JCT: D&B 2016 - Can I extend the final date for payment period?

I am using the JCT D&B 2016 and would like to extend the payment period because the client won’t pay the contractor’s invoices within 14 days from the due date (their standard terms are 30 days). Ideally they would, but it’s not a realistic proposition.

Clause 4.9.1 States “The final date for payment of each Interim Payment and the final payment shall be 14 days from its due date”. To bring this more into line with the client’s payment terms (30 days), can I change this period from 14 to 21 days?

Assuming that you are currently drafting the contract i.e. you are pre-contract not post-contract then you are free to make this change. Once the contract has been entered into then both you and the Employer are stuck with the contractual payment period unless the Contractor is prepared to enter into a formal agreement amending the standard timescales, however why would they want to agree to something that would have a detrimental effect on their cashflow?