Building a single-family house with/without a basement on a small plot

Yes, on the upper floor we are planning our room, a bathroom, and another guest room. Roof terrace then 4m deep x 9m wide.

Or did I overlook something?

Federal state? Depending on that, you either have to maintain a border distance of 2.5 or 3 meters with the garage or place it directly on the border. Here, up to 9 m per border and 15 m in total are possible. So 10 m is not allowed – except in the theoretical case that your neighbor permits this in the land register; for that, he can use less of his also small property.

It really depends a lot on your habits, whether you store a lot (decorations, etc.) or rather less? Are you handy and have quite a bit of tools, maybe even want a small workshop and so on. If you store relatively little, then the basement would not be a must. But if you do have quite a bit to store and three children are generally desired, then I would lean more towards a basement.

There is certainly room for optimization in the layout, also regarding the [staffelgeschoss]. With a 4m roof terrace, plus bedroom, guest room, and bathroom? That will be quite a challenge, since a staircase and hallway also need to be taken into account.

and I want to ease the fear a bit. It certainly sounds scary when a basement is flooded. But that is rare. Even with a high groundwater level. Generally, the basement remains dry inside with a "weiße Wanne." That is what this technology is for. Provided it is executed properly. In general, it is advisable to involve a construction supervisor during the critical phases. Then a basement is no problem even with "difficult" ground conditions.

The definition of a [Staffelgeschoss] is usually 2/3 of the area of the floor below. So 6x9m would also be possible. Or a terrace with 3x9m.

Two rooms and a bathroom would fit in well there, but it would also depend heavily on the exact position of the staircase.

you are certainly right, it is not fear but rather skepticism