diff --git a/FairSoftConfig.cmake b/FairSoftConfig.cmake
index 115c11fc..497c3245 100644
--- a/FairSoftConfig.cmake
+++ b/FairSoftConfig.cmake
@@ -37,6 +37,23 @@ set(CMAKE_INSTALL_PREFIX "${CMAKE_SOURCE_DIR}/install" CACHE PATH "Install prefi
 # set(GEANT4MT OFF CACHE BOOL "Build Geant4 in multi-threading mode" FORCE)
 # set(GEANT4MT ON CACHE BOOL "Build Geant4 in multi-threading mode" FORCE)
 
+#
+# CI Build Mode (default OFF)
+#
+#  Enable this to reduce peak disk usage during builds. When enabled:
+#  - Packages are built sequentially (one at a time) instead of in parallel
+#  - Each package's build and source directories are removed after installation
+#  - Disk usage is logged (space reclaimed + total build tree size)
+#
+#  The -j<ncpus> argument still parallelizes compilation within each package build.
+#  Trade-offs:
+#  - Builds take longer due to package serialization
+#  - Incremental rebuilds after failure are longer (removed packages must fully rebuild)
+#  Recommended for CI environments or systems with limited storage.
+#
+# set(CI_BUILD_MODE OFF CACHE BOOL "Serialize builds and remove package dirs after install" FORCE)
+# set(CI_BUILD_MODE ON CACHE BOOL "Serialize builds and remove package dirs after install" FORCE)
+
 #
 # Python
 #
diff --git a/cmake/legacy.cmake b/cmake/legacy.cmake
index 5c89f7cc..6aa96261 100644
--- a/cmake/legacy.cmake
+++ b/cmake/legacy.cmake
@@ -551,6 +551,44 @@ add_custom_target(source-cache
   COMMENT "Creating source cache at ${tarfile}"
 )
 
+# CI_BUILD_MODE: Serialize builds and remove package directories after install
+if(NOT DEFINED CI_BUILD_MODE)
+  set(CI_BUILD_MODE OFF)
+endif()
+
+if(CI_BUILD_MODE)
+  foreach(pkg IN LISTS packages)
+    ExternalProject_Add_Step(${pkg} reclaim_diskspace
+      COMMAND ${CMAKE_COMMAND}
+        -DPACKAGE_NAME=${pkg}
+        -DPKG_BUILD_DIR=${CMAKE_BINARY_DIR}/Build/${pkg}
+        -DPKG_SOURCE_DIR=${CMAKE_BINARY_DIR}/Source/${pkg}
+        -DCMAKE_BINARY_DIR=${CMAKE_BINARY_DIR}
+        -P ${CMAKE_SOURCE_DIR}/cmake/reclaim-diskspace.cmake
+      COMMENT "Removing ${pkg} package directories to reclaim disk space"
+      DEPENDEES install
+      ALWAYS OFF
+    )
+  endforeach()
+
+  # Serialize package builds to reduce peak disk usage in CI environments.
+  # This ensures only one package builds at a time, preventing multiple large
+  # package directories from existing simultaneously.
+  # Exclude optional packages (EXCLUDE_FROM_ALL) from the serial chain.
+  set(prev_pkg "")
+  foreach(pkg IN LISTS packages)
+    get_target_property(is_excluded ${pkg} EXCLUDE_FROM_ALL)
+    if(is_excluded)
+      continue()
+    endif()
+    if(prev_pkg)
+      ExternalProject_Add_StepDependencies(${pkg} configure ${prev_pkg})
+    endif()
+    set(prev_pkg ${pkg})
+  endforeach()
+  unset(prev_pkg)
+endif()
+
 include(CTest)
 
