Volume data and changed methodology

One reason explains most cases: the methodology for calculating a building's volume has changed over time. The same building may therefore be recorded in the register with one figure and produce another under today's methodology.

This does not mean anyone made a mistake, only that the data come from different periods. Professional sources specifically recommend checking volume data separately, because they are the most frequent source of discrepancy in the building register.

Here is the practical core of the article, a chain in which every link only becomes visible in hindsight. Volume data affect the level of procedure.

Step	Consequence
The extent of an extension is calculated against the existing volume	the base figure comes from the register
If the existing volume is recorded incorrectly	the extension percentage is wrong
The type of procedure may depend on that percentage	a wrong percentage leads to the wrong procedure

A wrong base figure usually emerges during the procedure rather than before it, and that is the most expensive moment to discover it, because the design has already been commissioned and the solution chosen. The dependence of the three levels of formality on the extent of an extension is covered by the article on levels of formality.

Three steps, of which the third settles the question unambiguously and avoids guesswork. The third step is an enquiry to the authority.

Compare the figure in the register with the data in the design or in a survey. That tells you whether a discrepancy exists at all, and how large it is.

Check what period the register entry dates from, since the age of the entry points to the methodology used and often explains where the difference comes from. An older entry may follow a different methodology.

Ask the local authority which figure will be taken as the basis in the procedure where a discrepancy exists. That answer is binding in a way no assumption can substitute for.

Two situations worth distinguishing, because the first is correction and the second is legalisation. The second is legalisation and follows another procedure.

Origin of the difference	What to do	Typical size
Methodology	establish which figure applies in the procedure and work from it throughout the design	a small difference
Actual changes to the building	points to works done without a procedure, moves into legalisation	a large difference

The size of the difference usually gives a hint, but it is not proof: a few square metres points to methodology, tens of square metres points to something having been done to the building. A large difference points rather to building work.

Three consequences, affecting above all projects near a threshold. A small difference can change the level of procedure.

A project near an extension threshold is particularly sensitive, because a small error in the base data changes the type of procedure, and with it the schedule and the cost. The data must be checked before design.

In such a case the volume data must be established before choosing the solution rather than after. A solution chosen on a wrong base figure may have to be redone precisely when time is most expensive.

The designer needs to know which figure applies, because they calculate from it; two different figures in two different documents produce two different designs. The wrong starting figure brings the wrong procedure.

A discrepancy in the volume data recorded in the register usually arises because the methodology for calculating a building's volume has changed over time, not because anyone erred. That would remain unimportant if the extent of an extension were not calculated against the existing volume: a wrong base figure gives a wrong percentage and hence the wrong procedure, which normally emerges once the design has already been commissioned.

Four rules: check the volume data separately rather than as part of a general look at the register. Establish with the local authority which figure applies in the procedure where a discrepancy exists. Distinguish a difference arising from methodology from actual changes, using its size as a hint. Carry out this check before choosing the solution where the project is near an extension threshold.