What quantity variances do to the final cost

The first question to ask about a variance is: what type of item does it concern? The distinction between provisional quantities and lump-sum quantities is explained in the article on provisional and lump-sum quantities; only its effect on the budget is considered here.

Type of item	What moves in the forecast	Where to look
Provisional quantity	the amount follows the quantity actually carried out, at the tender unit price	progress statements, jointly agreed measurements
Lump-sum quantity	the variation is not treated as a provisional quantity variance	the specification and its limits, see the article cited
Item changed by the client	the variance is a modification, not a quantity variance	the change register

The third case is the one most often misclassified. A quantity that increases because the maître d'ouvrage (bouwheer, the client) has changed the project is not a quantity variance: it is a modification, with its own form and regime. In a public contract, that regime is described in the article on changes to a public contract in progress. Classifying each variance correctly avoids counting it twice, or not counting it at all.

The provisional quantity items most exposed are those that depend on what cannot be seen: earthworks, foundations, removal of soil, work on the existing building. That is where projection must start.

In a public contract, article 81 of the RGE (general implementing rules) governs large variances. Independently of any modification made by the pouvoir adjudicateur (contracting authority), where the quantities actually carried out for a provisional quantity item exceed three times the provisional quantities or fall below half of them, either party may request a revision of the unit prices and of the initial periods. The revised price only applies to the quantities carried out beyond three times.

For the forecast, this divides each item into three zones.

Zone	Applicable price	Effect on the forecast
Between half and three times the provisional quantity	tender unit price	cost follows quantity, linearly
Beyond three times	tender unit price up to three times, then a price open to revision	the share beyond three times has an uncertain price: project it as a range
Below half	revision of unit prices possible at the request of either party	a quantity saving may be partly offset by a higher unit price

Beyond three times, revision can work both ways. A unit price built on a given quantity may prove too high for a much larger quantity; the contracting authority then has an interest in requesting revision. Conversely, the contractor will request it if the unit cost rises.

Below half, it is generally the contractor who has an interest in revision, because its fixed costs on the item (set-up, equipment, mobilisation and demobilisation) are spread over fewer units. In the forecast, a sharp drop in quantity should therefore not be counted as a net saving until the unit price question is settled.

The rule does not replace the forecast, it complements it. An item that goes from 100% to 250% of its provisional quantity stays at the tender price, but its cost has more than doubled: that is a major budget variance, even without any price revision.

The progress statement is the detection tool. In a public contract, article 95 of the RGE provides for a déclaration de créance (payment claim) accompanied by a detailed statement, checked and paid within 30 days (60 for certain contracting authorities in the healthcare sector). It is within that period that the projection must be made, before payment confirms the quantities. Reading a statement is covered in the article on variations and interim valuations.

Two figures per item are enough to raise the alert: the cumulative quantity carried out compared with the contract quantity, and the actual physical progress of the item on site. If 90% of the quantity has already been invoiced while the item is only two thirds complete, the final variance is already written.

Three warning thresholds follow from article 81: an item approaching half of its provisional quantity when it is almost finished; an item exceeding its provisional quantity before the end; an item whose projection crosses three times. Each calls for a different action: check the unit price, revise the forecast, prepare the discussion on the revised price.

The method comes down to four actions. They are repeated at each progress statement:

1. Measure the work done: the cumulative quantities carried out per item, checked, not merely invoiced.
2. Estimate what remains, either by extrapolation (quantity carried out divided by physical progress) or, better, by remeasuring the remaining work on the construction drawings and the observed state of the site. When the two methods diverge, that is a signal: one of the assumptions is wrong.
3. Value: projected quantity multiplied by the unit price, with a range for the share beyond three times in a public contract.
4. Carry the variance forward. The sum of projected variances feeds the "quantity variances" line of the dashboard described in the article on tracking the final cost during the works, against the contingency.

Fictitious example, rounded amounts excluding VAT, to illustrate the method.

Item	Provisional quantity	Unit price	Carried out to date	Physical progress	Projected quantity	Contract amount	Projected amount
Earthworks (m³)	1,000	€20	900	60%	1,500	€20,000	€30,000
Foundation concrete (m³)	200	€250	120	50%	240	€50,000	€60,000
Soil removal (t)	100	€80	280	70%	400	€8,000	€32,000 (of which 100 t beyond three times)
Slab demolition (m²)	600	€30	200	80%	250	€18,000	€7,500 (below half)

Reading. Earthworks and concrete drift within the linear zone: plus 20,000 euros at tender prices. Soil removal crosses three times: 300 tonnes at the tender price, then 100 tonnes at a price open to revision, to be projected as a range. Demolition falls below half: the apparent saving of 10,500 euros is not secured, since a request to revise the unit price is possible. In total, the forecast rises from 96,000 to about 129,500 euros on these four items, a variance to be set against the contingency item by item, and not only against the total.

In a private contract, article 81 of the RGE does not apply as such. The effect of a quantity variance depends on what the contract says: marché à forfait (lump-sum contract), unit price contract on provisional quantities, or mixed contract. The question of who bears a quantity error is dealt with in the article on who bears the risk of a quantity error.

The projection method, however, is the same. In a unit price contract, item-by-item projection is essential; in a lump sum, it mainly serves to spot items where an increase in quantity actually conceals a change to the plan, which then falls under the rules on extras. In both cases, providing in the contract for a clause on large variances, whether or not inspired by the public mechanism, avoids discovering the rule when the dispute arises.