Programmatically change editor layout

Unity does not expose a function to adjust the layout of the Unity editor manually. However there is a hack which someone kindly told me about on the IRC channel. I would give them credit but I honestly cannot remember who told me :S

WARNING - You cannot load Unity 3.x layouts in Unity 4.x because it will crash. In fact, you cannot even place a Unity 3.x layout asset in a Unity 4.x project because it will crash.

Example of Usage:

Ensure that the folder Assets/Editor/Layouts/ exists before using the following hack:

 using UnityEngine;
 using UnityEditor;

 public static class LayoutHackExample {

     [MenuItem("Layout Hack/Save Layout")]
     static void SaveLayoutHack() {
         // Saving the current layout to an asset
         LayoutUtility.SaveLayoutToAsset("Assets/Editor/Layouts/Your Layout.wlt");
     }

     [MenuItem("Layout Hack/Load Layout")]
     static void LoadLayoutHack() {
         // Loading layout from an asset
         LayoutUtility.LoadLayoutFromAsset("Assets/Editor/Layouts/Your Layout.wlt");
     }

 }

Installing a Layout:

If you want to install a layout into the drop-down list then you can programatically copy the layout. Here is how to determine the path for your layout:

 string targetPath = Path.Combine(LayoutUtility.LayoutsPath, "Layout Name.wlt");

Utility Class Source:

 using UnityEngine;
 using UnityEditor;
 
 using System.IO;
 using System.Reflection;
 
 using Type = System.Type;
 
 public static class LayoutUtility {
 
     private static MethodInfo _miLoadWindowLayout;
     private static MethodInfo _miSaveWindowLayout;
     private static MethodInfo _miReloadWindowLayoutMenu;
 
     private static bool _available;
     private static string _layoutsPath;
 
     static LayoutUtility() {
         Type tyWindowLayout = Type.GetType("UnityEditor.WindowLayout,UnityEditor");
         Type tyEditorUtility = Type.GetType("UnityEditor.EditorUtility,UnityEditor");
         Type tyInternalEditorUtility = Type.GetType("UnityEditorInternal.InternalEditorUtility,UnityEditor");
 
         if (tyWindowLayout != null && tyEditorUtility != null && tyInternalEditorUtility != null) {
             MethodInfo miGetLayoutsPath = tyWindowLayout.GetMethod("GetLayoutsPath", BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static);
             _miLoadWindowLayout = tyWindowLayout.GetMethod("LoadWindowLayout", BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static, null, new Type[] { typeof(string) }, null);
             _miSaveWindowLayout = tyWindowLayout.GetMethod("SaveWindowLayout", BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static, null, new Type[] { typeof(string) }, null);
             _miReloadWindowLayoutMenu = tyInternalEditorUtility.GetMethod("ReloadWindowLayoutMenu", BindingFlags.Public | BindingFlags.Static);
 
             if (miGetLayoutsPath == null || _miLoadWindowLayout == null || _miSaveWindowLayout == null || _miReloadWindowLayoutMenu == null)
                 return;
 
             _layoutsPath = (string)miGetLayoutsPath.Invoke(null, null);
             if (string.IsNullOrEmpty(_layoutsPath))
                 return;
 
             _available = true;
         }
     }
 
     // Gets a value indicating whether all required Unity API
     // functionality is available for usage.
     public static bool IsAvailable {
         get { return _available; }
     }
 
     // Gets absolute path of layouts directory.
     // Returns `null` when not available.
     public static string LayoutsPath {
         get { return _layoutsPath; }
     }
 
     // Save current window layout to asset file.
     // `assetPath` must be relative to project directory.
     public static void SaveLayoutToAsset(string assetPath) {
         SaveLayout(Path.Combine(Directory.GetCurrentDirectory(), assetPath));
     }
 
     // Load window layout from asset file.
     // `assetPath` must be relative to project directory.
     public static void LoadLayoutFromAsset(string assetPath) {
         if (_miLoadWindowLayout != null) {
             string path = Path.Combine(Directory.GetCurrentDirectory(), assetPath);
             _miLoadWindowLayout.Invoke(null, new object[] { path });
         }
     }
 
     // Save current window layout to file.
     // `path` must be absolute.
     public static void SaveLayout(string path) {
         if (_miSaveWindowLayout != null)
             _miSaveWindowLayout.Invoke(null, new object[] { path });
     }
 
 }