The development of written expression includes transcription, text generation, and executive functions (including planning) interacting within working memory. However, executive functions are not formally measured in school-based written expression tasks, although there is an opportunity for examining students' advance planning--a key manifestation of executive functions. We explore the influence of advance planning on Grade 2 written expression using curriculum-based measurement in written expression (CBM-WE) probes with a convenience sample of 126 students in six U.S. classrooms. Controlling for transcription, which is typically the primary focus of instruction in early elementary grades, we found that a score on advance planning explained additional significant variance in writing quantity and accuracy. Results support that planning may be an additional score to add to the use of CBM-WE. Implications for assessment and further research on the early development of planning and executive functions related to written expression are explored.