The repertoire and its symptoms
| Error | Typical mechanics | Detectable symptom |
|---|---|---|
| Omission | Whole items (assistance, trims, testing, certifications) or forgotten building zones | Chapters with anomalously low weight; ratios per m² out of range |
| Duplicate | The same work measured in two chapters (waterproofing in roofing and in its own chapter) | Anomalously high weight; cross-check of similar descriptions |
| Mixed rules | Part of the document deducts openings and part does not; excavations at different profiles | Inconsistent ratios between comparable zones |
| Inherited quantities | Amounts copied from a previous project and never re-measured | Global quantities without detail lines; round numbers |
| Mismatch with drawings | The design was revised and the quantities did not follow | Systematic divergences in the modified zones |
| Unit error | lm instead of m², unit instead of set | Absurd unit prices when crossed with the price database |
Proportionate control: three levels
Level 1, ratios (one hour). Global quantities per built m² against the typology's usual ranges: m² of partitions, of flooring, of façade, electrical points, lm of pipework. Every ratio out of range is a directed question. It is the filter that captures the most errors per hour invested.
Level 2, weights (one hour). Chapter structure against the usual weights, as developed in the budget by chapters. It detects omissions and duplicates by masses before reading a single item.
Level 3, directed sampling (half a day). Re-measurement against drawings of a sample: the highest-value items, those flagged by levels 1 and 2, and the historically conflictive ones for the works type. On the sample, rule consistency and the existence of detail lines are also verified.
The two structural preventions
Cheaper than detecting is not producing. Complete measurement lines (dimensions and location, never global quantities) make the document self-auditable and are the vaccine against inherited quantities. And the synchronised revision rule: no drawing revision closes without its quantities pass; the design-quantities mismatch is by far the family's most expensive error because contracts are signed on it.
The essentials
Six error families, known symptoms, and a three-level control (ratios, weights, directed sampling) that captures the essential risk in a day's work. The two preventions that reduce the problem at source: mandatory detail lines and quantities-drawings synchronisation at every revision. Whatever survives these filters is paid for on site: that is why control happens before requesting offers, when correcting costs hours rather than new rates.
Note: usual practices in Spain in 2026; control ranges and ratios vary by typology and project.