1. Why structural surprises cost more on site than in the design
A structural surprise discovered during the works costs on average two to four times what the same work would have cost in the tender package. Three reasons explain the multiplier. The frame contractor cannot price work that has not been defined, so it applies its schedule rates to unknown quantities, usually loaded to cover the risk. The site is already open, and interference with the other trades, partitions, services, finishes, creates knock-on costs. And the pressure of the programme prevents any normal competition, leaving the contractor already on site in a strong position to negotiate.
The rule is simple: every euro spent on a structural survey before the tender saves two to four euros in variations during construction. It is the most profitable preventive investment in a refurbishment project.
ℹ Survey against contingency: the trade-off Option A, a structural survey at 500 to 3 000 €: a certain cost, findings in 5 to 15 days, allowing the strengthening work to be priced precisely and written into the tender package. The contractors know what they are pricing, and the contingency can be reduced. Option B, a higher contingency with no survey, at 15 to 20 %: no cost, but a provision that may well be understated if a significant defect is present. The frame contractors price on their own assumptions, which diverge, and the tenders are further apart and less comparable. The recommendation: on any building from before 1960, or showing visible signs of a defect, option A is systematically the better economics.
2. The seven commonest structural defects
| Defect | Visible signs | Buildings at risk | Cost of remedy | Blocks the tender? |
|---|---|---|---|---|
| Active cracking in the façade | Stepped cracks, cracks through the wall, render detaching | Old masonry, clay soils, shallow foundations | 5 000 – 80 000 € for foundation works, up to 200 € a linear metre | Yes, a geotechnical survey is needed before the frame can be priced |
| Decayed floors | Deflection, creaking, abnormal flexibility, visible rot | Buildings before 1950 with timber or early hollow-pot floors | 30 – 120 €/m² to repair or replace | Yes where structural work is involved; it governs the finishing trades |
| Decayed roof structure | Insect or fungal attack, visible deflection, rotten bearings | Any period; chronic damp is the trigger | 20 – 80 €/m² for treatment plus partial replacement | Yes where replacement is needed; it governs the roof covering |
| Failing lintels | Cracking above openings, visible deflection, patched mortar | Old masonry with timber or stone lintels | 500 – 3 000 € each by span and material | Advisable before forming any new opening |
| Underpinning required | Differential settlement, floors dropping, out of level | Shallow foundations, compressible ground | 50 000 – 300 000 € for piling or grouting | Yes; a specialist contract precedes every other package |
| Dry rot | White growth, a mushroom smell, friable timber | Damp buildings, cellars, voids, failed façade joints | 5 000 – 50 000 € by extent, treatment plus timber replacement | Yes; the treated zone has to dry before any rebuilding |
| Façade biological growth | Vegetation, efflorescence, facing detaching | North-facing façades with no roof overhang | 3 – 15 €/m² to treat, 20 – 60 €/m² for full renewal | No, but it belongs in the façade package |
The defects marked as blocking must not be sidestepped by a lump sum allowance inside the frame contract. Contractors answering a tender that carries an item reading "possible strengthening: lump sum" return prices that cannot be compared and variations that cannot be avoided. The rule is that every blocking item has to be resolved before the tender goes out.
3. Provisions by type of building
The provision for structural surprises is not uniform. It depends on the type of building, its period, and the defects identified on the preliminary visit. The table below gives the orders of magnitude.
| Type of building | Period | Main structural risks | Recommended provision | Priority surveys |
|---|---|---|---|---|
| Nineteenth-century apartment block | 1850–1914 | Timber floors, projecting chimney breasts, limestone façades, shallow foundations | 12–15 % | Structural engineer plus asbestos survey |
| Early reinforced concrete block | 1920–1960 | Unprotected reinforcement corroding, poor concrete, early slabs | 15–20 % | Concrete carbonation testing plus structural survey |
| Older individual house | Before 1948 | Stone or brick walls, timber floors, traditional roof structure | 12–18 % | Structure, asbestos, lead, dry rot |
| Concrete block from the 1970s | 1960–1980 | Sprayed asbestos common, poor insulation, ageing services | 15–20 % | Asbestos survey first, then energy performance |
| Converted industrial building | Various | Unanticipated loadings, altered structure, contaminated ground | 20–30 % | Structure, contamination, asbestos, lead |
| Recent building | 1990–2010 | Low risk; isolated defects of workmanship | 8–10 % | Energy audit where a retrofit is planned |
These provisions apply to the total works budget and cover both structural and non-structural hazards. The provision on the directly exposed packages can be higher: 20 to 30 % on the frame alone in a nineteenth-century block with suspect foundations, against 8 % on the finishing trades of the same building.
