# Ifc Errors Property Set Mistakes

> Use when an IFC file fails validation on a property set, quantity set or material association, or when a custom property set is rejected, ignored, or collides with the standard library, or a layered-material assignment breaks a WHERE rule. Prevents naming a custom property set with the reserved Pset_ prefix, pointing IfcRelDefinesByProperties at a type object, attaching IfcMaterialLayerSetUsage to a type instead of an occurrence, hand-writing the derived TotalThickness, putting a length unit on an area quantity, and leaving IfcMaterialList in a new IFC4 file. Covers the Pset_ and Qto_ naming conventions, the NoRelatedTypeObject WHERE rule, the occurrence-only rule for material usage entities, derived attributes, quantity unit-type WHERE rules, and the IfcMaterialList legacy smell, across IFC2x3, IFC4 and IFC4.3. Keywords: property set error, Pset_ prefix, custom property set, IfcPropertySet, IfcRelDefinesByProperties, NoRelatedTypeObject, attach Pset to type, quantity set, IfcElementQuantity, Qto_ prefix, Ifc

- Skill: `impertio-studio/ifc-errors-property-set-mistakes` (Agent Skill, multi-file: 4 files)
- Install (CLI): `npx skillmds@latest add impertio-studio/ifc-errors-property-set-mistakes`
- Raw SKILL.md: https://api.skillmd.com/api/skills/impertio-studio/ifc-errors-property-set-mistakes/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- License: MIT
- Author: Impertio-Studio (https://skillmd.com/u/impertio-studio)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/impertio-studio/ifc-errors-property-set-mistakes

---


# IFC Property Set Mistakes

Diagnose and fix the six recurring mistakes in IFC property sets, quantity sets
and material associations : a reserved-prefix name collision, a property set
attached to a type through the wrong relationship, a material-usage entity on a
type, a hand-written derived attribute, a quantity carrying the wrong unit type,
and the legacy `IfcMaterialList` in a modern file.

Two of these are schema-valid but wrong (a "smell") ; three break a `WHERE`
rule (a hard error) ; one is a derived-attribute violation. ALWAYS classify the
mistake before fixing it.

For the property and quantity entity model see `ifc-syntax-property-sets`. For
the material entity model see `ifc-syntax-materials`. For units see
`ifc-syntax-units`. For schema-validation mechanics see
`ifc-errors-schema-validation`.

## Quick Reference

### The six mistakes

| # | Mistake | Severity | Rule or mechanism | Fix |
|---|---------|----------|-------------------|-----|
| 1 | Custom property set named `Pset_...` | Smell : schema-valid, collides with the standard library | `Pset_` is reserved for published sets | Rename without the `Pset_` prefix |
| 2 | `IfcRelDefinesByProperties` `RelatedObjects` holds an `IfcTypeObject` | Hard error (IFC4+) | `WHERE` rule `NoRelatedTypeObject` | Use `IfcTypeObject.HasPropertySets` for types |
| 3 | `IfcMaterialLayerSetUsage` / `IfcMaterialProfileSetUsage` assigned to an element type | Hard error | Usage entities are occurrence-only | Put the bare set on the type, the usage on the occurrence |
| 4 | A literal value written into the derived `IfcMaterialLayerSet.TotalThickness` | Hard error : derived slot | The value is computed by `IfcMlsTotalThickness` | Write `*` in the STEP slot ; never a number |
| 5 | A length unit on `IfcQuantityArea` (or any quantity unit mismatch) | Hard error | `WHERE` rule `WR21` : the unit must match the quantity type | Use an `AREAUNIT`, or omit `Unit` to inherit the project unit |
| 6 | `IfcMaterialList` in a new IFC4 / IFC4.3 file | Smell : schema-valid, legacy mechanism | Superseded by `IfcMaterialConstituentSet` | Replace with `IfcMaterialConstituentSet` |

### Severity, restated

- **Hard error** : a schema validator rejects the file (mistakes 2, 3, 5) or the
  STEP line is malformed (mistake 4). The file does not pass validation.
- **Smell** : the file passes validation, but the model is wrong by convention
  and breaks downstream tools or data exchange (mistakes 1, 6). NEVER read
  "validation passed" as "the property data is correct."

### Property set versus quantity set : same attachment, different content

`IfcPropertySet` (named properties) and `IfcElementQuantity` (measured
quantities) are both subtypes of `IfcPropertySetDefinition`. Both attach to an
**occurrence** through `IfcRelDefinesByProperties`, and to a **type** through
`IfcTypeObject.HasPropertySets`. The naming conventions are parallel : `Pset_`
for property sets, `Qto_` for quantity sets. Every rule in this skill that names
`Pset_` applies equally to `Qto_`.

### The reserved prefixes

| Prefix | Reserved for | A custom name MUST |
|--------|--------------|--------------------|
| `Pset_` | buildingSMART published property sets | NEVER start with `Pset_` |
| `Qto_` | buildingSMART published quantity sets | NEVER start with `Qto_` |

A published set name is an exact identifier, for example `Pset_WallCommon`
("properties common to all occurrences of `IfcWall`") or `Qto_WallBaseQuantities`.
A custom set uses a project or vendor prefix instead, for example
`ACME_WallThermal`.

