The schedule assumes the supplier will provide clean data. The supplier assumes the team will clean it. Neither statement appears in the plan. A project assumptions log records beliefs being used for planning before they have been sufficiently confirmed, along with how and when they will be checked.
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Its value comes from exposing decisions that depend on uncertain information. It is not a list of every unknown in the organisation.
Find assumptions inside commitments
Read the current forecast and ask what must be true for it to work. Look at resource availability, client response times, supplier inputs, existing systems and the scope of operational support.
Write each assumption as a statement that can be investigated. “Client engagement” is too broad. “The client can nominate an authorised reviewer before the first acceptance session” gives someone a question to resolve.
Use a short validation record
Capture:
Assumption and the decision or estimate that relies on it.
Why the team currently considers it reasonable.
Consequence if it is wrong.
Owner responsible for obtaining evidence.
Validation method and needed-by date.
Current status, evidence reference and resulting action.
Prioritise assumptions with significant consequences or an approaching decision date. An uncertain item needed next week deserves more attention than a low-impact detail that can be resolved later.
A hypothetical validation
A project plans to reuse an existing training room. The assumption owner checks availability and finds that it is booked during the intended rollout. The project can now move the training, choose another location or revise the rollout sequence.
Record the confirmed constraint and update the affected work. Do not leave a disproved assumption marked “open” while continuing to use the original plan. Where uncertainty remains, connect the possible consequence to the risk review and decide how much exposure is acceptable.
Review at decision points
Revisit assumptions when committing dates, approving changes and approaching major checkpoints. Ask whether old evidence still applies. A previously available person may have been reassigned since the plan was agreed.
Related guidance: small-team project planning guide.
Keep confirmed items for traceability, but make the unresolved decisions easy to find. If your project depends on assumptions nobody owns, Hili Consulting can help expose the critical unknowns and give each one a practical validation route.
