1. What accuracy actually depends on
One principle from which everything else follows: an estimate can only be as accurate as its basis. Accuracy grows with the maturity of the design.
If the shape of the building, the materials and the level of finish have not been decided, their cost is not known however carefully the calculation is done. More calculation on an undecided basis does not increase accuracy, it increases only apparent accuracy.
This is the distinction hardest to explain to a client and the most important to explain. It separates accuracy from apparent accuracy.
2. Why more calculation does not help
Three observations. A detailed calculation on undecided solutions gives a detailed result, not an accurate one.
Apparent accuracy is more dangerous than an honest range, because it creates confidence that is not warranted. A range says honestly what is still open.
A figure given to three decimal places on an undecided project is misleading through its presentation, even where the calculation itself is correct. Rounding is more honest here.
Practical rule: the way an estimate is presented should match its basis. A rounded range on an undecided project is more accurate than a precise figure.
3. What becomes fixed at each stage
Four things are added in sequence, each narrowing the range. They are the design, the ground, the completion level and the quotations.
| What is decided | When | Effect on the range |
|---|---|---|
| Volume and shape of the building | first | sets the basic order of magnitude |
| Structural solution and foundations | requires a ground survey | narrows substantially |
| Building services and energy solution | middle | narrows |
| Level of finish and material choices | usually last | largest price difference |
The fourth row is notable, because it is decided last yet affects price extensively: completion level explains roughly a twofold difference. It concerns finishes and fittings.
The range therefore does not narrow evenly but in steps, when the level of finish is decided. Before that the range stays wide.
4. How to explain accuracy to a client
Three sentences are usually enough. These things are decided at present, and these are open.
The price difference on the open items is roughly this, and that sets the width of the range.
The range narrows when those decisions are made, not when more calculation is done.
The third is the most useful, because it gives the client something they can act on: if they want a narrower range, they now know what to do about it. It names the decision that narrows the range.
5. What this does not mean
Two misreadings are worth pre-empting. It does not mean an early estimate is useless. An early estimate decides whether the project proceeds at all, and that is its job.
It does not mean a range is an excuse. A range must be justified, not arbitrary.
The difference is clear: a justified range names what is open; an arbitrary range simply adds percentages. The first is a tool, the second is caution.
In summary, and four practical rules
The accuracy of an estimate depends on how much has been decided, not on the volume of calculation, and a detailed calculation on an undecided basis gives a detailed rather than an accurate result. Four things become fixed stage by stage, and the level of finish, decided last, affects price most, so the range narrows in steps rather than evenly. An early estimate is not useless, and a range is not an excuse.
Four rules: tie accuracy to decisions, not to the volume of calculation. Round according to the basis, because presentation makes a promise. Name what is open, rather than simply widening the range. Explain that the range narrows with decisions, not with the passage of time.
This article serves professional orientation at the date of checking. It is not a quotation.