We present Mockup Builder, a semi-immersive environment for conceptual design which allows virtual mockups to be created using gestures. Our goal is to provide a familiar way for people conceive, create and manipulate three-dimensional shapes. To this end, we developed on-and-above-the-surface interaction techniques based on asymmetric bimanual interaction for creating and editing 3D models in a stereoscopic environment. Our approach combines both hand and finger tracking in the space on and above a multitouch surface. This combination brings forth an alternative design environment where users can seamlessly switch between interacting on the surface or in the space above it to leverage the benefit of both interaction spaces. A formal user evaluation with our experimental setup shows very promising avenues for further work towards providing an alternative to nowadays CAD system user interfaces.