Abstract: Parameter-efficient finetuning (PEFT) has become ubiquitous to adapt foundation models to downstream task requirements while retaining their generalization ability. However, the amount of additionally introduced parameters and compute for successful adaptation and hyperparameter searches can explode quickly, especially when deployed at scale to serve numerous individual requests. To ensure effective, parameter-efficient, and hyperparameter-robust adaptation, we propose the *ETHER* transformation family, which performs Efficient fineTuning via HypErplane Reflections. By design, *ETHER* transformations require *a minimal number of parameters*, are *less likely to deteriorate model performance*, and exhibit *robustness to hyperparameter and learning rate choices*. In particular, we introduce *ETHER* and its relaxation *ETHER+*, which match or outperform existing PEFT methods with significantly fewer parameters ($\sim$$10$-$100$ times lower than LoRA or OFT) across multiple image synthesis and natural language tasks without *exhaustive hyperparameter tuning*. Finally, we investigate the recent emphasis on Hyperspherical Energy retention for adaptation and raise questions on its practical utility. The code is available at https://github.com/mwbini/ether.