diff --git a/PKHeX.Core/PKM/HOME/PKH.cs b/PKHeX.Core/PKM/HOME/PKH.cs
index f54e4e260..85f4c8675 100644
--- a/PKHeX.Core/PKM/HOME/PKH.cs
+++ b/PKHeX.Core/PKM/HOME/PKH.cs
@@ -15,14 +15,13 @@ public sealed class PKH : PKM, IHandlerLanguage, IFormArgument, IHomeTrack, IBat
     public GameDataPB8? DataPB8 { get; private set; }
     public GameDataPK9? DataPK9 { get; private set; }
 
-    private const int HeaderSize = 0x14;
     public override EntityContext Context => EntityContext.None;
 
     public PKH(byte[] data) : base(DecryptHome(data))
     {
-        var mem = Data.AsMemory(HeaderSize);
+        var mem = Data.AsMemory(HomeCrypto.SIZE_1HEADER + 2);
         var core = mem[..CoreDataSize];
-        var side = mem.Slice(core.Length, GameDataSize);
+        var side = mem.Slice(core.Length + 2, GameDataSize);
 
         _coreData = new GameDataCore(core);
         ReadGameData1(side);
@@ -266,20 +265,22 @@ public sealed class PKH : PKM, IHandlerLanguage, IFormArgument, IHomeTrack, IBat
 
         // Header and Core are already in the current byte array.
         // Write each part, starting with header and core.
-        int ctr = HeaderSize;
+        int ctr = HomeCrypto.SIZE_1HEADER + 2;
         ctr += _coreData.WriteTo(result.AsSpan(ctr));
+        var gameDataLengthSpan = result.AsSpan(ctr, 2); ctr += 2;
+        int gameDataStart = ctr;
         if (DataPK8 is { } pk8) ctr += pk8.WriteTo(result.AsSpan(ctr));
         if (DataPB7 is { } pb7) ctr += pb7.WriteTo(result.AsSpan(ctr));
         if (DataPA8 is { } pa8) ctr += pa8.WriteTo(result.AsSpan(ctr));
         if (DataPB8 is { } pb8) ctr += pb8.WriteTo(result.AsSpan(ctr));
         if (DataPK9 is { } pk9) ctr += pk9.WriteTo(result.AsSpan(ctr));
+        WriteUInt16LittleEndian(gameDataLengthSpan, GameDataSize = (ushort)(ctr - gameDataStart));
 
         // Update metadata to ensure we're a valid object.
         DataVersion = HomeCrypto.Version1;
         EncodedDataSize = (ushort)(result.Length - HomeCrypto.SIZE_1HEADER);
         CoreDataSize = HomeCrypto.SIZE_1CORE;
-        GameDataSize = (ushort)(ctr - GameDataStart);
-        Data.AsSpan()[..HeaderSize].CopyTo(result); // Copy updated header.
+        Data.AsSpan()[..(HomeCrypto.SIZE_1HEADER + 2)].CopyTo(result); // Copy updated header & CoreData length.
 
         return result;
     }