What differs from new build
Three structural differences.
Not all quantities follow from the drawing. The actual thickness of a wall, the nature of a substrate or the condition of an element govern quantities that a survey does not give.
Items exist with no new-build equivalent, foremost strip-out and disposal.
Work takes place in a constrained environment, which affects outputs without changing quantities.
That last point is often confused with the previous one. A correct quantity paired with a new-build unit rate produces a false estimate, without the bill being at fault.
The general principles of measurement belong to another guide, on measurement and specification. The contract documents are analysed there.
Strip-out items
These are the worst-handled items in renovation bills, and the greatest source of disagreement. They concern strip-out and making good.
Four reasons.
They are often lumped together. A single demolition line allows neither comparison of tenders nor checking of invoices.
Their quantity depends on what is found. Stripping out sometimes reveals several undocumented successive layers.
Disposal is a separate item, whose cost depends on the nature of materials and their sorting, and which is regularly forgotten.
Some materials require particular handling, which belongs to the preliminary surveys, covered in the article on asbestos and its identification.
The rule that follows: itemise strip-out by nature and location, and treat disposal as an item in its own right.
How to describe an uncertain quantity
An honest bill distinguishes three statuses, and says so. It separates the measured, the estimated and the assumed.
Measured quantities, established by survey of the existing, with high reliability.
Assumed quantities, estimated on stated assumptions, to be adjusted against what is found.
Conditional quantities, for works to be carried out only if a condition is verified.
Those three statuses must appear in the document, with the assumption underlying each assumed quantity.
A written assumption can be checked and corrected. An implicit assumption gets argued about.
The legal regime of assumed quantities belongs to the guide on measurement and specification. Their contractual treatment is explained there.
| Status of the quantity | Origin | What must appear in the bill |
|---|---|---|
| Measured | Survey of the existing fabric | The quantity and how it was surveyed |
| Assumed | Stated assumption | The assumption adopted and the adjustment rule |
| Conditional | Works subject to a condition | The condition and the agreed unit price |
What makes tenders comparable
Four conditions, whose absence explains most unexplained gaps between quotations. They bear on the traceability of the quantities.
The same items, with the same breakdown, notably on strip-out.
The same assumptions, stated, failing which each firm prices a different project.
The same units, no trivial matter in renovation where the choice between area, length and lump sum changes the result.
The same treatment of contingency, each tender stating whether it includes a provision, covered in the article on pricing with a contingency.
Two tenders thirty per cent apart often cover two different scopes, not two price levels.
The choice of units
A technical point with important practical effects. It concerns the method of measurement adopted.
Area units suit homogeneous work and make checking straightforward.
Length units suit linear work, notably junction repairs, frequent in renovation.
Lump sums suit what cannot usefully be measured, but they transfer the contingency to whoever accepts them, and prevent adjustment against what is found.
An entirely lump-sum renovation bill is a signal. It indicates either very detailed knowledge of the building, or an undiscussed transfer of risk.
What this means for a professional
Four rules.
Itemise strip-out by nature and location, and count disposal separately.
State the status of each quantity, measured, assumed or conditional.
Write down the assumptions, so they can be checked rather than debated.
Give the same basis to every firm invited, the only way to obtain comparable tenders.
This article sets out principles of professional method and serves as orientation. It does not replace technical advice or assessment of the individual case.