I get this question a lot, and the short answer is: yes, you can install an outdoor bench on a grassy slope. You do not need a flat concrete base, but you do need to give the bench a stable, level footing. If you just set it on the grass, you will likely end up with a wobbly bench that sinks, slides, or rots over time.
Start by checking how steep the slope is. A gentle slope is easy to work with. A steep hillside may need more work, like a small retaining edge or terraced pad, but it is still usually doable without pouring concrete.
The first step is to remove the sod and organic topsoil where the bench legs will sit. Then compact the soil lightly. For a simple and effective base, dig two shallow level pads, fill them with about 3 to 4 inches of compacted gravel or paver base, and set precast concrete deck blocks or pressure-treated timber pads into the gravel. This gives you a level, well-draining foundation that will not heave as much as bare grass.
If you want to keep the bench from creeping downhill, anchor it or add a small retaining lip. You can use timber or stone edging on the downhill side, or drive ground anchors through the bench feet into the soil. On steeper slopes, consider building a small level terrace with landscape timbers or a low retaining wall, then set the bench on that flat area.
In most cases, a full flat concrete slab is overkill unless you are already planning a patio or you have very unstable soil. A well-prepared gravel pad with concrete piers is usually enough, looks more natural, and drains better.
So yes, an outdoor bench can work beautifully on a grassy slope. Just level the footing, improve drainage, and anchor it so it stays put. That way you get a comfortable seating spot without the cost and mess of a full concrete base.