The rule families
| Family | The question it resolves | Examples of convention |
|---|---|---|
| Deductions | Which openings and voids are discounted | Deduct openings above a certain area threshold; deduct none; deduct all |
| Measurement profile | On which geometry measurement is done | Excavation at theoretical profile or with real slopes; coverings by visible or developed surface |
| Unit | In what the quantity is expressed | m², m³, lm, unit, kg; the unit conditions the price and the certification |
| Reference document | From where measurement is taken | Execution design drawings; on site, what is actually executed under the same rules |
| Inclusions | Which accessory work travels inside the unit | Trims, junctions, the unit's own auxiliary means |
Every reference price database publishes its rules associated with each standard item: inheriting the item means inheriting its rule, and that is the cheapest way to have complete rules known by the whole market. Writing bespoke rules only pays where the works have particularities no database covers.
Why the rule is part of the price
The classic example: a partition measured gross (without deducting openings) generates more m² than the same partition with openings deducted, and the contractor who knows this tenders a lower unit price for the gross measurement, because the price compensates the quantity. Neither price is more honest: they are incomparable with each other. The operating consequence is twofold. In procurement, all offers must be calculated on the same declared rules, or the comparison table compares apples with oranges. On site, certification measures under the contract's rules, not those each party prefers that month: the agreed rule is the rulebook for the project's entire economic life.
How to declare them without friction
Three practices cover the need with little effort. Declare the project's reference price database, which carries its default rules. Make explicit in the item description every deviation from the database's rule (the exception is written, the rule is inherited). And resolve in writing, before requesting offers, the historically conflictive points of the works type: opening deductions in façades and partitions, excavation profiles, services measurement (by point, by lm, by unit) and where builder's assistance sits.
The essentials
There is no quantity without a rule and no interpretable price without one. The complete discipline fits in three gestures: a declared reference database, exceptions written into the item and conflictive points resolved before the consultation. With that, quantities are comparable at tender and executable at certification. The document that materialises these rules is developed in the bill of quantities, and its typical failures in frequent errors.
Note: usual conventions in Spain in 2026; the rules applicable to each contract are those it declares.