Construction Contract Addendum UK: How to Amend

What this document is

An addendum is a supplementary written agreement that modifies, clarifies, or adds to an existing construction contract. It does not replace the original contract. Instead, it records a specific change that both parties have agreed to, and once signed it becomes part of the contractual framework governing the project.

Addendums are commonly used on projects running under JCT, NEC, or bespoke forms of contract, and they sit alongside variation instructions and change orders rather than replacing them. The difference matters: a variation is usually issued under a mechanism built into the contract itself, whereas an addendum is a separate agreement that can alter the contract terms themselves, not just the works.

That makes addendums particularly useful when the change goes beyond what the variation clause was designed to cover, for example a fundamental shift in payment terms, an extension of the contract period, or the introduction of new parties. Because an addendum changes binding obligations, it needs to be drafted with the same care as the original contract.

How to use this document

1. Identify exactly what is changing. Before drafting anything, write down what the parties have actually agreed. Is it a change to the scope of works, the completion date, the contract sum, the payment schedule, or something else? Vague intentions produce vague addendums, which is where disputes start. Get the commercial position nailed down first. 2. Reference the original contract clearly. The addendum must identify the contract it amends by date, parties, and project reference. Without this link, the addendum floats free and can be challenged on grounds of uncertainty. Include the contract title, the execution date, and any unique reference numbers used on the project. 3. Draft the amendment in precise language. Spell out which clauses are being amended, deleted, or added, and quote the new wording in full. If the contract sum is changing, state the old figure, the new figure, and the net adjustment. If dates are shifting, give the new dates clearly rather than saying things like 'a reasonable extension'. 4. Confirm what stays the same. Include a clause stating that all other terms of the original contract remain in full force and effect. This prevents an argument later that the addendum somehow discharged or replaced the whole agreement. It is a short clause but an important one for preserving the original bargain. 5. Execute the addendum properly. Both parties should sign, date, and ideally witness the addendum using the same formalities as the original contract. If the original was executed as a deed, the addendum should usually be executed as a deed too. Keep signed copies with the main contract file so the project record stays complete.

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