BIM concepts, dimensions and levels of development

Building information modelling is often described as 3D design, but the geometry is only part of it. A BIM model is a database of objects: walls, doors, beams, ducts, pumps, rooms and zones, each with a type, properties and relationships to other objects. Drawings, schedules and quantities are views of that database. If a door type changes in the model, every plan, elevation and door schedule that shows it changes too. CON-120 introduced this idea and the basic file formats. This lesson builds the vocabulary a construction manager needs to specify, judge and use models: objects and properties, the so-called dimensions of BIM, levels of development, and level of information need.

Objects, types and properties

In a BIM authoring tool, every element belongs to a category (such as wall, door or duct), a family or type (for example, a 200 mm concrete block wall, or a single-flush steel door 915 × 2134 mm) and an instance (one particular wall or door at one location). Properties can belong to the type, so they are shared by every instance (fire rating, manufacturer, assembly code), or to the instance (room number, mark, phase). Good modelling practice puts each property in the right place so that changing a type changes every instance consistently.

Objects also carry relationships. A door is hosted by a wall, a wall is bounded by a level, a room is defined by its bounding walls, and a pump is connected to a pipe system. These relationships are what let the software update dependent elements automatically and calculate quantities, areas and system flows. They are also why a model made of dumb 3D shapes, such as a mesh exported from a visualization tool, is not BIM even if it looks identical.

Classification systems give objects consistent codes so data can be sorted, costed and compared across projects. In North America the most common are MasterFormat (work results, used for specifications and many cost reports), UniFormat (building elements, useful for early estimates) and OmniClass (a broader set of tables that includes products, spaces and phases). A model in which every element carries a UniFormat or MasterFormat code can produce an elemental estimate or a trade-by-trade quantity report with little manual sorting.

The dimensions of BIM

Industry marketing often describes BIM in dimensions. The first three are geometry: x, y and z. Beyond that, the numbers are labels for uses, not mathematical dimensions, and different sources number them differently. The most widely used meanings are shown below. Treat them as shorthand, and always define what you mean in a contract.

A more precise approach, used by the National BIM Standard–United States and many owners, is to describe BIM uses: specific purposes such as design authoring, design review, 3D coordination, cost estimation, phase planning, site utilization planning, digital fabrication, field layout, record modelling and asset management. Each use implies certain model content, people and software. Lesson 4 shows how a BIM execution plan selects uses and assigns responsibility for each.

Level of development (LOD)

A model element can be anything from a placeholder box to a fabrication-ready assembly. Without a shared way of describing how far an element has been developed, a contractor cannot tell whether a duct in the design model can be used for coordination or is merely an indication of intent. Level of development (LOD) answers that question. The North American reference is the LOD Specification published by BIMForum; its 2025 edition was released at the end of 2025.

LOD describes how much the geometry and information of an element can be relied on, not how detailed the graphics look. A beautifully rendered chair can be LOD 100 if its size and position are only indicative, and a plain grey box can be LOD 300 if it accurately represents the space reserved for a piece of equipment.

Several points often cause confusion. LOD is assigned to elements, not to whole models: a model at the end of design development might have walls at LOD 300, ducts at LOD 200 and furniture at LOD 100. LOD 350 was added because trade coordination needs supports, hangers, clearances and connections that LOD 300 does not require. And LOD 500 is about verification against the built work, not a further step in detail; BIMForum does not define it as a progression level.

Some practitioners distinguish level of detail (the amount of graphic detail) from level of development (how much can be relied on), and level of accuracy for survey and scan data, specified separately by the U.S. Institute of Building Documentation. In contracts, always say which you mean and refer to a named specification and edition.

Model element tables

The practical tool for LOD is a model element table (sometimes called a model progression specification or LOD matrix). Rows list element types, often organized by UniFormat; columns list project milestones; each cell gives the LOD required and the party responsible for modelling the element (the model element author). The table becomes part of the BIM execution plan and, on many projects, part of the contract.

Level of information need

LOD has a weakness: it bundles geometry and information into a single number, which tempts people to over-specify. A client who asks for everything at LOD 400 may pay for detail it never uses. The ISO approach, set out in ISO 7817-1:2024 (which builds on the earlier European standard EN 17412-1), is level of information need. Instead of a single number, each information requirement is defined by purpose and by three components: geometrical information (detail, dimensionality, location, appearance and parametric behaviour), alphanumerical information (identification and the properties needed) and documentation (drawings, certificates, manuals and other documents).

Level of information need starts from the question "What decision or task will this information support?" and specifies only what is needed for that purpose at that milestone. For example, to support a fire strategy review at design stage, a fire door needs its location, rating and direction of swing but not its hardware supplier; for handover, it needs its rating, manufacturer, model, installation date, inspection certificate and maintenance requirements, but the geometry can stay simple.

The two approaches are not exclusive. Many Canadian and American projects use BIMForum LOD for geometry and a separate property list (for example, COBie fields or the owner's asset attributes) for information, which moves toward the ISO idea in practice.

Model quality: what makes a model trustworthy

A model is only useful if it is reliable. Model quality checks fall into three groups. Visual checks look for obvious errors such as missing elements or elements on the wrong level. Rule-based checks use software to test properties and geometry against rules: every door has a fire rating, no wall is shorter than 2.1 m, every room has a number, every element has a classification code. Coordination checks find clashes between disciplines (Lesson 5).

A model also needs a defined coordinate system, consistent levels and grid, agreed units and a shared origin. Mismatched origins are a classic problem: the architectural model sits at one point and the structural model 300 m away, or rotated to true north while the other uses project north. The BIM execution plan should fix the project base point and survey point, tied to the site control described in CON-250.

Who owns and authors each element

Every element has an author responsible for its geometry and data at each stage. When responsibility moves (for example, from the design engineer to the trade detailer at LOD 350), the handover must be explicit so that the design model and the trade model do not both claim to be authoritative. Many disputes arise from ambiguity here: a design engineer assumes the contractor will finalize sizes, the contractor assumes the design model is correct, and nobody checks the gap.

Authoring responsibility is distinct from intellectual property and from contractual reliance, which Lesson 11 covers. The construction manager's practical job is to make sure the model element table names an author for every element at every milestone, and that the schedule allows time for each transfer.