How is classification stored in an IFC file?

Through a relationship: IfcRelAssociatesClassification links one or more elements to an IfcClassificationReference, which holds the code (for example Ss_25_10_30) and points to an IfcClassification that names the system and edition (for example Uniclass 2015). It is not a property in a property set, although some exporters also write the code as one.

Which classification system should I use, Uniclass or OmniClass?

The one your client and contract require. Uniclass 2015 is standard in the UK and on many ISO 19650 projects; OmniClass is common in North America; CCI is spreading in the Nordics and Central Europe; NL-SfB is used in the Netherlands and Belgium. Mixing systems on one project is what causes trouble.

How can I check that every element has a classification code?

Write an IDS specification with a classification facet in its requirements — for example every IfcWall must have a Uniclass code starting with Ss_25 — and run it against the model. IFC Viewer Online evaluates the classification facet, including hierarchical codes, in the browser.

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Validation · 2026-10-03 · 9 min

IFC Classification Explained: Uniclass, OmniClass and How Codes Live in the Model

A classification code is the bridge between a model and everything that happens after it — cost plans, specifications, asset registers. In IFC it is not a property but a relationship, which is why so many exports lose it. How it works, which system to use, and how to check it.

IFC Classification Explained: Uniclass, OmniClass and How Codes Live in the Model — IFC Viewer Online article cover

Ask a quantity surveyor, a specification writer and a facilities manager what they need from a model, and the first answer from all three is the same: tell me what each thing is, in my terms. That is what classification does. A wall is not just an IfcWall; it is Ss_25_10_30 in Uniclass, a specific system with a cost line, a specification clause and a maintenance regime.

IFC carries that code — but not where most people look for it, which is why so many exports arrive without it.

Diagram of IFC classification: IfcWall linked by IfcRelAssociatesClassification to IfcClassificationReference Ss_25_10_30 and IfcClassification Uniclass 2015, with a table comparing Uniclass, OmniClass, CCI and NL-SfB — A classification code reaches an element through a relationship, not a property. The reference holds the code; the classification names the system.

Key takeaways

In IFC, classification is a relationship to a classification reference, not a property.
A code means nothing without its system: Ss_25_10_30 is Uniclass; 21-02 10 10 is OmniClass.
Check codes with an IDS classification facet — system and code together — on every delivery.

How the code is stored

IfcClassification names the system and edition — Uniclass 2015, OmniClass, CCI.
IfcClassificationReference holds one code (Identification) and its name, and points to the system — directly, or through parent references for hierarchical codes.
IfcRelAssociatesClassification links that reference to one or many elements, or to element types.

Which system?

System	Where it is common	Structure
Uniclass 2015	UK; many ISO 19650 projects	Tables by prefix (Ss systems, Pr products, EF elements…) with hierarchical codes
OmniClass	North America	Numbered tables (21 elements, 23 products…)
CCI	Denmark, Estonia, Czechia and others	Classes by component and construction
NL-SfB	Netherlands and Belgium	Numeric element codes

The right answer is the one in the contract. The wrong answer is two systems on the same project.

Hierarchy: why Ss_25 matches Ss_25_10_30

Codes in Uniclass and similar systems are hierarchical: Ss_25_10_30 sits under Ss_25_10, which sits under Ss_25. A requirement such as "every wall is classified under Ss_25" should accept the deeper code. Good checkers follow the chain of references; naive ones compare strings and fail elements that are perfectly classified.

Checking classification on a delivery

Agree system and depth in the BEP "Uniclass 2015, Systems table, to at least level 3, on all physical elements."
Write it as an IDS specification Applicability: the element classes that must be classified. Requirement: a classification facet with the system and the code pattern.
Run it on every revision Failures come back per element, with the reason — missing code, wrong system, code outside the allowed range.
Watch it across versions A comparison between revisions shows classification changes as their own category — useful when cost or FM teams depend on the codes.

Writing the specification is covered in IDS explained; comparing revisions in how to compare two IFC versions; and why classification matters at handover in COBie from IFC.

IFC Classification Explained: Uniclass, OmniClass and How Codes Live in the Model