diff --git a/build.ps1 b/build.ps1
index 3ea0b34c1..1b9b1e56d 100644
--- a/build.ps1
+++ b/build.ps1
@@ -13,9 +13,9 @@ $PSScriptRoot = Split-Path $MyInvocation.MyCommand.Path -Parent
 # CONFIGURATION
 ###########################################################################
 
-$TempDirectory = Join-Path $PSScriptRoot '.fallout/temp'
+$TempDirectory = "$PSScriptRoot/.fallout/temp"
 
-$DotNetGlobalFile = Join-Path $PSScriptRoot 'global.json'
+$DotNetGlobalFile = "$PSScriptRoot/global.json"
 $DotNetInstallUrl = "https://dot.net/v1/dotnet-install.ps1"
 $DotNetChannel = "STS"
 
diff --git a/src/Fallout.Cli/Commands/AddPackageCommand.cs b/src/Fallout.Cli/Commands/AddPackageCommand.cs
index d0258a33d..fffe67520 100644
--- a/src/Fallout.Cli/Commands/AddPackageCommand.cs
+++ b/src/Fallout.Cli/Commands/AddPackageCommand.cs
@@ -1,3 +1,4 @@
+using System.IO;
 using System.Linq;
 using System.Threading.Tasks;
 using Fallout.Common;
@@ -6,13 +7,14 @@
 using Fallout.Common.Tooling;
 using Fallout.Common.Tools.DotNet;
 using Fallout.Solutions;
+using Microsoft.Build.Evaluation;
 
 namespace Fallout.Cli.Commands;
 
 /// <summary>
 /// <c>fallout :add-package</c>: adds (or upgrades) a NuGet package reference in the build project.
 /// </summary>
-internal sealed class AddPackageCommand(IConfigurationReader configuration, IPackageManager packages) : IFalloutCommand
+internal sealed class AddPackageCommand(IPackageManager packages) : IFalloutCommand
 {
     public string Name => "add-package";
 
@@ -31,8 +33,7 @@ await NuGetVersionResolver.GetLatestVersion(packageId, includePrereleases: false
                  .ToString())
             .NotNull("packageVersion != null");
 
-        var configuration1 = configuration.Read(buildScript, evaluate: true);
-        var buildProjectFile = configuration1[ConfigurationReader.BuildProjectFileKey];
+        var buildProjectFile = FindBuildProject(rootDirectory);
         Host.Information($"Installing {packageId}/{packageVersion} to {buildProjectFile} ...");
         packages.AddOrReplacePackage(packageId, packageVersion, PackageManager.DownloadType, buildProjectFile);
         DotNetTasks.DotNet($"restore {buildProjectFile}");
@@ -49,4 +50,43 @@ await NuGetVersionResolver.GetLatestVersion(packageId, includePrereleases: false
         Host.Information($"Done installing {packageId}/{packageVersion} to {buildProjectFile}");
         return 0;
     }
+
+    internal static AbsolutePath FindBuildProject(AbsolutePath rootDirectory)
+    {
+        var buildProject = rootDirectory.GlobFiles("**/*.csproj")
+            .Where(x => HasMatchingRootDirectory(x, rootDirectory))
+            .OrderBy(x => x.ToString().Length)
+            .FirstOrDefault();
+
+        Assert.True(buildProject != null,
+            $"Could not find a build project with a FalloutRootDirectory property pointing to '{rootDirectory}'.");
+
+        return buildProject;
+    }
+
+    private static bool HasMatchingRootDirectory(AbsolutePath projectFile, AbsolutePath rootDirectory)
+    {
+        ProjectProperty rootDirectoryProperty;
+        try
+        {
+            rootDirectoryProperty = ProjectModelTasks.ParseProject(projectFile).NotNull()
+                .GetProperty("FalloutRootDirectory");
+        }
+        catch
+        {
+            return false;
+        }
+
+        if (string.IsNullOrWhiteSpace(rootDirectoryProperty?.EvaluatedValue))
+        {
+            return false;
+        }
+
+        var configuredRootDirectory = rootDirectoryProperty.EvaluatedValue;
+        var resolvedRootDirectory = Path.IsPathRooted(configuredRootDirectory)
+            ? (AbsolutePath)configuredRootDirectory
+            : projectFile.Parent / configuredRootDirectory;
+
+        return resolvedRootDirectory == rootDirectory;
+    }
 }
diff --git a/src/Fallout.Cli/Commands/SetupCommand.cs b/src/Fallout.Cli/Commands/SetupCommand.cs
index 5c879a814..e59cf199b 100644
--- a/src/Fallout.Cli/Commands/SetupCommand.cs
+++ b/src/Fallout.Cli/Commands/SetupCommand.cs
@@ -156,6 +156,8 @@ public async Task<int> ExecuteAsync(string[] args, AbsolutePath rootDirectory, A
                     new
                     {
                         RootDirectory = buildDirectory.GetWinRelativePathTo(rootDirectory),
+                        BuildDirectory = buildProjectRelativeDirectory,
+                        BuildProjectName = buildProjectName,
                         ScriptDirectory = buildDirectory.GetWinRelativePathTo(WorkingDirectory),
                         TargetFramework = TARGET_FRAMEWORK,
                         FalloutVersion = falloutVersion,
diff --git a/src/Fallout.Cli/templates/build.ps1 b/src/Fallout.Cli/templates/build.ps1
index f66198b3e..95d31a787 100644
--- a/src/Fallout.Cli/templates/build.ps1
+++ b/src/Fallout.Cli/templates/build.ps1
@@ -13,9 +13,9 @@ $PSScriptRoot = Split-Path $MyInvocation.MyCommand.Path -Parent
 # CONFIGURATION
 ###########################################################################
 