## Decision Trees

### Naming a property set or quantity set

```
You are naming an IfcPropertySet or IfcElementQuantity.
|
+-- Is the name an EXACT published Pset_ / Qto_ identifier for this element,
|   verified against the buildingSMART property library?
|     YES -> use it as-is. The content must match the published template.
|     NO  -> continue.
|
+-- Does the name start with Pset_ or Qto_ ?
      YES -> COLLISION (mistake 1). Rename with a project / vendor prefix.
      NO  -> custom set, correctly named. Proceed.
```

### Attaching a property set to the right host

```
You are attaching an IfcPropertySet (or IfcElementQuantity).
|
+-- Is the host an occurrence (IfcWall, IfcSlab, ... an IfcObject) ?
|     -> create IfcRelDefinesByProperties
|        RelatedObjects = the occurrence(s),
|        RelatingPropertyDefinition = the set.
|
+-- Is the host a type object (IfcWallType, ... an IfcTypeObject) ?
      -> add the set to IfcTypeObject.HasPropertySets directly.
      -> NEVER put the type into IfcRelDefinesByProperties.RelatedObjects
         (mistake 2 : WHERE rule NoRelatedTypeObject, IFC4+).
```

### Associating a structured material

```
You are associating a material that has internal structure.
|
+-- Layered element (wall, slab, roof, plate) ?
|     type      -> IfcRelAssociatesMaterial.RelatingMaterial = IfcMaterialLayerSet
|     occurrence-> IfcRelAssociatesMaterial.RelatingMaterial = IfcMaterialLayerSetUsage
|                  (the usage ForLayerSet points back at the same set)
|
+-- Profiled element (beam, column, member, pile) ? [IFC4+ only]
|     type      -> RelatingMaterial = IfcMaterialProfileSet
|     occurrence-> RelatingMaterial = IfcMaterialProfileSetUsage
|
+-- Non-layered, non-profiled composite (window, door) ?
      IFC4+  -> IfcMaterialConstituentSet
      IFC2x3 -> IfcMaterialList (the only option in 2x3)

NEVER put a *Usage entity on a type : it is occurrence-only (mistake 3).
```

## Patterns

### Pattern 1 : Name a custom property set

ALWAYS reserve `Pset_` and `Qto_` for buildingSMART published sets. A custom
set ALWAYS uses a distinguishing project or vendor prefix.

- Correct published use : `Pset_WallCommon`, content matching the published
  template (each property name, type and value type as specified).
- Correct custom use : `ACME_WallAcoustics`, `ProjectX_CostData`.
- NEVER : `Pset_WallAcoustics` for a custom set. It is schema-valid, so no
  validator stops it, but it shadows or collides with the standard library and
  receiving software cannot tell custom data from standard data.

A custom `Pset_` name is mistake 1. The fix is a rename ; see
`references/examples.md` Example 1.

### Pattern 2 : Attach a property set to a type, not through the wrong relationship

`IfcRelDefinesByProperties` attaches a set to **occurrences**. In IFC4 and
IFC4.3 its `RelatedObjects` is `SET [1:?] OF IfcObjectDefinition`, which
syntactically admits an `IfcTypeObject` ; the `WHERE` rule `NoRelatedTypeObject`
("There shall be no related object being of type `IfcTypeObject`") forbids it.

ALWAYS attach a type's property sets through the `HasPropertySets` aggregation
on `IfcTypeObject` itself. NEVER list a type in
`IfcRelDefinesByProperties.RelatedObjects`. See `references/examples.md`
Example 2.

Version note : in IFC2x3 `RelatedObjects` is `SET [1:?] OF IfcObject`, and
`IfcTypeObject` is not an `IfcObject`, so the schema type alone blocks the
mistake and no `WHERE` rule is needed. The widening to `IfcObjectDefinition` in
IFC4 is exactly why `NoRelatedTypeObject` was added.

### Pattern 3 : Bare set on the type, usage on the occurrence

`IfcMaterialLayerSet` and `IfcMaterialProfileSet` are the bare structured sets.
`IfcMaterialLayerSetUsage` and `IfcMaterialProfileSetUsage` add placement and
are **occurrence-only** : the IFC4.3 specification states the usage "is always
assigned to an individual occurrence object (and only to relevant subtypes of
`IfcElement`)."

ALWAYS associate the element TYPE (`IfcWallType`, `IfcBeamType`) with the bare
`IfcMaterialLayerSet` / `IfcMaterialProfileSet`. ALWAYS associate each
OCCURRENCE with a `...SetUsage` whose `ForLayerSet` / `ForProfileSet` points
back at that same bare set. NEVER associate a `...SetUsage` with a type. See
`references/examples.md` Example 3.

### Pattern 4 : Leave TotalThickness derived

`IfcMaterialLayerSet.TotalThickness` is a **derived attribute** : its value is
computed from the layer thicknesses by the schema function `IfcMlsTotalThickness`.
A derived attribute is never stored. In a STEP file its positional slot ALWAYS
holds the token `*`. Writing a literal number there is mistake 4 : the value is
either rejected as malformed or silently recomputed and discarded.

