diff --git a/arduino-mk/Arduino.mk b/arduino-mk/Arduino.mk
index 70637ca..315492f 100644
--- a/arduino-mk/Arduino.mk
+++ b/arduino-mk/Arduino.mk
@@ -494,7 +494,8 @@ LIB_OBJS      = $(patsubst $(ARDUINO_LIB_PATH)/%.c,$(OBJDIR)/libs/%.o,$(LIB_C_SR
 USER_LIB_OBJS = $(patsubst $(USER_LIB_PATH)/%.cpp,$(OBJDIR)/libs/%.o,$(USER_LIB_CPP_SRCS)) \
 		$(patsubst $(USER_LIB_PATH)/%.c,$(OBJDIR)/libs/%.o,$(USER_LIB_C_SRCS))
 
-CPPFLAGS      = -mmcu=$(MCU) -DF_CPU=$(F_CPU) -DARDUINO=$(ARDUINO_VERSION) \
+# Using += instead of =, so that CPPFLAGS can be set per sketch level
+CPPFLAGS      += -mmcu=$(MCU) -DF_CPU=$(F_CPU) -DARDUINO=$(ARDUINO_VERSION) \
 			-I. -I$(ARDUINO_CORE_PATH) -I$(ARDUINO_VAR_PATH)/$(VARIANT) \
 			$(SYS_INCLUDES) $(USER_INCLUDES) -g -Os -w -Wall \
 			-ffunction-sections -fdata-sections