diff --git a/CMakeLists.txt b/CMakeLists.txt
index aaaede7..3ea5f94 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -5,33 +5,29 @@ if (NOT TARGET _pico_extras_inclusion_marker)
 
     # Pull in PICO SDK (must be before project)
     include(pico_sdk_import.cmake)
+    if (PICO_SDK_VERSION_STRING VERSION_LESS "1.2.0")
+        message(FATAL_ERROR "Require at least Raspberry Pi Pico SDK version 1.2.0")
+    endif()
 
     project(pico_extras C CXX)
 
     set(CMAKE_C_STANDARD 11)
     set(CMAKE_CXX_STANDARD 17)
 
-    # Initialize the SDK
-    pico_sdk_init()
-
     pico_is_top_level_project(PICO_EXTRAS_TOP_LEVEL_PROJECT)
 
-    add_library(pico_extras_included INTERFACE)
-    target_compile_definitions(pico_extras_included INTERFACE
-            -DPICO_EXTRAS=1
-    )
-
-    pico_add_platform_library(pico_extras_included)
+    # The real work gets done in post_init which is called at the end of pico_sdk_init
+    list(APPEND PICO_SDK_POST_LIST_FILES ${CMAKE_CURRENT_LIST_DIR}/post_init.cmake)
+    if (PICO_EXTRAS_TOP_LEVEL_PROJECT)
+        message("pico_extras: initialize SDK since we're the top-level")
+        # Initialize the SDK
+        pico_sdk_init()
+    else()
+        set(PICO_SDK_POST_LIST_FILES ${PICO_SDK_POST_LIST_FILES} PARENT_SCOPE)
+    endif()
 
     if (NOT PICO_EXTRAS_PATH)
         set(PICO_EXTRAS_PATH ${CMAKE_CURRENT_LIST_DIR})
     endif()
     set(PICO_EXTRAS_PATH "${PICO_EXTRAS_PATH}" CACHE PATH "Path to Pico Extras")
-
-    add_subdirectory(src)
-
-    if (PICO_EXTRAS_TESTS_ENABLED OR PICO_EXTRAS_TOP_LEVEL_PROJECT)
-        add_subdirectory(test)
-    endif ()
-
 endif()
\ No newline at end of file
diff --git a/post_init.cmake b/post_init.cmake
new file mode 100644
index 0000000..668535a
--- /dev/null
+++ b/post_init.cmake
@@ -0,0 +1,14 @@
+add_library(pico_extras_included INTERFACE)
+target_compile_definitions(pico_extras_included INTERFACE
+        -DPICO_EXTRAS=1
+        )
+
+pico_add_platform_library(pico_extras_included)
+
+# note as we're a .cmake included by the SDK, we're relative to the pico-sdk build
+add_subdirectory(${CMAKE_CURRENT_LIST_DIR}/src ${CMAKE_BINARY_DIR}/pico_extras/src)
+
+if (PICO_EXTRAS_TESTS_ENABLED OR PICO_EXTRAS_TOP_LEVEL_PROJECT)
+    add_subdirectory(${CMAKE_CURRENT_LIST_DIR}/test {CMAKE_BINARY_DIR}/pico_extras/test)
+endif ()
+