 foreach(ver IN ITEMS 18.6 18.8 19.0)
@@ -604,6 +642,12 @@ if(SOURCE_CACHE)
 else()
   message(STATUS "  ${Cyan}SOURCE CACHE${CR}       using upstream URLs (generate cache by building target 'source-cache' and pass via ${BMagenta}-DSOURCE_CACHE=...${CR})")
 endif()
+message(STATUS "  ")
+if(CI_BUILD_MODE)
+  message(STATUS "  ${Cyan}CI_BUILD_MODE${CR}      ${BGreen}ON${CR} (serialized builds, package directories removed after install)")
+else()
+  message(STATUS "  ${Cyan}CI_BUILD_MODE${CR}      ${BGreen}OFF${CR} (enable with ${BMagenta}-DCI_BUILD_MODE=ON${CR} to reduce disk usage)")
+endif()
 if(CMAKE_OSX_SYSROOT)
   message(STATUS "  ")
   message(STATUS "  ${Cyan}OSX_SYSROOT${CR}        ${BGreen}${CMAKE_OSX_SYSROOT}${CR} (change with ${BMagenta}-DCMAKE_OSX_SYSROOT=...${CR})")
diff --git a/cmake/reclaim-diskspace.cmake b/cmake/reclaim-diskspace.cmake
new file mode 100644
index 00000000..b7bc3b13
--- /dev/null
+++ b/cmake/reclaim-diskspace.cmake
@@ -0,0 +1,64 @@
+# reclaim-diskspace.cmake
+# Remove package build/source directories after install to reclaim disk space
+# Expected variables: PACKAGE_NAME, PKG_BUILD_DIR, PKG_SOURCE_DIR, CMAKE_BINARY_DIR
+
+if(NOT DEFINED PACKAGE_NAME OR NOT DEFINED PKG_BUILD_DIR OR NOT DEFINED PKG_SOURCE_DIR)
+  message(FATAL_ERROR "PACKAGE_NAME, PKG_BUILD_DIR, and PKG_SOURCE_DIR must be defined")
+endif()
+
+# Helper function to measure directory size in MB
+function(measure_dir_size_mb dir out_var out_success)
+  set(${out_success} FALSE PARENT_SCOPE)
+  set(${out_var} 0 PARENT_SCOPE)
+  if(UNIX)
+    find_program(DU_EXECUTABLE du)
+    if(DU_EXECUTABLE)
+      execute_process(
+        COMMAND ${DU_EXECUTABLE} -sk "${dir}"
+        OUTPUT_VARIABLE du_output
+        ERROR_QUIET
+        OUTPUT_STRIP_TRAILING_WHITESPACE
+      )
+      if(du_output)
+        string(REGEX REPLACE "^([0-9]+).*" "\\1" size_kb "${du_output}")
+        math(EXPR size_mb "${size_kb} / 1024")
+        set(${out_var} ${size_mb} PARENT_SCOPE)
+        set(${out_success} TRUE PARENT_SCOPE)
+      endif()
+    endif()
+  endif()
+endfunction()
+
+# Measure sizes before removal
+measure_dir_size_mb("${PKG_BUILD_DIR}" build_mb build_ok)
+measure_dir_size_mb("${PKG_SOURCE_DIR}" source_mb source_ok)
+
+if(build_ok AND source_ok)
+  math(EXPR total_mb "${build_mb} + ${source_mb}")
+  message(STATUS "[${PACKAGE_NAME}] Reclaiming ~${total_mb} MB (build: ${build_mb}, source: ${source_mb})")
+elseif(build_ok)
+  message(STATUS "[${PACKAGE_NAME}] Reclaiming ~${build_mb} MB")
+elseif(source_ok)
+  message(STATUS "[${PACKAGE_NAME}] Reclaiming ~${source_mb} MB")
+else()
+  message(STATUS "[${PACKAGE_NAME}] Removing package directories...")
+endif()
+
+# Remove package directories
+execute_process(COMMAND ${CMAKE_COMMAND} -E rm -rf "${PKG_BUILD_DIR}" RESULT_VARIABLE build_rm)
+execute_process(COMMAND ${CMAKE_COMMAND} -E rm -rf "${PKG_SOURCE_DIR}" RESULT_VARIABLE source_rm)
+
+if(NOT build_rm EQUAL 0)
+  message(WARNING "[${PACKAGE_NAME}] Failed to remove package build directory")
+endif()
+if(NOT source_rm EQUAL 0)
+  message(WARNING "[${PACKAGE_NAME}] Failed to remove package source directory")
+endif()
+
+# Report total build tree size
+if(DEFINED CMAKE_BINARY_DIR)
+  measure_dir_size_mb("${CMAKE_BINARY_DIR}" tree_mb tree_ok)
+  if(tree_ok)
+    message(STATUS "[${PACKAGE_NAME}] Build tree size: ${tree_mb} MB")
+  endif()
+endif()