09 December 2021

What is BIM?

BIM is a process for creating and managing information on a construction project, throughout the project’s life cycle. As part of this process, a coordinated digital description of every aspect of the built asset is developed, using a set of appropriate technology. It is likely that this digital description includes a combination of information-rich 3D models and associated structured data such as product, execution and handover information.

Internationally, the BIM process and associated data structures are best defined in the ISO 19650 and 12006 series of standards.


What are BIM dimensions?

BIM dimensions have evolved from a need to differentiate between modelling geometry in two or three dimensions. This has been part of the modelling evolution, moving from drawing boards to the first 2D CAD systems, to 3D modelling packages.

Adding further aspects to this modelling can help project teams understand what information they are setting out to model. 4D is commonly known as ‘modelling scheduling information to model construction sequences’. 5D is known as ‘adding financial cost’. There is little international consensus beyond this, and arguably cost isn’t a ‘dimension’ at all – it is just a further information field.

At NBS, and in the international standards, these dimensions are not typically referred to. If specific information is required to be modelled, it is far better to be clear on precisely what this information is than to use terminology such as 5D, 6D or 7D, etc.

For example, if an appointing party wants health and safety, fire safety, inclusive design and sustainability information modelled then this information need should be identified clearly in the information requirements that are issued to the project team. Furthermore, they should state precisely which information fields are required, and in which format. For example, for sustainability, is it the embodied carbon in CO2e/kg measured to standard X for all structural materials? And for cost, is it the replacement cost and expected life measured in pounds sterling and months?

That said, the more commonly used ‘dimensions of BIM’ are looked at in this article. They may be useful phrases to help begin conversations with clients, to demonstrate the various BIM use cases and to help develop more precise requirements.



2D BIM is a digital geometric model that constitutes an X and a Y axis associated with further information. Early CAD systems were 2D models, where plans and sections could be developed on computers more quickly and more accurately than manually, on a drawing board.

More advanced modelling tools now allow parameters, constraints and concepts to be attached to the 2D model. However, most in the industry would not consider 2D geometry models as BIM.



3D BIM is a digital geometric model that constitutes an X, Y and Z axis associated with further information. 3D modelling tools have been a huge success because:

  • 2D views of geometric information can be generated from the 3D model at different levels of detail.
  • Schedules can be generated, reporting on objects of different types within the 3D model. 
  • Multiple 3D models can be combined to report on any geometric clashes.

All of these features greatly improve accuracy and efficiency, and reduce the risk of errors occurring on projects.

Furthermore, where specific information is added or linked to these models then further benefits can be seen.

At NBS, we encourage all objects in a 3D model to be classified consistently using the Uniclass 2015 classification scheme (the UK implementation of ISO 12006-2). We also encourage specification information to be linked to the objects to provide a richer set of coordinated information. If specification information is stored in a cloud tool such as NBS Chorus then detailed decisions can be made and stored in a database that is separate from the 3D model. This has the advantage of keeping the 3D model more simple, while still coordinating and reporting on a complex information set.



4D BIM is adding scheduling information to model construction sequences. Adding a dimension of time allows the project team to better visualize how the construction will be sequenced. From a contractor point of view, this is vital. 4D BIM was a huge step forward for the industry when first made possible through the use of new modelling tools – it demonstrated collaboration between the design and construction team through coordination and sharing of 3D models.



5D BIM is generally considered to be adding cost information to a model. If discussing 5D BIM, it is advised to clearly set out these specific requirements. For example, is the team expected to be providing capital or operational costs? Are these costs expected to be pre-tender estimates or a record of as-built costs? Who is responsible for adding this information? What method of measurement is to be used?



6D BIM is considered by some to be adding facility management to the information set. However, there is little industry consensus on this, and arguably this isn’t a ‘dimension’ at all. If discussing 6D BIM, it is strongly advised to set out precisely what is required so that all parties have a clear understanding.



7D BIM is considered by some to be adding sustainability information to the information set. As with 6D BIM, be sure to carefully define the specific information required in terms of data types, scope, units, rules of measure, etc.



8D BIM is considered by some to be adding health and safety information to the information set. As with 6D BIM, be sure to carefully define the specific information required in terms of data types, scope, units, rules of measure, etc.


Final thoughts

Using dimensions to define information requirements can be helpful to start conversations in terms of extracting client requirements or understanding potential deliverables. However, what may be more helpful is using a guide such as the RIBA Plan of Work, in combination with the ISO 19650 and ISO 12006 series of standards, to understand what types of information are required throughout a project. The RIBA Plan of Work lists the key project strategies such as fire safety, sustainability, inclusive design, planning for use, etc. These are then broken down; for example, sustainability is explored in terms of operational carbon or connectivity and transport. It is advised to examine the needs of a project, then to understand the information that is required, when it is required and who is responsible for developing it.