-$TempDirectory = Join-Path $PSScriptRoot '_ROOT_DIRECTORY_/.fallout/temp'
+$TempDirectory = "$PSScriptRoot/.fallout/temp"
 
-$DotNetGlobalFile = Join-Path $PSScriptRoot '_ROOT_DIRECTORY_/global.json'
+$DotNetGlobalFile = "$PSScriptRoot/global.json"
 $DotNetInstallUrl = "https://dot.net/v1/dotnet-install.ps1"
 $DotNetChannel = "STS"
 
diff --git a/tests/Fallout.Cli.Specs/Commands/AddPackageCommandSpecs.cs b/tests/Fallout.Cli.Specs/Commands/AddPackageCommandSpecs.cs
new file mode 100644
index 000000000..c5b48d100
--- /dev/null
+++ b/tests/Fallout.Cli.Specs/Commands/AddPackageCommandSpecs.cs
@@ -0,0 +1,91 @@
+using System;
+using System.IO;
+using Fallout.Cli.Commands;
+using Fallout.Common.IO;
+using FluentAssertions;
+using Xunit;
+
+namespace Fallout.Cli.Specs.Commands;
+
+public class AddPackageCommandSpecs
+{
+    [Fact]
+    public void Project_with_root_directory_property_is_selected()
+    {
+        // Arrange
+        using var root = TempRoot.Create();
+        root.WriteProject("src/app.csproj", "<Project />");
+        var expected = root.WriteProject("build/custom.csproj",
+            "<Project><PropertyGroup><FalloutRootDirectory>..</FalloutRootDirectory></PropertyGroup></Project>");
+
+        // Act
+        var actual = AddPackageCommand.FindBuildProject(root.Path);
+
+        // Assert
+        actual.Should().Be(expected);
+    }
+
+    [Fact]
+    public void Missing_root_directory_property_gives_clear_error()
+    {
+        // Arrange
+        using var root = TempRoot.Create();
+        root.WriteProject("build/build.csproj",
+            "<Project><!-- <FalloutRootDirectory>..</FalloutRootDirectory> --></Project>");
+
+        // Act
+        var action = () => AddPackageCommand.FindBuildProject(root.Path);
+
+        // Assert
+        action.Should().Throw<Exception>()
+            .WithMessage("*Could not find a build project*FalloutRootDirectory*");
+    }
+
+    [Fact]
+    public void Project_with_shortest_path_is_selected()
+    {
+        // Arrange
+        using var root = TempRoot.Create();
+        root.WriteProject("build/nested/first.csproj",
+            "<Project><PropertyGroup><FalloutRootDirectory>../..</FalloutRootDirectory></PropertyGroup></Project>");
+        var expected = root.WriteProject("build/second.csproj",
+            "<Project><PropertyGroup><FalloutRootDirectory>..</FalloutRootDirectory></PropertyGroup></Project>");
+
+        // Act
+        var actual = AddPackageCommand.FindBuildProject(root.Path);
+
+        // Assert
+        actual.Should().Be(expected);
+    }
+
+    private sealed class TempRoot : IDisposable
+    {
+        public AbsolutePath Path { get; }
+
+        private TempRoot(AbsolutePath path) => Path = path;
+
+        public static TempRoot Create()
+        {
+            var path = (AbsolutePath)System.IO.Path.Combine(
+                System.IO.Path.GetTempPath(), "fallout-add-package-" + Guid.NewGuid().ToString("N"));
+            path.CreateDirectory();
+            return new TempRoot(path);
+        }
+
+        public AbsolutePath WriteProject(string relativePath, string content)
+        {
+            var project = Path / relativePath;
+            project.Parent.CreateDirectory();
+            File.WriteAllText(project, content);
+            return project;
+        }
+
+        public void Dispose()
+        {
+            if (Directory.Exists(Path))
+            {
+                Directory.Delete(Path, recursive: true);
+            }
+        }
+    }
+}