An interface, with a property that has a varying interface

First, a link to my stackblitz: https://stackblitz.com/edit/angular-hnfgcx

I have written a more technical description of my issue in stackblitz aswell, so please read that for any clarification :)

So the title might be a bit confusing, but let me try to elaborate.

I have a basic interface for my "items", that has 2 properties:

id - a unique identifier item - a varying type (Either CarInterface or HouseInterface)

However, when I try to access properties of a CarInterface or HouseInterface, the type isn't recognized.

Thanks for your time!