ALWAYS write `*` in the `TotalThickness` slot. To change the total thickness,
change the `LayerThickness` of the layers. See `references/examples.md`
Example 4.

### Pattern 5 : Match the quantity unit to the quantity type

`IfcQuantityArea` carries `WHERE` rule `WR21` : "If a unit is given, the unit
type shall be area unit" (`Unit.UnitType = IfcUnitEnum.AREAUNIT`). It also
carries `WR22` : `AreaValue >= 0`. The sibling quantities behave the same way :
`IfcQuantityLength` needs a `LENGTHUNIT`, `IfcQuantityVolume` a `VOLUMEUNIT`,
and so on.

The `Unit` attribute of every `IfcPhysicalSimpleQuantity` is OPTIONAL. ALWAYS
EITHER omit `Unit`, so the quantity inherits the matching unit from the project
`IfcUnitAssignment`, OR supply a unit whose `UnitType` matches the quantity
entity. NEVER put a `LENGTHUNIT` on `IfcQuantityArea`. See
`references/examples.md` Example 5.

### Pattern 6 : Replace IfcMaterialList in IFC4 and IFC4.3

`IfcMaterialList` (single attribute `Materials : LIST [1:?] OF IfcMaterial`) is
a bare ordered list with no per-part semantics, fractions or geometry. It is the
IFC2x3 mechanism for a non-layered composite. It remains in IFC4 and IFC4.3
ONLY for round-tripping IFC2x3 data.

ALWAYS use `IfcMaterialConstituentSet` for a non-layered, non-profiled composite
in a new IFC4 or IFC4.3 file : each `IfcMaterialConstituent` names a part, its
`Material`, an optional `Fraction` and `Category`. An `IfcMaterialList` in a new
IFC4+ file is mistake 6 : schema-valid, but a smell that loses constituent
semantics. See `references/examples.md` Example 6.

## Version Notes

| Entity / rule | IFC2x3 | IFC4 / IFC4.3 |
|---------------|--------|---------------|
| `IfcRelDefinesByProperties.RelatedObjects` | `SET [1:?] OF IfcObject` ; no `WHERE` rule needed | `SET [1:?] OF IfcObjectDefinition` ; `WHERE` rule `NoRelatedTypeObject` |
| `IfcMaterialProfileSet` family | does not exist | introduced in IFC4 |
| `IfcMaterialConstituentSet` family | does not exist | introduced in IFC4 |
| `IfcMaterialList` | the standard non-layered mechanism | legacy ; superseded by `IfcMaterialConstituentSet` |
| Quantity-set prefix | `BaseQuantities` convention | `Qto_` prefix (IFC4+) |
| `IfcQuantityNumber` | does not exist | introduced in IFC4 |
| `IfcMaterialProperties` | split into typed subtypes (`IfcMechanicalMaterialProperties`, ...) | one consolidated non-abstract entity |

Material usage entities (`IfcMaterialLayerSetUsage`, `IfcMaterialProfileSetUsage`)
are occurrence-only in every version that defines them. The element-supertype
rename `IfcBuildingElement` to `IfcBuiltElement` (IFC4.3) does not affect any
entity in this skill : property sets, quantities and materials attach through
`IfcObject` / `IfcObjectDefinition` and `IfcTypeObject`, whose names are stable.

## Reference Links

- `references/methods.md` : entity and attribute signatures for `IfcPropertySet`,
  `IfcRelDefinesByProperties`, `IfcElementQuantity`, the quantity subtypes, the
  material set and usage entities, with cardinalities and `WHERE` rules.
- `references/examples.md` : the wrong STEP and the corrected STEP for each of
  the six mistakes, plus ifcopenshell detection snippets.
- `references/anti-patterns.md` : the mistakes in detail, why each fails, and
  the deterministic rule.

### Official sources (verified 2026-05-20)

- IfcPropertySet : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcPropertySet.htm
- IfcRelDefinesByProperties (NoRelatedTypeObject) : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcRelDefinesByProperties.htm
- IfcRelDefinesByProperties (IFC2x3) : https://standards.buildingsmart.org/IFC/RELEASE/IFC2x3/TC1/HTML/ifckernel/lexical/ifcreldefinesbyproperties.htm
- IfcElementQuantity : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcElementQuantity.htm
- IfcQuantityArea (WR21, WR22) : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcQuantityArea.htm
- IfcMaterialLayerSet (derived TotalThickness) : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMaterialLayerSet.htm
- IfcMaterialLayerSetUsage : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMaterialLayerSetUsage.htm
- IfcMaterialConstituentSet : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcMaterialConstituentSet.htm
- IfcMaterialList (IFC2x3) : https://standards.buildingsmart.org/IFC/RELEASE/IFC2x3/TC1/HTML/ifcmaterialresource/lexical/ifcmateriallist.htm
- Pset_WallCommon (Pset_ convention) : https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/property/Pset_WallCommon.htm