow.xaml
+++ b/ucalc/NewWindow.xaml
@@ -13,8 +13,6 @@
         PreviewMouseUp="OnPreviewMouseUp"
         Icon="logo.ico">
 
-    <!-- TODO: Wrap in scroll to handle small screens! -->
-    
     <StackPanel Margin="12">
 
         <Label Content="Geben Sie den Zeitraum ein, für den die neue Abrechnung gelten soll."
diff --git a/ucalc/Pages/SideBar.xaml.cs b/ucalc/Pages/SideBar.xaml.cs
index d9fc60e..b6dc024 100644
--- a/ucalc/Pages/SideBar.xaml.cs
+++ b/ucalc/Pages/SideBar.xaml.cs
@@ -38,17 +38,17 @@ namespace UCalc.Pages
             }
         }
 
-        public void OnSaveClick(object sender, RoutedEventArgs e)
+        private void OnSaveClick(object sender, RoutedEventArgs e)
         {
             ParentWindow.Save();
         }
 
-        public void OnPrintClick(object sender, RoutedEventArgs e)
+        private void OnPrintClick(object sender, RoutedEventArgs e)
         {
             ParentWindow.Print();
         }
 
-        public void OnAboutClick(object sender, RoutedEventArgs e)
+        private void OnAboutClick(object sender, RoutedEventArgs e)
         {
             throw new System.NotImplementedException();
         }
diff --git a/ucalc/ucalc.csproj b/ucalc/ucalc.csproj
index be94a1b..7e626fb 100644
--- a/ucalc/ucalc.csproj
+++ b/ucalc/ucalc.csproj
@@ -95,4 +95,8 @@
         <Resource Include="logo.ico"/>
     </ItemGroup>
 
+    <ItemGroup>
+        <PackageReference Include="Newtonsoft.Json" Version="12.0.3"/>
+    </ItemGroup>
+
 </Project>
\ No newline at end of file