Overview

Exporting a Revit model to FBX for import into Unity is not as straight forward as exporting and importing FBX files when it comes to maintaining materials. There are some paid-for third-party applications that facilitate this workflow but it has proven quite difficult to find a cost-free workflow that does this correctly.

By using 3DS Max and some tricks in Revit, it is indeed possible by using the standard Revit, 3DS Max and Unity out-of-the-box features.

 

Solution(s)

The following steps will import Revit models into Unity while maintaining the Revit Materials:

 

In Revit, switch to a 3D view, set the visual style to "Realistic" and the LOD to "Medium"

Export the Revit model to FBX, check "LOD" when exporting and use the latest FBX version

In 3DS Max, Go to "Reference" and select "Manage Links"

Go to the "Presets" tab and select the desired preset, then select "Modify" and modify as shown below:

Now select the FBX link (exported from Revit) and attach it

Bind the link after attaching it so that we have access to edit the materials

Access the 3DS Max "Scene Converter" (3DS Max 2018 and up)

Browse for the following script in the Scene Converter

From the Scene Converter, select the "Standard Material to Physical Material" option, then select "Apply Only Selected" and then "Convert Scene" (some missing file errors might appear)

Export the 3DS Max scene to FBX

Disable Animation, Camera and Lights. Check "Embed Media", use FBX 2018 Binary format and export

In Unity, import the asset, then select it and go to the "Materials" tab, click "Extract Textures" and select / create a folder in the Unity project to place it in. Do the same for "Extract Materials" which should be enabled after the previous step

 

You should now have a model with the correct materials! 

Note(s) 
This will work for most (if not all) materials applied to the Revit model

Conclusion 
Exporting from Revit (using the correct settings), to 3DS Max (using the Scene Converter) and finally to Unity (using the Textures- and Materials Extraction) yields the desired results - in my case at least.