We propose a novel design for a guided-mode resonance (GMR) grating sensor that extends the sensitivity to a large region of space, possibly several tens of microns away from the grating surface. This type of sensors has high sensitivity in the half-space above the grating, close to the theoretical limit, together with a controllable - potentially very high - quality factor. It relies on a resonance caused by a "confined" mode of a sub-wavelength thick grating slab, a mode that is largely expelled from the grating itself into the grating environment. The small thickness assumption allows us to derive a simple yet accurate analytical model for the sensor behavior, which is tested numerically using a rigorous coupled-wave analysis (RCWA) method as well as in preliminary grating transmission measurements.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.