Scalable video coding consists in compressing the video sequence into a layered bitstream where each layer refers to different spatial, temporal or quality representation of the video. Scalability enables compression gain compared to the simulcast encoding of layers thanks to inter-layer predictions. The scalable HEVC extension (SHVC) is the latest scalable technology promising up to 30% bitrate gains under the common test conditions, defined by JCT-VC. These conditions do not consider UHD and use fixed quantization step, which is not relevant in operational environment. In this paper, we propose an innovative adaptive rate control algorithm for SHVC. We consider HD as a base layer and UHD as an enhancement layer, with a constant global bitrate and a dynamic bitrate ratio adjustment between layers. The proposed algorithm is evaluated on a UHD data set where enables on average a BD-BR gain of 4.25% compared to a fixed-ratio encoding.