Supplementary Agreement

Once a contracting authority and its contractor have agreed that a change to a public contract is necessary and legally permissible, that agreement needs to be documented in a form that is legally binding, auditable, and transparent. The supplementary agreement is the standard instrument for this purpose: it is the document that records, in contractually binding terms, exactly what has been agreed to change and on what basis.

What is a supplementary agreement?

A supplementary agreement (also called a contract amendment, amending agreement, or addendum in different European jurisdictions) is a bilateral contract executed by both the contracting authority and the contractor. It modifies one or more terms of the original contract, specifying what is being changed, what the new terms are, the effective date of the change, and the legal ground (under Article 72 of Directive 2014/24/EU or equivalent national law) on which the modification is permissible.

The supplementary agreement is distinct from a contract variation order, which is typically a unilateral instruction issued by the authority under a contractual power. A supplementary agreement requires the consent of both parties and therefore provides a higher level of mutual commitment. It is the appropriate vehicle for changes that are agreed rather than instructed, such as scope reductions, term extensions activating a contractual option, or agreed restructuring of payment terms.

A well-drafted supplementary agreement should include: a recital or preamble explaining the background to the change and the legal ground relied on; the precise text of the modified contract clauses; confirmation of all terms not modified; the effective date; and, where required, a reference to any modification notice publication that has been or will be made. It should also record the cumulative modification value to date, which is necessary for demonstrating compliance with the 50% cap under Article 72(1)(b) and (c).

Without proper documentation through a supplementary agreement or equivalent instrument, modifications are difficult to audit and may be challenged on procedural grounds even where the substantive legal basis is sound. Contracting authorities across Europe are required by their own internal governance frameworks (and in some jurisdictions by statute) to maintain a complete modification record for each contract.

In the UK under the Procurement Act 2023, the obligation to publish certain contract changes as modification notices has increased the importance of clear internal documentation that supports what is published.

Why it matters for bidders

A supplementary agreement is your primary protection as a contractor if a dispute arises later about what was agreed, when, and on what terms. Verbal agreements to change contract scope, price, or duration are legally precarious in a public procurement context, where the original contract is a formal instrument and any change to it should be equally formal.

When negotiating a modification, insist on a properly drafted supplementary agreement before you begin performing the changed scope. Doing additional work in anticipation of an agreement that is never finalised leaves you without contractual cover for the additional cost.

Example

A French regional council holds a social care staffing contract. Following a regulatory change requiring enhanced training for care workers, the council and contractor agree to add a funded training programme to the contract, activated under a review clause in the original documents. They execute a supplementary agreement recording: the regulatory background, the review clause relied on, the additional scope (quarterly training days and the associated cost), the revised annual contract value, and the effective date. The modification is lawful, documented, and auditable.

Frequently Asked Questions

Does a supplementary agreement need to be in any particular form?

No statutory form is prescribed in EU procurement law, but the agreement must be in writing and signed by authorised representatives of both parties. Many authorities have standard templates. The substance is what matters: the recital, the precise change, the legal basis, and the cumulative modification record.

Can a supplementary agreement contradict the original contract?

It can modify the original contract, but it should do so explicitly. A supplementary agreement that modifies a clause should either reproduce the new clause text in full or expressly state what is being deleted and replaced. Ambiguous drafting that leaves both the original and amended text in force simultaneously creates interpretation disputes.

Is a supplementary agreement sufficient to implement a modification that requires a publication notice?

The supplementary agreement implements the modification between the parties. It does not substitute for any applicable publication obligation. Where a modification notice publication is required in TED or Find a Tender, that notice must be published separately, either before or alongside execution of the supplementary agreement.