In this paper, a novel spatial resolution adaptation approach for video compression is proposed. Its ability to dynamically apply downsampling to frames exhibiting low spatial detail delivers improved rate distortion performance, together with a reduction in computational complexity of the encoding process. This method is based on an experimental investigation of the dependence between the QP threshold, which determines when to encode lower resolution frames, and the distortion obtained after downsampling/upsampling. The proposed approach is integrated with the High Efficiency Video Coding (HEVC) reference codec for intra coding, and evaluated on 15 high-resolution test sequences with varying levels of spatial detail. The results show a promising average bitrate savings of approximately 4% (B-D measurements), and significant complexity reduction (29% on average).