Hi Mcarton -- Since these are private indoor areas and will not be used for routing, I'd probably just be inclined to draw them as parts of the buildings, or make them an entirely new building themselves. Just tag with `layer=1` and they should not interfere with the road and walking paths. They don't seem to be present on the aerial imagery yet ([here's the area in question][1]) so you may just have to guesstimate.
_Edit: Adding the `bridge=yes` tag, as suggested by the "skyway" answer linked by __neuhausr__, is also a good idea._
It might be satisfying to try to show how high they are and where on the buildings they connect using the [simple 3D building scheme][2]. You can draw a bridge in several parts to show its incline from the first floor of one building to the second floor of another -- eg, first a segment with `min_height=10` and `height=13` meters, then transitioning to a segment with `min_height=11` and `height=14`, etc. This will work better if your buildings are tagged with height, and you may want to add a `building=yes` or `building=parking` tag (as well as height) to your parking structure.
You could go all out and try to apply the [indoor tagging scheme][3] but again, since these are private areas, it would probably be a wasted effort.
[1]: https://www.openstreetmap.org/#map=18/51.49950/5.45956&layers=N
[2]: https://wiki.openstreetmap.org/wiki/Simple_3D_buildings
[3]: https://wiki.openstreetmap.org/wiki/Simple_Indoor_Tagging