4. The structural survey: who, when and how
Who carries it out
A structural survey is carried out by a structural engineer or a building surveyor. For defects involving the foundations and active cracking, a geotechnical diagnostic of the existing site is the reference, and it includes ground investigation where needed. For roof structures and floors, a carpenter or a timber engineer can carry out the survey.
When to commission it
The survey should be commissioned as soon as the preliminary visit reveals signs of a defect, and systematically on high-risk buildings, before 1960, with known structural alterations, or on clay soils. It has to be received before the developed design estimate is produced and before the tender goes out. Commissioning a survey during the tender period lengthens the programme and creates an information asymmetry between the contractors.
What the survey has to deliver
For the economist: quantities that can be priced, linear metres of repair, area of floor to replace, number of piles, not merely a description of condition such as "floors in poor condition".
For the architect: precise technical recommendations that can be worked into the drawings and the specification.
For the client: a ranking of urgency and a cost estimate item by item, with the residual ranges of uncertainty stated.
5. Carrying structural risk in the estimate
Case 1, a defect identified and quantified
The strengthening work is priced line by line in the estimate, on the quantities given by the survey. It goes into the frame package or into a dedicated enabling works package. The contingency on that package can be reduced to 8 to 10 %, since the survey has removed the uncertainty.
Case 2, a defect identified but not quantified
The survey has detected something, active cracking, doubtful floors, but could not quantify the work precisely, whether for want of access or because further investigation is needed. In that case a conditional sum is created in the estimate, with a low and a high figure and an explicit statement of what will resolve it. The contingency on that package stays high, at 20 to 25 %.
Case 3, a building never surveyed
No investigation has been carried out. The contingency is at its maximum, 15 to 20 % of the total, and the estimate states explicitly that the absence of a structural survey is a major source of uncertainty. Where the signs of a defect are significant, the economist should recommend not going to tender before investigating.
ℹ The contract clause to include as a matter of course Every refurbishment project should carry an unforeseen conditions clause in the conditions of contract, stating the trigger, a joint record by written instruction, the time allowed to price, the applicable basis of price, whether the schedule of rates or a new rate built up from net cost, and the ceiling of the dedicated provision. Without that clause, unforeseen works discovered on site are valued case by case with no established framework, which lengthens the negotiation and weakens the client's position.
6. Underpinning: the special case
Underpinning, piling, ground grouting, cast concrete pads, is the most expensive and least predictable structural work in refurbishment. It becomes necessary where the existing foundations cannot take the new loads, in a roof extension or an extension, or where the ground turns out not to bear.
Its cost runs from 50 000 € for a few localised piles to 300 000 € and beyond for underpinning a whole building. It depends on the number of points, the depth required, the technique used and the conditions of access. On a building in a dense urban setting, the access constraints for the drilling rig can treble the cost against an open site.
The absolute rule: never start works requiring underpinning without a validated geotechnical investigation. Its cost, 2 000 to 6 000 €, is negligible against the risk of discovering on site that the foundations allowed for are inadequate.
In summary
Structural surprises cost two to four times more on site than in the tender package, which makes the structural survey the most profitable preventive investment in a refurbishment. The seven commonest defects are active cracking, decayed floors, decayed roof structure, failing lintels, underpinning, dry rot and façade growth. The recommended provisions run from 8 to 10 % on a recent building to 20 to 30 % on a converted industrial building or an old building with multiple defects. Every blocking item has to be resolved before the tender, not carried as a lump sum inside the frame contract. The conditions of contract should always include an unforeseen conditions clause with an established valuation procedure.
The nature of the figures quoted
The percentages and amounts quoted on this page are orders of magnitude for framing, not measurements. They illustrate mechanisms and proportions, and they substitute neither for a tender exercise nor for an estimate prepared on drawings.
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Sources: the BT construction cost indices published monthly by INSEE on its 2010 base, used to update the figures · construction cost statistics from SDES, the statistical service of the ministry responsible for construction · regulatory texts published in the Journal officiel and consolidated on Légifrance for the requirements cited.