Hi there! I'm an experienced DIYer in Western Washington midway through a basement flip and trying to figure out how to deal with this slope in our concrete slab. Please see my drawing - it shows how the grade slopes 3/4"+ towards a glass sliding door.
We're now going to put dri-core and then LVP on the slab to warm up the basement. This is the only area of the whole basement that has a slope. I expect it was intended for drainage, but it can't drain the whole floor anyway, so I'm not too worried about eliminating the slope. We haven't had any water intrusion issues and even the sill plates of all our walls have stayed dry for 60+ years.
So now to eliminate the slope. I'm thinking we should just fill it with concrete and then build up the threshold with PT or composite 2x4s to match the height of the concrete. We're replacing the single-pane sliding glass door with a new one (can raise the header or buy a shorter custom door to fit).
My husband is considering not completely filling it and leaving some sort of air gap, but I think that introduces more issues than it solves. Anyways, I'd love any advice y'all have to offer!