diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5bc5b9d..518387c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -3,6 +3,10 @@
 
 cmake_minimum_required(VERSION 2.8...3.8)
 
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR)
+	message(FATAL_ERROR "In-tree builds are not supported. Run CMake from a separate directory: cmake -B build")
+endif()
+
 # Regenerate CMakeLists.txt automatically in the root project
 set(CMKR_ROOT_PROJECT OFF)
 if(CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
diff --git a/include/project_parser.hpp b/include/project_parser.hpp
index 376322b..e86cd92 100644
--- a/include/project_parser.hpp
+++ b/include/project_parser.hpp
@@ -126,6 +126,7 @@ struct Project {
     std::string build_dir = "build";
     std::string generator;
     std::string config;
+    bool allow_in_tree = false;
     Condition<std::vector<std::string>> project_subdirs;
     std::vector<std::string> cppflags;
     std::vector<std::string> cflags;
diff --git a/src/cmake_generator.cpp b/src/cmake_generator.cpp
index acb3a29..b7ff456 100644
--- a/src/cmake_generator.cpp
+++ b/src/cmake_generator.cpp
@@ -464,6 +464,14 @@ int generate_cmake(const char *path, bool root) {
     if (root) {
         cmd("cmake_minimum_required")("VERSION", project.cmake_version).endl();
 
+        if (!project.allow_in_tree) {
+            // clang-format off
+            cmd("if")("CMAKE_SOURCE_DIR", "STREQUAL", "CMAKE_BINARY_DIR");
+                cmd("message")("FATAL_ERROR", "In-tree builds are not supported. Run CMake from a separate directory: cmake -B build");
+            cmd("endif")().endl();
+            // clang-format on
+        }
+
         comment("Regenerate CMakeLists.txt automatically in the root project");
         cmd("set")("CMKR_ROOT_PROJECT", "OFF");
         // clang-format off
diff --git a/src/project_parser.cpp b/src/project_parser.cpp
index 882f6f4..9750c8a 100644
--- a/src/project_parser.cpp
+++ b/src/project_parser.cpp
@@ -79,6 +79,7 @@ Project::Project(const std::string &path, bool build) {
             get_optional(cmake, "generator", generator);
             get_optional(cmake, "config", config);
             get_optional(cmake, "arguments", gen_args);
+            get_optional(cmake, "allow-in-tree", allow_in_tree);
         }
     } else {
         if (toml.contains("cmake")) {