Ground-level terrace, any experiences in planning the slope?

so far I have only built a fence

How did it actually go with the foundations there?

Yes, that hardly saves any effort. Otherwise, your description is pretty much a qed for what I meant. Beam substructure with drilled point foundations requires much less heavy equipment. But I’m also taking a big risk here, so far I have only built fences and no terraces...
I will report back (in one to two years...)

Yes, but what do you need? If necessary, you can do it with a wheelbarrow. The rule of thumb is one wheelbarrow per square meter. Otherwise, such a dumper costs €80/day and can easily handle a full semi-trailer load. At the same time, have someone who distributes and compacts the material. You can rent a compactor for a week at €100, and for a terrace a smaller one is sufficient; you don’t need the 500-kilo compactor. So the equipment stays very manageable.

How did it actually go with the foundations?

Pretty well. I only connected one foundation. So I exposed an old wall foundation, drilled thick, deep holes into it and glued in reinforcing bars. Then I poured a new foundation over it. For the others, I either bypassed the old foundation, or I simply made a medium foundation longer and wider. It's now about 40x60x80. We've already had several storm gusts up to 100 km/h, and the fence is still standing.

Yeah, but what do you need?

Rule of thumb is one wheelbarrow per square meter

But that doesn’t reach frost-free depth, or what kind of monster wheelbarrows do you have? So let’s assume you already removed the topsoil due to house construction and only need 50 cm to frost-free depth. That means you have 0.5 m³ per m² (by the way, 3-4 wheelbarrows of my type – and those already have two wheels). Let’s take a small terrace of 30 m². That’s 15 m³ of excavation. One trip to remove it and one to bring frost protection (for me 100 EUR gross just delivery costs). The frost protection might not even fit on one trip due to tonnage. A 250-kilo compactor costs me 50 EUR per day. But one day is enough. About 2 tons of gravel is either another 80 EUR delivery or you haul it yourself twice with a trailer. Whether wood or stones are delivered can be balanced out against each other. But before that, you definitely have many more trips and more equipment that you don’t usually have but have to rent. And let’s be honest, digging out 15 m³ of soil with a shovel won’t happen in one day. Depending on the soil, that’s quite hard work. So you’d add an excavator for 120 EUR a day (gross). Or several days of hard labor. For a substructure for the area, you make 20 spot foundations (with counter battens) with a soil auger (yeah, depending on the soil not ideal, but easier than digging everything out completely and you can still spread the excavation). Pour in the concrete mix (theoretically you can do this with bags and water). The rest is just screwing with a cordless drill. You also place the foundations or later the beams in water level or slight slope more precisely than you would cleanly level your entire base. With a terrace 5 m deep, there’s no way to level it all at once with a screed, meaning you always have inaccuracies at the intersections of your “pull paths,” which you must later compensate when laying the concrete slabs. I still maintain that a wooden terrace is easier to do yourself. At least if you haven’t done either yet.

I maintain that a wooden terrace is easier to do yourself. At least if you haven't done either before.

Agreement from experience. Especially if you work with mortar bags instead of screwing, aligning is an absolute breeze.

It also always depends on the level of perfection one expects. I do not demand absolute perfection for personal work; you don’t get that even from a craftsman for a reasonable price...

Sure, everyone is entitled to that. I am by no means a perfectionist. However, consistently, even with the same low demand for accuracy, in my opinion, this is easier to achieve with a wooden terrace for the DIY amateur than with stone. The only thing that speaks in favor of the stone terrace, in my opinion, is the generally longer durability. Although that of wooden terraces is longer than the common opinion suggests (at least if constructive wood protection is observed) and that of stone terraces is often overestimated.