The simple answer is that the costs can only be charged once they are incurred, and are shown to be in the books (remember to include the costs of the final audit certificate in your books before the audit is carried out).
The Commission official answer is:
“…According to Article II.26 of the FP6 model contract, an audit certificate is a document which certifies that the costs incurred during a reporting period (and declared in Form C) meet the conditions required by the contract. Costs related to the audit certificates may be charged in the actual reporting period as far as they are recorded in the accounts of the contractor (see Article II.19 .1, d of the contract) or they can be estimated in this reporting period and adjusted in the next period (if incurred during that period) or they can be reported in the following period depending on when they are incurred.
If the cost of audit certificate was incurred after the end of current reporting period, it cannot be charged to the project in current reporting period. However, the table included in Point 6 of the Form C: “Audit certificate of the contractor” should be filled to indicate the cost of the audit certificate provided for that period even though the cost of the audit certificate will not be claimed as an eligible in Point 2 of the Form C (“Declaration of eligible costs”) in the current reporting period. This cost will be included in the declaration of eligible costs in next reporting period.
As regards the cost of the audit certificate for the last reporting period:
In general, costs must be incurred during the duration of the project. This is one of the eligibility requirements stipulated in Article II.19.1 of the FP6 model contract. However, Article II.19.1. c) contains an exemption from the general rule: costs incurred in drawing up the final reports (also audit certificates) may be incurred during the period of up to 45 days after the end of the project…”