IfcDiff is both a CLI utility and library that lets you compare the changes between two IFC models. Changes are made on the assumption that the GlobalId of an element in one model is consistent with the same element in another model. You may compare geometric changes, and changes in various IFC relationships and properties. IfcDiff supports comparing across different IFC schema versions.

Changes will be sorted into three lists:

  • Added: a list of GlobalIds of elements present in the new file but not present in the old file.

  • Deleted: a list of GlobalIds of elements present in the old file but not present in the new file.

  • Changed: A list of GlobalIds of elements present in both the old and new file, but changes were detected. A list of changes are provided.

There are different methods of installation, depending on your situation.

  1. Source installation is recommended for users wanting to use the latest code as a library or a CLI utility.

  2. Using the BlenderBIM Add-on is recommended for non-developers wanting a graphical interface.

Source installation#

  1. Install IfcOpenShell

  2. Clone the source code.

  3. cd /path/to/IfcOpenShell/src/ifcdiff

  4. pip install -r requirements.txt

Here is a minimal example of how to use IfcDiff as a Python module or CLI utility:

$ python -m ifcdiff -h
usage: ifcdiff.py [-h] [-o OUTPUT] [-r RELATIONSHIPS] old new

Show the difference between two IFC files

positional arguments:
  old                   The old IFC file
  new                   The new IFC file

  -h, --help            show this help message and exit
  -o OUTPUT, --output OUTPUT
                        The JSON diff file to output. Defaults to diff.json
                        A list of space-separated relationships, chosen from "type", "property", "container", "aggregate", "classification"
$ python -m ifcdiff old.ifc new.ifc
$ cat diff.json

Here is a minimal example of how to use IfcDiff as a library:

from ifcdiff import IfcDiff

ifc_diff = IfcDiff("/path/to/old.ifc", "/path/to/new.ifc", "/path/to/diff.json")

See also

For more information on how to use IfcDiff as a library, check out the API reference.

You can also alias it to a command:

$ alias ifcdiff='python -m ifcdiff'

Using the BlenderBIM Add-on#

The BlenderBIM Add-on is a Blender based graphical interface to IfcOpenShell. Other than providing a graphical IFC authoring platform, it also comes with IfcOpenShell, its utilities, and a Python shell built-in. This means you don’t need to install Python first, and you also can compare your IfcOpenShell scripting to what you see with a visual model viewer, or use a graphical interface to access the IfcOpenShell utilities.

  1. Install the BlenderBIM Add-on by following the BlenderBIM Add-on installation documentation.

  2. Launch Blender. Change to the Scene Properties tab in the Properties Panel. Scroll down to the IFC Quality Control > IFC Diff panel.

  3. Browse to your old IFC file, new IFC file.

  4. Optionally add any relationships you want to check.

  5. Optionally type in a filter query.

  6. Press Execute IFC Diff

TODO: add pictures and make this clearer for non-developers.

Geometry changes#

IfcDiff compares geometry changes using the underlying IFC geometric definition. This means that if a shape is described in one file as an extrusion, and as a mesh in another file, it is considered to be a change in geometry, even if they resolve to be the same boundary representation.

Geometric tolerance is defined using the precision defined in the new IFC model.


By default, IfcDiff only compares changes in attributes and geometry. You may wish to optionally specify more relationships to compare. You may choose from:

  • type: detects changes in the type relationship, such as when an occurrence now belongs to a different type.

  • property: detects changes in property sets, properties, quantity sets, and quantities. Also includes detected changes in inherited properties.

  • container: detects changes in the spatial container, handling indirect containment such as when an element is part of an aggregate.

  • aggregate: detects changes in aggregation.

  • classification: detects changes in classification references. Also includes detected changes in inherited classifications.