Price revision and the budget's base date

Every amount in the budget carries a value month, the month of the prices on which it was based. The initial budget, each stage estimate and each tender has its own date; comparing them without bringing them back to the same date is like comparing amounts in different currencies.

The date is set from the very first figure. It is written next to the budget in the architect's agreement and carried over onto each estimate. An updated estimate with no change of value date mixes two things: the development of the project and the movement of prices.

Price revision is an item in the total project cost. It appears, together with fees, costs and the contingency, in the list dealt with in the article on moving from works budget to total project cost. Failing to provide for it means assuming that prices will stay frozen throughout design and construction.

Before comparing tenders with the budget, the budget must be brought to the tender date. Between the budget's value month and the month the tenders are submitted, prices have moved; the gap between tenders and budget must be read after that update, not before.

The method is simple in principle. The budget is updated using indices representative of construction costs, applying their movement between the two dates. The indices used and how to combine them are explained in the article on price revision and indices in the guide on construction prices.

Fictitious example. A works budget of 2,000,000 euros HTVA (excluding VAT) set at January values; tenders are submitted in October. If the chosen indices rose by 4% between the two dates (fictitious rate), the updated budget is 2,080,000 euros. A tender at 2,150,000 euros exceeds the updated budget by 70,000 euros, not by 150,000: the gap attributable to the project or the market is 70,000, the rest reflects the general price level.

That breakdown changes the decision. A gap due to the price level cannot be corrected by renegotiating a tender; a gap specific to the project can be corrected through the brief, the finishes or the packaging, before signature.

Once the contract is signed, revision is governed by the contract clause. It generally applies a parameter formula (fixed share, wages, materials) at each état d'avancement (progress statement). The formula and the parameters, including the I-2021 materials index, are explained in the guide on prices and, for how they relate to the items, in the article on measurement and price revision.

In the dashboard, revision has its own line. It has two parts: the revision already invoiced, which is known, and the revision projected on the remaining works, to be estimated according to the programme. This line is not mixed with extras or quantity variances, otherwise it becomes impossible to say why the final cost is moving. The structure of the dashboard is described in the article on tracking the final cost during the works.

The programme weighs on revision. Works that slip push execution into months when indices are higher. The revision projection is therefore redone at every programme update, not only at every invoice; who bears the revision linked to a delay depends on the contract.

Moment	Value date	What the client does
Initial budget	month it was set	date the budget, write it into the agreement
Estimate at each stage	month of the estimate	re-date, separate the price effect from the project effect
Tender submission	month of submission	update the budget before comparing
Signature	reference date set by the revision clause	check the formula, the indices, the base date
Works	each progress statement	check the revision invoiced, project the remainder
Final account	end of the works	reconcile total revision with the forecast

In public works contracts, price revision is the rule. Article 38/7 of the RGE (general implementing rules, Royal Decree of 14 January 2013) requires a price revision clause in works contracts. It only exempts a contract where two conditions are both met: an estimated amount below 120,000 euros, and an initial execution period below 120 working days or 180 calendar days.

For the pouvoir adjudicateur's (contracting authority's) budget, the consequence is direct. For any works contract outside that exception, revision is not a possibility but an expense to be provided for from the estimate onwards, with an explicit assumption on index movements and on the programme. Where the exception applies, the clause is not compulsory; if the contract contains none, the price is firm, and tenderers may build the risk of increases into their prices: the cost does not disappear, it moves.

In a private marché à forfait (lump-sum contract), cost increases are not passed on automatically. Article 1793 of the old Civil Code provides that the contractor responsible for the lump-sum construction of a building, on the basis of a plan drawn up and agreed, "may not claim any increase in price" (unofficial translation, as are all quotations of legal texts on this page), in particular for increases in labour or materials. A revision therefore only exists if the contract provides for it in a clause.

The clause is something to negotiate and to check. For the client, three points matter: the parameters of the formula and their weighting, the base date, and what happens to revision during a delay attributable to the contractor. A vague clause on the base date can, on its own, shift a significant share of the revision.

Where the Breyne Act applies, revision is regulated by law. That regime, its caps and how they relate to payments are dealt with in the article on payments, guarantees and security; it is not repeated here.

The ABEX index is a public reference for the movement of construction costs. Published every half-year by the Association belge des experts (Belgian Association of Experts), it is used in particular to index insured amounts. According to Assuralia (update of 21 May 2026), the ABEX index referenced II/2026 stands at 1,071.

It is an indicator, not a revision formula. It helps to place an old budget or an insured amount in time. Revision of a contract, on the other hand, rests on the parameters set by the contract, and it is against those that the revision invoiced is measured.