BIM-based cost determination by cost group

Model elements are allocated to cost groups, quantities are taken from the model and priced with benchmarks or unit rates. On that basis a cost determination structured by cost group can be generated.

What matters is how the costs reach the model: they are as a rule not held as attributes within the model but assigned to the elements afterwards through a link to external databases. Valuation therefore stays outside the model and can be exchanged at any time.

There is a good reason for this. A price is not a property of an element but a property of element, place, date and market conditions together. A price held in the model would be wrong within months and could not be adjusted for region. Separating geometry from pricing is therefore not a detour but the right architecture.

Open formats are used for exchange, in particular IFC for the model data. Separate bodies of rules and schema definitions exist for connecting model objects to commercial data.

A dedicated set of guidelines exists for model-based quantity take-off, defining five completion levels for cost determination, from 100 to 500. They describe which information state of the model elements supports which kind of cost determination.

Completion level	Supports	Information state at least
100	estimating total cost and building cost	briefing
200	quantities of a building volume, structured at an upper level	concept design
300	quantities of individual element types, finer structure	developed design
400 and 500	quantities at item and execution level	detailed design and procurement

This grading is useful because it answers a question constantly asked in practice: from which model state is which cost statement possible. The answer is that the information state of the model and the stage of cost determination must match.

A note on application: the mapping of completion levels onto the levels of the cost structure should be reconciled with the current edition of DIN 276 before use. The relevant guideline and the reissue of the standard date from the same year, and the standard deepened the required treatment for the cost estimate and cost calculation each by one level. Anyone using both bodies of rules together should check the current editions and fix the governing requirement within the project.

Three limits act together, and the first hits precisely the fee-relevant cost group. None of them disappears with better software.

Limit	What it consists of
The level of detail varies between disciplines	in building services the item level, meaning ducts, fittings, components and insulation, is reliably modelled only from detailed design onwards
Not everything in a building is a model element	site set-up, shoring, protection measures, demolition in existing fabric and a substantial part of KG 200 and 700 have no geometry
Model quality determines result quality	walls modelled storey by storey or continuously, openings deducted or not, the handling of layered build-ups: all of it decides the quantity

An asymmetric situation follows, matching one already familiar: KG 300 can be determined from a model earlier than KG 400. Since it is KG 400 that triggers the fee-relevant threshold rule, the critical part of the determination stays benchmark-based the longest.

The benefit lies less in speed than in three other things. The third is economically the most significant, because it acts in the early phases where the room for management is greatest.

Gain	What it means
Traceability	every quantity remains attributable to a model element; on a variance it is visible whether quantity or price was the cause
Updatability	a design change acts directly on the quantities, so the principle that costs mirror the design is technically embodied rather than asserted
Variant comparison	two design options can be set against each other on the same valuation basis, because valuation is separated from geometry

The system of interventions is covered in the article on cost management. That article also explains why the room maneuver shrinks with every step of design progress.

On projects in existing buildings, model-based cost determination meets an additional difficulty: there is no model, only a building. Here the model is not the starting point but the first deliverable.

An as-built model must first be created, from measured survey, laser scan or existing documents. That effort falls before the cost determination, is a service to be separately agreed, and is routinely underestimated in project planning.

A fundamental limit is added: a model can only represent what is known. Concealed elements, service routes and the actual condition of the fabric cannot be modelled before opening up. An as-built model therefore suggests a completeness it does not have, and in existing buildings that suggestion is more dangerous than openly stated uncertainty.

In practice it follows that an as-built model should carry its state of knowledge, that is which elements derive from survey, which from documents and which from assumption. That distinction is precisely the basis on which contingencies are justified, as described in the article on cost risks and contingencies.

Model-based cost determination presupposes agreements going beyond a conventional design contract. All four are settled before the project starts, or not at all.

#	To be settled	Why
1	The modelling conventions	because they determine the quantities
2	The information level required per design phase	matched to the stages of cost determination
3	The mapping of model elements to cost groups	including the question of who maintains it
4	The handling of cost shares not modelled	particularly KG 200 and 700

The fourth is most frequently overlooked and creates the largest gap, because the non-modelled components are precisely those also missing from conventional benchmarks. It therefore decides the completeness of the entire determination.