Eligible cost rules sit in the award letter, the funder’s standard conditions, and any later variations you have accepted in writing. Teams often treat “project-related” as enough. That reading fails when a cost is related but still excluded—capital items below a threshold that was never approved, entertainment, or staff time charged without a timesheet method the funder recognises.
Start with the budget schedule. Every line you claim should map to an approved category. If you moved spend between lines, check whether the funder required prior approval above a percentage. Keep a short eligibility note for unusual items: what the cost was, which clause permits it, and where the invoice lives.
When in doubt, ask before you claim. A clarification email filed with the evidence pack is cheaper than a clawback after payment.