The ABEX index
This is the usual Belgian reference for measuring the evolution of construction costs. It is published periodically and underpins the revision formulas.
Published by the association giving it its name, it is established half-yearly. Its value is 1056 for the first half of 2026, with an update due for the second half.
The index serves two distinct purposes that should not be confused. One updates an estimate, the other revises a contractual price.
Updating an estimate. A benchmark established at a given index can be transposed to another half-year by a simple ratio of indices. This is useful for reusing in-house references.
Indexing insured values. An insured reinstatement sum is generally linked to this index, so that cover follows the evolution of costs.
An in-house benchmark kept without its reference index is unusable, exactly like a benchmark kept without its area convention.
Material indices
Alongside the general index, sector indices track the evolution of material and labour prices. The choice of index is fixed in the contract rather than afterwards.
They are published periodically and serve contractual revision formulas. Their granularity allows the real structure of a contract to be reflected, where the labour share and the material share do not follow the same curves.
In Belgian public contracts these revision mechanisms are the rule rather than the exception, with formulas framed by the applicable regulation.
How a revision formula works
The principle is simple, even if its written form looks technical. The formula sets current indices against those of the reference date.
A revision formula splits the price into parts, typically a labour part, a material part and a non-revisable part. Each part evolves under its own index, between a reference date and the date of execution.
Three elements must appear in the contract for a formula to be workable. Without them the revision cannot be applied in practice.
The reference date of the indices, which is not necessarily the date of signature.
The weighting of the parts, which must reflect the real nature of the works.
The frequency of application, monthly, by interim valuation or otherwise.
A formula missing any of those three elements produces disputes, because each party will read it in its own favour.
| Element | Role in the formula | Where to find it |
|---|---|---|
| Reference index | Value at the date of the offer | Contract, revision clause |
| Application index | Value at the date of invoicing | Periodic publication by the body |
| Fixed non-revisable share | Portion of the price excluded from revision | Contract, expressed as a percentage |
| Frequency | How often the revision is calculated | Contract, monthly or per interim statement |
Fixed or revisable price
Intuition pushes towards a fixed price. It is sometimes a poor guide.
A fixed price is not free. A contractor committing to a fixed sum over eighteen months prices the risk of movement into the offer. The client therefore pays a risk premium, whether prices rise or not.
A revisable price is fairer but less predictable. It follows real cost but exposes the budget to a variation that must be provided for.
Three elements guide the choice.
The foreseeable duration. On a short project the stake is small and a fixed price is comfortable.
Current volatility. In a period of sharp material movement, the risk premium on a fixed price becomes high.
The capacity to absorb a variation. A client without budget margin may prefer paying the premium for certainty.
What consent duration adds
A point specific to Belgium, where consent periods run from sixty to one hundred and sixty days by region, with the time before a file is declared complete on top. That gap alone justifies providing for indexation at the estimate stage.
The longer the period, the wider the gap between the initial estimate and prices at tender. A Brussels project is therefore structurally more exposed to that gap than a Flemish project under the simplified procedure.
This effect is covered in the branch on regional cost. The periods are given there region by region.
What this means for an estimate
Four rules.
Date and index every estimate. State the reference index alongside the area convention.
Provide for revision rather than hoping it is nil, in proportion to the foreseeable duration.
Check the formula before comparing two offers. A fixed offer and a revisable offer do not commit to the same final sum.
Update in-house references at each index publication, or the price base drifts silently.
This article reflects professional practice and indices at the date of checking and serves as orientation. It does not constitute legal advice and does not replace assessment of the individual case.