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

so many useful infos at once thanks a lot, guys!!
I also initially thought about the base slab - vertically from outside -, - horizontally from inside - & then accept the thermal bridges... However, I would have to remove a concrete staircase for that & excavate a ~2.5m deep trench along 40% of the exterior wall. But if that is much better than pure internal insulation, we should tackle that first Who should be my first contact person here? Architect, civil engineer or energy consultant?

the accuracy of the plans is a good point. I will personally check them with 3D scans but if we can’t find the structural plans soon, everything will have to be measured again anyway.

Internal insulation will not work. The usual energy advisor programs are not sufficient because they only calculate according to Glaser. We use programs that perform extended calculations for internal insulation. It makes sense to have a colleague who is both an architect and an energy advisor. This person must be legally competent to implement the funding program 151 at the KFW. That way, you also receive the full €4,000 for the planning services.

Does the trench and stairs cost more than €48,000 in subsidy for the 2nd housing unit?

Otherwise, remove the basement stairs if present and insulate the basement ceiling; the living space may be lost anyway due to the too low ceiling.

In other words, calculate what really is the cheapest way. With program 151, it is definitely worth trying to keep the 2nd housing unit. Which federal state or state building code are we talking about here? The window area seems small in relation to the living space, if I am interpreting the pictures correctly.

Rhineland-Palatinate. No, a €48,000 grant would be amazing - that would already be 1/8 of our target budget for the whole house?! - and turning it into a granny flat was already our goal.

What costs can I roughly expect for a basement renovation like this? I would do the earthworks & insulation mostly myself with some occasional purchase of expertise. Heating is initially out of the question because it should be coordinated with the house construction above. Water and electricity should certainly be completely renewed. The windows are only double-glazed but still look good, though I suspect they are not tight enough for any "modern" insulation?
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If the second residential unit doesn't work out, why remove the basement stairs? Simply for maximum insulation between the living space & basement?

You receive €48,000 per housing unit as a repayment grant for KfW55 plus €4,000, totaling €100,000. Therefore, it probably wouldn't make sense to sacrifice the 2nd housing unit. Then the windows will just be removed,...

Insulating cellar staircases is often a much too great effort and associated with many thermal bridges, all of which would have to be proven.

Okay, so I’m going to assume that I will dig out the basement all around. Out of curiosity: which company are you with and where are they from? - if no advertising is wanted here, then feel free to send me a PM.

Architecture and engineering firm (you have PM)