Cells constantly adjust the sizes and shapes of their organelles according to need. In this study, we examine endoplasmic reticulum (ER) membrane expansion during the unfolded protein response (UPR) in the yeast Saccharomyces cerevisiae. We find that membrane expansion occurs through the generation of ER sheets, requires UPR signaling, and is driven by lipid biosynthesis. Uncoupling ER size control and the UPR reveals that membrane expansion alleviates ER stress independently of an increase in ER chaperone levels. Converting the sheets of the expanded ER into tubules by reticulon overexpression does not affect the ability of cells to cope with ER stress, showing that ER size rather than shape is the key factor. Thus, increasing ER size through membrane synthesis is an integral yet distinct part of the cellular program to overcome ER stress.