Extras in a private lump-sum contract

The text covers a specific case: the lump-sum construction of a building, on the basis of a plan drawn up and agreed with the owner of the land. In that case, the architect or contractor who has undertaken it "may not claim any increase in price" (unofficial translation, as are all quotations of legal texts below), whether on the grounds of an increase in the cost of labour or materials, or on the grounds of changes or additions made to that plan, "if those changes or additions have not been authorised in writing, and the price agreed with the owner".

Two cumulative conditions are therefore required for a change to give rise to an extra: written authorisation of the change, and agreement with the owner on its price. One without the other is not enough under the terms of the text. A written order saying "make this change" without a price leaves the door open to a dispute over the amount; an agreed price with no written authorisation leaves open the question of who asked for the change. In practice, it is best to record both in the same document. If one is missing, the contractor cannot, under the terms of the text, claim a price increase for that change, and the dispute then turns on proof of what was requested, accepted and priced.

Condition	What it assumes	The client's point of vigilance
Lump-sum contract	an overall price for the work, not unit prices applied to quantities	in a mixed contract (lump sum and items in provisional quantities), only the lump-sum part has an overall price
Construction of a building	the subject of the contract is a building	renovation follows its own logic, dealt with in a separate guide
Plan drawn up and agreed	a fixed plan accepted by both parties at signature	incomplete drawings at signature weaken the lump sum
Written authorisation of the change	a written decision by the owner	an oral agreement at a site meeting does not replace it
Price agreed with the owner	an amount accepted by the owner	fix it before execution: a price "to be settled in the final account" is not an agreed price

The lump sum covers increases in the contractor's costs. The text expressly excludes an increase in labour or materials as grounds for an extra. If the contract includes a price revision clause, that clause applies, and it is tracked on its own line of the dashboard (see the article on price revision in this branch).

The lump sum covers what the agreed plan describes, and only that. The more precise the signature file, the clearer the scope of the lump sum. A plan that defers choices until later (finishes, equipment, options) creates as many future extras, which article 1793 does not prevent if they are requested and accepted in the proper form.

The lump sum does not cover changes wanted by the client. Every change to the brief during the works is a legitimate extra as long as it is authorised in writing and its price agreed. The discipline of the text is not meant to prevent changes, but to make them visible and priced before they are carried out.

The best application of article 1793 is a procedure written into the contract before the works. The useful clauses are detailed in the article on the clauses that protect; their effect on the budget comes down to four steps.

1. The request. Every change, whether it comes from the client, the architect or the contractor, is recorded on a numbered change form describing the change and the items affected.
2. The pricing. The contractor submits a price, upwards or downwards, with any effect on the programme. Omissions are tracked as rigorously as additions: an item removed and not deducted is a hidden extra.
3. The decision. The client authorises the change in writing and accepts the price, on the same document. The client signs: the architect advises and checks, but the spending decision belongs to the client, unless there is a written, limited mandate.
4. Execution, then the invoice. No changed work starts before signature. In a genuine emergency, an immediate written authorisation, even by email, followed by the price within a period set in the contract, is better than an oral agreement.

Document	Minimum content	Signed by
Change form	number, description, items affected, origin of the request	author of the request
Price quotation	amount HTVA (excluding VAT), addition or omission, effect on the programme	contractor
Authorisation	agreement on the change and on the price, date	client
Change register	running total of approved extras and extras under discussion	kept by the client or the client's adviser

The register feeds directly into the dashboard described in the article on tracking the final cost during the works. Signed forms go on the approved extras line; forms that are priced but not signed go on the extras under discussion line.

The text works in the client's favour, but in practice the dispute turns on evidence. Who asked for the change, who accepted it, at what price: those are the facts the discussion turns on. Ambiguous emails, site minutes that record a change without reservation, and interim payments made without objection may be relied on against the client.

Three reflexes limit that risk. Dispute in writing, without delay, any invoice containing an unauthorised item. Have site minutes reread and have them record that a change discussed remains subject to written authorisation and price. Never pay an extra "to keep things moving" without a document fixing its amount.

In the dashboard, a disputed extra stays entered at its probable amount. Removing it because article 1793 seems to favour the client amounts to betting on the outcome of a dispute.

The same change often has two effects: on the contractor's price, and on the architect's appointment. The code of ethics of the Ordre des architectes (Order of Architects) of 16 November 2022 (art. 16) requires the architect to design "within the limits of the brief set in the appointment and of the budget", and provides that any change to the brief "must be the subject of an additional agreement stating its financial impact" (the convention additionnelle, or additional agreement).

For the client, this is a second safety net. A change to the brief requested during the works should therefore leave two traces: the authorisation and the price on the contractor's side, the additional agreement on the architect's side. When both are missing, that is the scenario in which the budget drifts without anyone having priced it.

The client may end the lump-sum contract at will. Article 1794 of the old Civil Code allows this, provided the client compensates the contractor for all its expenses, all its work and everything it could have earned from the undertaking.

It is an exit, not a saving. The cost of termination includes what has been carried out, the expenses incurred (orders, supplies) and the lost profit on the part not carried out, to which must be added the cost of the contractor who will complete the work. Before using that lever to escape a drifting budget, it must be priced as an option in the trade-off note, alongside a reduction of the brief. A fixed compensation clause in the contract should be read in that light.

In renovation, extras arise mainly from the existing building, discovered during the works. The subject is dealt with in the article on addenda and additional works in the guide on renovation.

In a public contract, the logic is different: article 1793 gives way to the general implementing rules, which require written orders and set ceilings on modifications. See the article on changes to a public contract in progress.

Everything above applies to contracts concluded before 1 September 2027. Book 7 "Les contrats spéciaux" (Special contracts) of the Civil Code, inserted by the act of 22 July 2026 (Moniteur belge, Belgian Official Gazette, of 23 September 2026), will apply to contracts concluded from that date; unless the parties agree otherwise, the previous rules remain applicable to the future effects of earlier contracts.

Book 7 keeps the lump-sum principle, but does not carry over, in its section on immovable works, the written authorisation rule of article 1793. That does not mean nothing in writing will be useful any more: evidence and the contract remain decisive. Hence the recommendation, from today for contracts that will be signed after that date, to write into the contract the written change procedure described above. The detail is dealt with in the article on what changes in 2027 with Book 7 of the Civil Code.