In context-aware systems, user interface adaptation often occurs automatically, offering limited transparency and minimal user control. To overcome this limitation, this research elaborates on the concept of the Extra-User Interface, an additional user interface layer designed to empower users by enabling real-time observation, customization, and steering of adaptation behaviours. Positioned above the primary user interface, an Extra-User Interface extends the meta-user interface concept by providing structured services, such as inspecting and modifying underlying user interface artefacts. To assess the impact of Extra-User Interfaces on user experience, we adopt a Goal-Questions-Metrics approach, focusing on user satisfaction, comprehension, and performance. The research follows a multi-layered method, combining conceptual development, model-based design, and empirical validation. As a proof of concept, a prototype was developed and tested in a mid-air gesture-controlled environment, showing how an Extra-User Interface can enhance user autonomy and efficiency in controlling adaptation. Moreover, the research explores the applicability of extra-user Interfaces in high-stakes domains through the development of a second prototype for a data visualization interface designed to support critical decision tasks. Contributions of this work are threefold: (1) a theoretical framework defining Extra-User Interface concepts and primitives; (2) a model-based approach for designing graphical user interfaces whose adaptation is controlled by an extra-user interfaces; and (3) the implementation and evaluation of Extra-User Interface environments in realistic settings. Preliminary results validate the feasibility and usefulness of Extra-User Interfaces as a user-centric solution for controllable user interface adaptation in ambient intelligence and ubiquitous computing.