Demolition of house with basement, new construction without basement

Why do you recommend that?

I am also in favor of demolition. Just be careful with the costs for breaking up and filling, and also with the pollutants.

And are there any other special considerations to keep in mind when demolishing a house with a basement and building a house without a basement on the same spot?

yes, when demolishing, make sure to disconnect all utilities,
electricity, water, sewage, TV, gas and possibly seal them off and reuse them in the new house,
this could be cheaper than doing everything new

Why do you recommend that?

There is a developed property with a mix of substance—still quite usable and a surprise egg—and an owner who wants to plow the building under the field and top it off with a villa. He wants to make it appealing to someone who currently trusts him as a tenant. By now, I know my beloved well enough to recognize her frown even from twenty kilometers away at this hour ;-) So wait until the planned refinement process is successfully completed and an expert has reviewed it and given their okay to buy it without reservations. For now, do nothing beyond a preliminary contract, plan the house in agreement, and agree to acceptance under the condition of flawless completion (after the bank has agreed, of course).

Just ask where the old soakaway pit is, what it is filled with (by any chance the old asbestos roof from the 70s),
surprise egg indeed,
why a white tank in the basement,
layer or groundwater?

In your summary, I recognize more than just a frown. The kidney stone and lunch are making themselves known. You are right. The alarm bells are ringing very, very loudly.

There is a developed plot with a substance mixed from still quite usable and surprise egg, and an owner who wants to plow the building under the field and top it with a villa. He wants to make it appealing to someone who currently trusts him as a tenant. By now I know my beloved well enough to recognize her frown even from twenty kilometers away at this hour ;-) So just wait until the planned refinement process is successfully completed and an assessor has reviewed it and given his okay to buy it without reservations. Now do nothing beyond a preliminary contract, plan the house in agreement, and agree to acceptance under the condition of flawless completion (once the bank agreed, of course).

The plot already belongs to me, not to the landlord.