It's located in Malibu, California.
After Stark disassembled the Avengers, he went to his mansion attempting to create a device which may expose the Skrulls from their disguises but fails everytime. That was until Doctor Doom arrived and gave Stark the chip needed for the device to work after having a short fight.
Background in other media
The appearance of the mansion is based on Tony Stark's mansion in the Marvel Cinematic Universe.