Measuring incomplete design is the normal condition rather than the exception. Programmes rarely allow the design to be finished before a price is needed, and a measurer who waits for complete information will never measure anything.
What separates a defensible measurement from a guess is not the completeness of the drawings. It is whether the gaps have been identified, filled deliberately, and recorded.
Three ways to handle a gap
Measure it on a stated assumption. Where the element is predictable, assume the likely solution, measure it, and record the assumption. This is the best option because it produces a measured quantity that can be corrected precisely when the design arrives.
Carry it as a provisional sum. Where the scope is known but the detail is not, a defined provisional sum keeps the item visible and lets the contractor plan around it. In cases where even the scope is uncertain, the sum will be undefined and carries programme risk, as covered in provisional sums, defined and undefined.
Exclude it and say so. Sometimes the honest answer is that an element cannot be priced at all yet. An explicit exclusion is far better than a silent omission, because it prompts a decision rather than a surprise.
The schedule of assumptions
Every measurement from incomplete information should be issued with a list of what was assumed. Not buried in a covering letter, but as a schedule the design team can read and correct.
Each entry states the element, the assumption made, and the effect if it is wrong. That last part is what makes the schedule useful rather than defensive; it tells the reader which assumptions matter.
The design team then has an opportunity to say that an assumption is wrong before it becomes a variation. In practice this catches a meaningful proportion of them, and the correction costs nothing at that point.
What is usually missing
Services, almost always. Mechanical and electrical design typically lags architectural design, and the quantities depend on it heavily through builder's work, riser sizes, plant space and distribution.
Substructure, where the design depends on ground investigation that may not be complete. Foundation type is the single largest assumption on most schemes.
Specification detail, where drawings show an element but not what it is made of. A wall shown as a line has an area but no cost until somebody decides what it is.
Junctions and interfaces, which are where design development adds most and where the drawings are least likely to be resolved.
Refurbishment is a different problem
On new build the missing information will eventually exist. Equally, on a refurbishment some of it cannot exist until the fabric is opened up, and no amount of design work resolves it.
That changes the response. Assumptions about existing structure and services should be paired with an opening up protocol and with provisional sums, because the resolution comes from the site rather than from the design team.
The wider approach is set out under refurbishment and reuse.
How the assumptions get used later
When the design arrives and differs from the assumption, the difference is a change. Whether it is a variation depends on the contract and on how the scheme was documented, but commercially the movement is identifiable and explicable.
That is the entire value of the schedule. Without it, the cost simply moves and nobody can say why, which is how a well managed project becomes an argument.
It also makes the reconciliation between cost plan issues possible, because each movement can be traced to an assumption that resolved rather than to a general increase.
Saying what the measurement will bear
A measurement from partial information supports a budget. It does not support a fixed price, and presenting it as though it does is the failure that produces most tender stage surprises.
The proportion of value that is measured, assumed, provisional and excluded is worth stating on the front of any such document. A client who knows that a third of the value is not measured will treat the number correctly.
That failure is described in incomplete design information.
What this means for you
Ask for the schedule of assumptions with every measurement. If there is not one, the assumptions were still made, they simply were not recorded.
Then circulate it to the design team and ask them to correct it. It is the cheapest quality control step available on any scheme with an unfinished design. Our approach sits under quantities and take-off.
Need quantities from a design that is not finished?
Send what exists. We will measure what can be measured and tell you precisely what has been assumed.