Ultrasonic techniques have been proposed to determine the extension of piezoelectric multilayer actuators which can be used to construct deformable mirrors for adaptive optics. In the time domain approach, pulse-echo measurements were carried out using an ultrasonic transducer integrated with the actuator. In the frequency domain approach, the frequency shift of the integrated transducer was used to estimate the extension by analysis of the resonant spectrum for impedance magnitude. Time and frequency domain measurements were carried out on large actuators (7×7×28 mm3). Frequency domain measurements were carried out on smaller actuators (2×2×2 mm3) which are more suitable for a miniaturised deformable mirror. Monolithic integration of the ultrasonic extension sensing transducer and the actuator was demonstrated using layers in the multilayer actuators. Strong correlation was observed between the travel time of the ultrasonic pulse through the actuator and the simultaneously-measured actuator extension. It was noted, contrary to common prediction, that the travel time of the signal was earlier when the actuator was extended. Results are presented on the development of the device design, the characteristics of the prototype device and the accuracy of the sensing techniques.