Buying a plot of land where a house previously burned down. What should be considered?

The (non)conformity of the plan and reality can be checked before building on an existing basement. I don't see where a timber frame general contractor would be inherently inferior to a stone-on-stone builder.

A carpenter would probably have found similar solutions as a master mason, only using wood as the material, at least where possible... but if a load-bearing pillar with a beam exists in the building, which in the planning, however, is about 30 cm in the wrong place and a load-bearing wall is planned on it, you can either think about how to solve it (build up stairs, concrete a column) or you have to change the planning. If the walls are already in production at a prefabrication company, that is suboptimal. (Almost) everything is solvable; the damage just gets bigger the later you discover it.

So first of all, there is a damage assessment report that should contain all the essential information, as compensation was based on that.

It's a pity about the financing; you would need to get in touch with the KfW to see if it still counts as a renovation.
Note, the KfW’s interpretation does not necessarily have to be the same as the state building code.
See [Wohneinheitendefinition], a permitted single-family house can be considered a multi-family house by the KfW, which is often used that way if the colleague here is knowledgeable enough. That means the 60% demolition rule may not apply for the KfW. Which is also how I would see it, but it depends on my colleague.

The Energy Saving Ordinance or KfW remains an issue, since the floor slab can only be insulated from the inside. The exterior wall in the floor can be exposed again to be insulated from the outside. However, the bad thermal bridge cancels the flat-rate thermal bridge surcharge (0.05 W/(m² K)) for the entire building.
However, it should be feasible to achieve a KfW55.
The subsidies of €100,000 alone are already attractive alongside the 0.75% interest rate.

but if a supporting pillar with a beam exists in the building, but in the planning it is about 30cm in the wrong place and a load-bearing wall is planned on it [...] then once the walls are already in production at a prefabrication company, that is suboptimal.

One does not generally derive anything from an unchecked plan, just as one does not simplify sums or write "nämlich" with an "h". The client really learns that in the first semester, so the building materials or the construction method are not to blame. Improvisation due to one's own sloppiness is always the harshest lesson.

The subsidies alone of €100,000 are already attractive alongside the 0.75% interest.

The interest rate is now 0.95%, unless you plan for a ten-year term. But it is still attractive.

no, it is still 0.75% with program 151

But this only applies to renovation, not new construction. So if it cannot be considered as renovation, it would still be interesting.