1. Five bases of comparison
These must be assessed together, not in sequence. Taken separately they give the wrong answer.
| Basis | In euros per square metre |
|---|---|
| Construction cost, for the structure, at the same completion level | yes |
| Construction time, affecting temporary accommodation and price movement risk | no, but a cost |
| Timing of decisions, how early the design must be final | no, but a cost |
| Scope for change during construction, and its price | no, but a cost |
| Constraints from ground and access | no, may exclude a solution |
Only the first is in euros per square metre. The other four are costs too, but they do not appear in a quotation, and so they drop out of a comparison made on quotations alone.
2. Which of them usually decides
The third basis, the timing of decisions. It decides which technology is realistic at all.
Where the design is not complete, a factory produced solution is not realistic, whatever its price advantage. A factory needs final dimensions.
Where the design is complete and final, the flexibility advantage of building on site disappears, which the client would otherwise be paying for. Only time and price then count.
The timing of decisions is therefore not one criterion among five but often the one that settles the choice in advance. That is why it should be assessed first, even though it is not financial.
3. What to leave out of the comparison
Three things do not belong in an economic comparison. They are obligations or preferences, not costs.
The technical suitability of the structural solution, which is for the designer to decide.
Compliance with energy performance requirements, which is an obligation, not an argument for choice. Both solutions must meet it.
A material preference without a cost justification, which is the client's right but not part of the comparison. It should be stated as a preference.
The third should nonetheless be stated, not hidden. If a client prefers timber for reasons that are not financial, that is legitimate and it ends the comparison. A hidden preference that bends the figures is worse than a stated preference that replaces the comparison.
4. How to present the result
Three documents, in order of usefulness. A comparison table across the five bases, each assessed separately.
The assumptions the comparison rests on, including completion level and ground conditions. They are written down with the result.
A recommendation with its reasoning, referring to the bases rather than to a general preference. That makes the recommendation checkable.
The second is the most important, because it shows what would overturn the result if it changed. Without it a recommendation is an opinion, not an analysis.
5. The commonest error
One error explains most wrong comparisons: comparing different completion levels. The scope must be harmonised before comparing.
A prefabricated quotation often covers the structure, which is 40 to 50 % of the budget; a quotation for building on site often covers more. The remainder must be priced separately.
Without harmonising the completion level, what is being compared is not technologies but the scope of quotations, as explained in the article on comparability. The conclusion is then unfounded.
In summary, and four practical rules
Technologies are compared across five bases, of which only one is in euros per square metre while the other four are costs that do not appear in a quotation. The timing of decisions usually decides, and it should be assessed first even though it is not financial. The commonest error is comparing different completion levels, which measures the scope of quotations rather than the technology.
Four rules: assess the timing of decisions first, even though it is not financial. Harmonise the completion level before any price comparison. Write the assumptions out, because they determine the result. State non-financial preferences rather than folding them into the figures.
This article serves professional orientation at the date of checking. It is not a quotation.