I'm doing some landscaping cleanup for my parents' house - for visualization, this house's surrounding layout is almost identical: 
The front and right side of the house is surrounded by lawn and mulched beds while the left side of the house has the garage underneath the first floor, with the adjacent lawn sloping downwards to it. And instead of a wood deck, the back of the house has a brick patio extending along the entire side.
TL;DR at bottom, picture album for the following write-up is here: https://imgur.com/a/7ufVM7f
I noticed that after a really heavy rainfall a small amount of water was leaking into the back of the garage and then draining out through the front. Apparently the water infiltrates like that only after it's been raining hard for a long time so I figured it was a drainage issue around the exterior walls. I cleaned out the gutters and extended the downspouts a bit further away and now I'm going to regrade the areas that are adjacent to the exterior wall. There are two sections of mulched beds in the front, one is sloped adjacent to the garage wall and the other is flat next to the front door.
Sloped and flat beds, might need to extend downspout even further out (green piece next to wall is not connected)
Back of the garage where some water leaks underneath after heavy sustained rain.
Seems to come in the joint where the wall meets the floor, under the sewer drain pipe
It turns out that the front siding where the beds are is EIFS (or a hybrid system), whereas the back is actual stucco. And it appears the foundation is much higher than the sloped bed, if the walls of the garage are any indication. Furthermore, the EIFS not only covers the foundation to the ground but also extends about 3-5 inches below grade next to the sloped bed, and about 2 inches below grade next to the flat bed. I can only think it was done for aesthetic purposes, but not sure why it had to be below grade. All in all, the below-grade siding portion seems to be in relatively great condition despite the soil/water contact for over 20 years.
Blue line marks garage wall and foundation top
Siding covers past foundation to below grade 3-5 inches in sloped bed
Foundation is visible underneath
Flat bed, blue line is foundation, ~6 inches above ground
Again, siding extends past foundation, about 2 inches below grade here. Seems like a concrete lip was cast at the base of the siding, maybe to help protect the bottom from water and dirt getting in?
For the sloped and flat garden beds, I want to regrade the area immediately adjacent to the walls. Can I simply add some more soil to the top and grade it away from the wall? It would be a good amount below the foundation but it would still touch the siding. Also, moving around the existing soil in some parts might be more difficult due to the existing plants in the way. I was also thinking of digging a few inches below the grade first and line some roofing material against the EIFS wall to also help keep water away and limit soil contact, something like this: 
Not sure if I can lay as deep or wide though.
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TL;DR - Water leaking in to garage/basement after heavy rain, want to regrade beds next to wall to divert more water away. Siding covers foundation and extends a bit below grade - but seems to be in great condition even with soil contact for over 20 years. Wondering if it's still ok to put soil next to wall as the foundation is already high enough, even though it'd make contact with the siding? Probably a more permanent fix is needed but wanted a short-term solution in the meantime.
Can I put some sort of underground flashing (e.g., roofing membrane) right next to the wall to keep water and dirt away? What material should I use, and how should I install it?
