How can the utility room in the basement be effectively integrated into the thermal envelope?

When I read this post, it could be feasible:
https://www.hausbau-forum.de/threads/kfw40-mit-externem-technikgebaeude.34706/

But for exact clarification, you have your energy consultant!

Thank you hanse987 for the link, I did not know the post yet.

It all seems very complex. Especially because the subsidies always face additional costs. Finding a financially worthwhile way in advance is difficult for me.

If the additional insulation would cost us a maximum of €6,000, the higher standard would be feasible at net zero cost due to the higher subsidy.

The mentioned example is always good to better assess such things. So always feel free to share them!

Ever thought that all the points are complete nonsense? The Energy Saving Ordinance 2014/16 provides clarification on this. The building envelope describes the entire building; even according to DIN EN 18599, nothing can be deducted from it because the design does not allow opening another zone. The KfW assumes the Energy Saving Ordinance, so it has nothing to do with KfW. I already feel sorry for the colleague who calculates this,...

... since I was quoted: The plans and exact information on how this is feasible will hopefully be received this week. I will gladly pass on this information then. There are only 2 alternatives. Either the external building is extremely insulated (like the rest of the house) and counts towards the thermal envelope despite the distance, or there are exceptions?

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For existing buildings, there is no exception regarding the building envelope. The KfW clearly bases this on the Energy Saving Ordinance.
For KfW55 in existing buildings, the thermal bridges must also be proven.

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If there is an external building, then it is a building on its own
and does not count towards it. A combined heat and power unit is often in an external building.
Which then simply is not a residential building.

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However, there is a basement staircase in the floor plan, so an external technical room does not help much, because the basement ceiling cannot be continuously insulated or sealed.

Unfortunately, what you wrote is not entirely correct. A flat rate value can also be applied for the thermal bridges. And yes, there is a basement staircase. However, you can install an insulated door at the top and insulate the stairwell.