The principle: the contract binds the parties
The implementing Grand-Ducal regulation of 8 April 2018 lays down the basic rule: "The contract binds the parties" (art. 103, unofficial translation). The same article adds that the contracting authority "undertakes nothing that would make the obligations of the successful tenderer more onerous". A modification is therefore neither a right of the client (maître d'ouvrage) nor a favour from the contractor: it is an exception governed by law.
The form is prescribed. Under article 120 of the implementing regulation, "the modification of the contract is made in the form of an amendment" (avenant), on the conditions of article 43 of the law (art. 119). An oral instruction at a site meeting, even one that is then carried out, is not an amendment.
The modification cases in article 43
Article 43 (1) distinguishes several cases, which differ in both origin and limit. The table below summarises them from the budget's point of view.
| Case | Condition | Limit | Budget reading |
|---|---|---|---|
| Review clause or option (a) | provided for clearly from the outset in the contract documents, including a price revision | that of the clause | budgeted from the estimate (devis), since it is known at the start |
| Additional works (b) | have become necessary, a change of contractor is impossible and would cause major inconvenience | price increase of no more than 50% of the value of the initial contract, per modification | funded from the contingency |
| Unforeseeable circumstances (c) | which a diligent contracting authority could not have foreseen, without altering the overall nature | 50% of the initial value, per modification | funded from the contingency |
| Non-substantial modification (e) | does not meet the definition in art. 43 (4) | no figure-based ceiling, but no substantial modification | tracked like any other variance line |
| Low value (§ 2) | below the European threshold and below 15% of the initial value for works | assessed on a cumulative net basis | the running total is kept on the dashboard |
The wording of the two most frequent cases is precise. For additional works, "any price increase may not exceed 50 per cent of the value of the initial contract", and that limit applies modification by modification. Unforeseeable circumstances are those "which a diligent contracting authority could not have foreseen": a gap in the tender documents that careful review would have revealed fits that case poorly on the wording of the text.
A low-value modification is tracked cumulatively. Article 43 (2) allows a modification without a new procedure if its value is below both the European threshold in article 52 (EUR 5,404,000 excluding VAT for works in 2026) and 15% of the initial value of the works contract, on a cumulative net basis. Omissions are therefore deducted, which means they must be tracked as rigorously as additions.
A substantial modification remains prohibited. Article 43 (4) defines it and article 43 (5) provides that any modification falling outside the authorised cases requires a new procedure. For the budget, this means that an unforeseen need that is too large cannot be funded by amendment: it becomes a new contract, with its own estimate, procedure and timetable. The technical description of additional works and their pricing in the bill are covered in the article on additional works.
With an indexation clause, the reference changes. Article 43 (3) provides that, where the contract includes an indexation clause, the updated price is the reference value for assessing the limits. The 15% or 50% calculation is therefore made on the updated value, not on the tender price.
A fictitious example of tracking the running total
The following example is entirely fictitious and serves only to show the calculation. A structural works contract falling under Book I is awarded at EUR 2,000,000 excluding VAT, with no indexation clause. The 15% ceiling in article 43 (2) amounts to EUR 300,000.
| Modification | Amount excl. VAT | Net running total | Share of initial value |
|---|---|---|---|
| Amendment 1: strengthened foundations | + EUR 120,000 | EUR 120,000 | 6% |
| Amendment 2: additional floor openings | + EUR 90,000 | EUR 210,000 | 10.5% |
| Amendment 3: plant room removed | - EUR 30,000 | EUR 180,000 | 9% |
| Request 4: basement extension | + EUR 150,000 | EUR 330,000 | 16.5% |
The fourth request takes the running total out of the low-value band. It can go through by amendment only if it falls under another case in article 43 (1), for example additional works meeting all the conditions of point b), within the limit of 50% of the initial value (EUR 1,000,000) for that modification. Otherwise, it requires a new procedure. In either case, the client needs to know this before giving the instruction, which presupposes an up-to-date change register.
When the successful tenderer can ask in turn
In contracts falling under Book I, the successful tenderer (adjudicataire) also has levers. Article 43 (2) allows it to request a modification if, because of the contracting authority, the start of the works is delayed by more than 40 days, if changes cause a variation of more than 20% in the total value of the contract, or if the contractual period is exceeded by more than 40 days. In the fictitious example, 20% represents EUR 400,000: an accumulation of changes requested by the client can thus open the way for the contractor to request a revision of its terms.
Before work begins, the same threshold opens the way to termination. Under article 44 (5), the successful tenderer may terminate if the start is delayed by more than 40 days because of the contracting authority, or if, before work begins, changes cause a variation of more than 20% in the total value. Article 44 (4) also allows either party to terminate in the event of significant and unforeseeable variations in prices, wages or conditions of performance following a case of force majeure.
Form determines validity. Article 43 (6) requires the request to be notified by registered letter, failing which it is void, and confines requests based on the second subparagraph of paragraph 2 to a one-month period. The client has every interest in recording the date of receipt of each letter in its register.
Municipalities
For a municipality, an amendment may not be enough. Article 147 of the implementing regulation provides: "Any subsequent significant departure from the detailed final design must be approved by the municipal council and the supervisory authority." A modification that substantially changes the approved project therefore takes the file back to the body that voted the appropriations. The full cycle of the municipal budget is described in the article on the budget of a public project, and accountability in the one on the final account against the estimate.
The effect on the budget
Each amendment uses up contingency, and amendments add up. On the dashboard described in the article on tracking the final cost during the works, signed amendments go on the approved amendments line, and priced but unsigned requests on the amendments under discussion line. In the final account, they reappear in the "additional contracts included" column of article 47.
Three indicators are enough to steer. The net running total of modifications as a share of the initial (or updated) value, to know what remains under the 15%. The variation in total value caused by the contracting authority's changes, to anticipate the 20% threshold. The balance of the contingency, to know whether the voted budget still holds.
Price revision is not an amendment. It flows from the clause in the special specifications (cahier spécial des charges) and is tracked on the statutory increases (hausses légales) line, separate from modifications. Mixing the two distorts both the article 43 running total and the justification required in the final account.
In private contracts, the logic is different: there are no legal cases or ceilings, but the double condition of article 1793 of the Civil Code, presented in the article on extras in a private lump-sum contract. The European consultation on the revision of the directives, which could one day affect these rules, is followed in the article on developments to watch.
This article sets out Luxembourg public procurement rules as they stood on the date of checking and is intended as professional guidance. It replaces neither legal advice nor an examination of the specific case.