<p>Prospective life cycle assessment (pLCA) is increasingly used to assess the potential environmental impacts of product systems under future scenarios. However, addressing uncertainties arising from future variability in multifunctional processes remains a methodological challenge. For example, waste heat that is considered nonfunctional at the laboratory scale may become functional at larger scales. While some pLCA case studies have addressed these issues, a systematic and comprehensive approach for identifying and handling multifunctional processes remains lacking. This paper introduces a structured, stepwise guidance that builds upon and complements existing methods for addressing multifunctionality under future scenario development in pLCA. Focusing on the uncertainty due to choices and epistemic uncertainties, the guidance covers the identification of relevant inventory parameters such as flow types, the number of functional flows, substituted products, substitution ratios, and allocation factors. Applying this framework to a case study on novel incinerator bottom slag valorization reveals that pLCA results are sensitive to future assumptions regarding potential variability of these inventory parameters. The proposed approach allows for a more comprehensive and transparent assessment of multifunctionality in pLCA, thereby supporting consistent environmental decision-making.</p>