How does a BIM model become a reliable basis for quantities, costs and bills of quantities? In this Ask the Expert interview, Felix Grau of NOVA BUILDING IT explains why IFC exchange and automated quantity take-off can already work well today – and why success in practice depends above all on clear modelling guidelines, pragmatic processes and a closer connection between design, cost estimation and tendering.
NOVA BUILDING IT provides digital platforms for construction cost management with BIM. Since 2014, the company has operated a native, GDPR-compliant cloud service in Germany. By integrating the IFC standard into its cloud modules, NOVA BUILDING IT supports BIM-based workflows from initial budgeting through to final account. Its focus is on connecting BIM models with quantities, costs, tendering, procurement and billing.
Felix Grau is the founder and shareholder of NOVA Building IT GmbH and has been working on IT solutions for tendering, procurement and construction cost management for more than 20 years. As the construction industry continues to digitalize, he actively contributes to key expert bodies such as GAEB for AVA and buildingSMART for BIM, where he supports the development of data models and the standardization of digital processes.
BIM World: A BIM model is more than a digital 3D drawing. It can also contain information about building components. Is that already a reliable basis for quantities and costs in the context of tendering?
Felix Grau: Absolutely. In the BIM-to-AVA process, we use geometric and technical information to visualize and process the project accordingly in the 3D viewer. The technical information then forms the basis for sound construction cost management.
The better the building components are described, the easier it becomes to assign quantities and costs automatically. For a wall, for example, it may be relevant whether it is load-bearing, what material or material layers it consists of, and whether it is an internal or external wall. Parameters like these can be evaluated using intelligent filters.
In most cases, this does not require a huge amount of information. Often, a few details are enough – as long as they are the right ones. At the same time, the better and more consistently the information is maintained, the more can be derived from it.
BIM World: To make this information usable in other programs as well, BIM models are often exchanged via IFC. How reliably does this exchange work in practice today – and where do problems still arise due to incomplete or differently structured model data?
Felix Grau: From our perspective, IFC exchange works very reliably in many cases. The quantities used for cost planning, cost calculation, tendering and billing of construction services are derived from the dimensions of the building components. CAD systems already write these dimensions reliably into the IFC file today. This makes it possible, for example, to calculate the total area of all internal walls.
In systems such as NOVA AVA, this data is then evaluated and assigned to the relevant specifications. In practice, it is usually enough to use the right settings when modelling in CAD. Together with our CAD partners, we provide short and easy-to-understand guidelines for this.
BIM World: That sounds as if the technical prerequisites have largely been solved. Where do the most common errors still occur in practice?
Felix Grau: The information content of IFC files is indeed sufficient. Different BIM authoring tools may write slightly different IFC “dialects”, but in practice these can be handled very well. We also see that the quality of BIM and IFC models is improving significantly in more and more architecture and planning offices.
The crux usually lies in the modelling. Users often do not work consistently, meaning that identical components within a project are not modelled in the same way and are not attributed in the same way. In such cases, there is often a lack of healthy pragmatism.
The guiding question should be: “Which few important modelling rules and attributes do we really need in order to obtain good model data for bills of quantities and construction data?”
BIM World: Construction projects are constantly changing: discipline-specific models are adjusted, design stages are revised, and quantities change. How can we prevent cost planning and bills of quantities from becoming outdated after only a short time?
Felix Grau: The connection between the models and the derived quantity and cost calculations is ensured through unique keys. When a new model version is available, the costs are updated directly on the basis of the changes.
So-called cost elements play an important role here. These are “packages” that contain all the services required to specify building components. If a component changes, this change is recognized and the costs can be updated accordingly.
BIM World: Technically, then, a great deal is already possible. Where is the biggest gap today between the technical BIM model and the commercial reality of a construction project?
Felix Grau: The biggest gap arises where the two sides – the BIM model and its commercial use – are not considered holistically. The basic knowledge of how to connect information from the BIM model with costs is technically available today.
And not everything has to be modelled down to the smallest detail. A simple example: the cost of a skirting board can be calculated automatically based on the dimensions of a room and its door openings. Many people still think BIM is complicated. But it does not have to be.
From my point of view, this is therefore primarily an organizational question. Many people apparently do not yet think of “drawing”, “cost estimation” and “tendering” as sufficiently interconnected. But that is exactly what can be improved.
BIM World: Do you have a concrete recommendation here?
Felix Grau: Some of our customers have had very good experiences when developing their workflows by not starting with the modelling itself, but with their standard bills of quantities. They look at their work items and service descriptions and then work backwards towards BIM modelling: What do I need to model and attribute as simply as possible in order to get from the BIM model to these items and descriptions?
In other words, the process starts with the desired result: the bill of quantities. The point is not to overload a model with as much information as possible, but to define exactly the information that is truly needed for the specific AVA process.
BIM World: How far can this process be automated? Can bills of quantities already be generated directly from the BIM model today?
Felix Grau: Yes, automated bill-of-quantities generation from the 3D model is already possible. The professional human input currently lies in assessing which services need to be carried out in order to realize the project.
Most of our customers have solid internal price databases or use tools such as STLB-Bau and DBD-BIM. This means the bridge between model, service description and costs has already been built. Of course, there will be further developments here in the future. We are currently working on implementing AI-supported features for BIM-based quantity and cost calculation. In the end, however, the results still need to be validated by the designer.
BIM World: If the process is already that advanced, can a BIM model already become the central source for the entire commercial management of a construction project – from the initial cost estimate through to the final account?
Felix Grau: Absolutely. And not only in the future. Today.
BIM World: At the same time, BIM-to-AVA is not yet standard practice everywhere. Why is that?
Felix Grau: We do not presume to have a complete overview of all the many facets of BIM application, let alone to judge them. But when it comes to BIM-to-AVA, we often see that BIM is approached in an overly complicated way. Too much is often modelled, and the model is overloaded with too much information. Keeping all of that consistent is difficult.
But it does not have to be that way. Many work steps can be carried out more easily and more quickly by cost and tendering specialists in the AVA software – within a BIM-to-AVA process in which they always keep the visualized model in view.