ᐅ How would you assess the soil quality based on the photo?

Great! A lot of work, but definitely worth it!

After the site development has been completed and the soil survey has been conducted, here is the evaluation.

BS1:
- 0.40 meters (1.3 feet) topsoil-fine sand-clay mixture
- 2.10 meters (6.9 feet) clay, silty, semi-solid, moist, gray

BS2:
- 0.40 meters (1.3 feet) clay, silty, heavily stony, stiff to semi-solid, moist, reddish-brown
- 2.60 meters (8.5 feet) clay, silty, no water encountered, semi-solid to solid, moist, gray

BS3:
- 0.20 meters (0.7 feet) clay, silty, heavily stony, stiff to semi-solid, moist, reddish-brown
- 2.30 meters (7.5 feet) clay, silty, semi-solid, moist, gray

Do I need to remove the soil and bring in topsoil? Or could I also till the soil in autumn and mix it, for example, with horse manure, sand, horn meal, etc.?

Something should normally grow in the soil, but it is too dense and impermeable; it lacks air and crumb structure. Mixing in sand is a good idea. After measuring the pH value, apply lime if necessary. pH test strips are available from garden centers, for example from Neudorff. All instructions are on the packaging. I believe you can still grow grass there without bringing in topsoil. K.

Now that the building area has been fully developed, unfortunately, there is hardly any topsoil left. They have mixed all the dirt (clay with sandstone) into the building plots, compacted it, and applied about 5cm (2 inches) of topsoil on top.

To revisit the original idea of converting clay soil into good soil: this process takes a lot of time, involving digging over, mixing in sand, and so on. But I can’t live in a clay desert for years...

Should the soil be dug over every winter and grass reseeded in spring? Or is it enough to dig over the first winter and then just add some sand regularly afterward?

No, only once. Once you have grass growing on it, the roots will loosen the soil, earthworms will settle in and further loosen it, so the process continues over time. You might need to fertilize the lawn a bit more at the beginning than elsewhere, but since the clay contains minerals, it also benefits the grass. We have also achieved good results on soil with very little topsoil, mostly clay and silt.