From cost estimate to cost calculation: the transition

Attribute	Cost estimate	Cost calculation
Basis	concept design, outline drawings	developed design, worked-up drawings
Depth of treatment	second level	third level
Quantities	areas and volumes under DIN 277	quantities of reference units by cost group
Pricing	benchmarks per area or volume unit	unit rates per reference unit
Work phase	2	3

The change in the third row is the real difference. In the cost estimate, costs are derived from overall quantities; in the cost calculation, quantities are measured by element group and priced.

It follows directly what simply carrying amounts forward misses: adopting the allowances of the cost estimate and merely presenting them in finer detail changes the level but not the method. The cost calculation is meant to rest on measured quantities, not on distributed estimates.

Since the 2009 revision, fees for all basic services are derived exclusively from the results of the cost calculation, exceptionally from those of the cost estimate. A single document therefore carries the remuneration of all nine work phases.

The fee derived from it must carry the practice's work across the whole project duration, that is into work phase 9. An allowance set too low cannot be corrected by any later stage, and cost increases during construction do not raise the fee basis.

In practice: the transition is the last point at which the basis of one's own remuneration can still be influenced, not by higher allowances but by complete capture. The system is covered in the article on the cost calculation.

Three areas are routinely not carried across, although they belong to the fee basis. What they share is that none appears in any drawing as a new quantity, which is how they escape the take-off.

Contributions from specialist design. Structure and building services form a substantial part of building cost. Where they continue to be carried through benchmarks for want of an appointment, cost group 400 in particular comes out too low, which simultaneously shifts the threshold of the special rule for technical installations.

Existing fabric being incorporated. In existing buildings it belongs to the fee basis and must be appropriately taken into account. It appears in no drawing as a new quantity, however, and is therefore overlooked at the transition. The system is covered in the article on existing fabric in chargeable costs.

The conditionally chargeable cost groups. Whether external works and furnishings are designed alongside governs their chargeability. The transition is the point by which that question should be settled.

Cost control requires a comparison between the two stages. Since they are prepared at different levels, comparison is possible only at the coarser level, that is by consolidating the third level onto the second.

Two requirements secure this, and both concern the stability of the breakdown. Without them the comparison measures reallocation rather than change.

The allocation stays stable. An item allocated to one group in the cost estimate stays there in the cost calculation. Moving it creates a deviation in both groups that is not one.

The scope stays the same. Where cost groups are added that were not carried in the cost estimate, this must be shown separately and not presented as a cost increase.

The comparison is simultaneously the first serious test of the cost estimate. Where the cost calculation diverges substantially, the cause must be named: advanced design, changed requirements, market movement, or a gap in the earlier determination. Building a sound comparison is covered in the article on target-actual comparison.

A structural conflict accompanies this transition: the cost calculation is to be prepared in work phase 3, while structural and building services design services are in practice frequently commissioned only after work phase 4 is complete. The specialist design is frequently not yet available at that point.

A substantial part of the figures that should feed the cost calculation therefore comes from design work that does not yet exist at that point. The conflict cannot be resolved, only decided.

Three routes are open, and none is without drawbacks. The choice belongs in the record, because it determines how robust the whole cost calculation is.

Route	Advantage	Drawback
Commission specialists earlier	robust figures, sound basis	more effort before the design decision
Work from benchmarks	quick to implement	plant scope tends to be set too low
Update later	figures become more accurate	the fee basis is fixed late

The second route is the commonest and the riskiest, because the error acts twice over: an understated allowance for cost group 400 reduces both the chargeable costs directly and the threshold up to which those costs are chargeable in full. It acts on the fee basis and on the threshold rule at once.

Anyone taking it should nonetheless state the plant standard assumed and flag the item as one to be updated. The system is covered in the article on KG 400.

The transition is the point at which documentation determines how the rest of the project goes. Four entries belong on the record: Afterwards the position can no longer be reconstructed.

#	Entry	What it consists of
1	Origin of every quantity	which drawing or calculation it comes from
2	Origin of every unit price	own experience, published benchmark or a specialist designer's figure
3	Price date	to which the valuation refers
4	Assumptions made	particularly where the design is still open

These four entries are the difference between a cost calculation that can be explained and one that can be attacked. Since the fee follows from it and it is simultaneously the yardstick for every later comparison, it is the longest-acting document in the whole of cost planning.