That looks just the thing for that gap. Pretty sure something like that went in when the walls were repaired, but the floor was left like that.
What's under the decking?
Space, mostly. It currently finishes about 20cm below that new door, then minus the joist height, set on slab pillars, with a membrane of some sort to stop anything growing.
@sacredhart started
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That looks just the thing for that gap. Pretty sure something like that went in when the walls were repaired, but the floor was left like that.