diff --git a/CMakeLists.txt b/CMakeLists.txt
index e31d8cc40..6dcc269f9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -2,6 +2,17 @@
 # Set the minimum version of CMake that's required 
 cmake_minimum_required(VERSION 3.12)
 
+# make sure the source and binary directories are different
+# this is here to prevent so-called "in-source" builds where the root directory of the source
+# is also the root directory of the build
+get_filename_component(srcdir "${CMAKE_SOURCE_DIR}" REALPATH)
+get_filename_component(bindir "${CMAKE_BINARY_DIR}" REALPATH)
+if ("${srcdir}" STREQUAL "${bindir}")
+  message(FATAL_ERROR "ldmx-sw does not support in-source builds.
+  Call 'ldmx cmake ..' from within a 'build/' subdirectory or
+  Tell cmake to use a different build directory with 'ldmx cmake -B build -S .'")
+endif()
+
 # Set the project name
 project(LDMX_SW VERSION 3.1.1
                 DESCRIPTION "The Light Dark Matter eXperiment simulation and reconstruction framework."