Cost calculation at the third level of the cost structure

Since the 2009 revision the so-called cost calculation model applies. Under it, the fees for all basic services within a service profile are determined exclusively from the results of the cost calculation, exceptionally from those of the cost estimate. It replaced the earlier stage-by-stage accounting.

An asymmetry follows that is underestimated in practice: a determination made in work phase 3 sets the remuneration for services running into work phase 9. An allowance set too low acts across the entire project duration and can no longer be corrected by later stages.

Where costs rise during construction without any change of scope, the fee basis stays unchanged. Where the commissioned scope changes by agreement and chargeable costs change as a result, a written agreement on adjusting the fee calculation is provided for.

The system of chargeable costs is covered in the branch on chargeable costs under the HOAI. That branch also sets out which cost groups enter in full, conditionally or not at all.

At this stage the divergence between the two bodies of rules becomes especially tangible. The standard describes a procedure, while the regulation attaches a legal consequence to it.

Up to the 2008 edition the standard likewise required only the second level. The deepening to the third level came with the reissue, and the ordinance did not follow.

In practice this means: working in conformity with the standard requires the third level and potentially delivers more than the basic service demands. Confining oneself to the basic service means working at the second level and departing from the current edition of the standard.

Both routes are defensible, but the choice belongs in the contract rather than in the final account. The boundary between basic and additional services is covered in the separate article.

The standard names the bases expressly, and they differ markedly from those of the preceding stage. The middle entry is the decisive one.

The second point is the decisive difference from the cost estimate. There, costs are derived from area and volume benchmarks; here, quantities are measured from the design and priced.

How costs are determined in detail is left to the designer. In practice a kind of coarse bill of quantities based on quantities and reference units frequently emerges. The results of the specialist designers for structure and building services are incorporated.

A note on structure: accuracy rises noticeably when calculation proceeds by building elements rather than by lump allowances per cost group. The side effect is transparency, since every item remains traceable to a quantity and a unit rate.

Beyond the depth owed, a cost calculation may be prepared and maintained by individual items, lead items, execution types or single building elements. That deepening is a special service rather than a basic one.

Such a deepened cost calculation is always an additional service to be remunerated separately. It must offer the client a discernible added value over the determination owed, or the separate remuneration is hard to justify.

That added value regularly lies in connectivity: a calculation built by building elements and execution types can be carried directly into the tender documents and eases subsequent cost control across procurement. The effort of the deepening is thus recovered in the following phase.

The lead designer's cost calculation also covers the cost of technical installations and the structure. Those figures are determined by the respective specialist designers, whose services in practice are frequently commissioned only after work phase 4 is complete.

A recurring situation arises: the cost calculation is to be prepared in work phase 3, but the specialist design from which a substantial part of its figures should come has not yet been commissioned at that point. The conflict cannot be resolved, only decided.

Three routes are open, and the choice belongs on the record. It determines how robust the basis for the fee ultimately is.

For deviation between the cost calculation and the later final cost statement several figures circulate side by side, which rules out their use as a calculation quantity. A tolerance that varies by half depending on the source is no yardstick for a breach of duty.

No prescribed tolerance limit exists. Neither the standard nor the ordinance contains one. What does exist is case law on liability for incorrect cost determinations, which provides orientation. The magnitudes cited there run at roughly 15 to 25 per cent, with the individual case always governing.

The distinction matters in practice because it changes the argument. A deviation is not impermissible because it exceeds a percentage; it becomes problematic where it rests on a faulty or incomplete determination. Conversely, even a smaller deviation can be assailable where the determination omitted obvious items.

The most effective safeguard follows: documenting the assumptions. A cost calculation showing, for every material item, the quantity applied, the unit rate and its origin remains explicable even where a deviation later arises. One showing only totals does not.

The standard permits both presentations and requires only that the form chosen be stated. For a client unable to reclaim input tax, such as in private housing, the gross presentation is the one that matters in practice.

For the chargeable costs derived from it, however, the HOAI governs: value added tax is not part of them. Working gross therefore requires converting back for the fee determination. The clean solution is to work net and carry value added tax as a separate line.

• The six stages of cost determination under DIN 276
• Cost framework and cost estimate: the early stages
• Chargeable costs under DIN 276 and the HOAI
• Trade-oriented cost structure by procurement package
• Basic services versus additional services