KFW 50EE or 40EE - Is the upgrade worthwhile?

The construction company strongly advised against KFW40 because additional costs of €18,000 have to be assumed here and also definitely no central ventilation with heat recovery and enthalpy heat exchanger.... They install a decentralized ventilation without heat recovery and that is "super"...

For me, that would rather be a reason to question it... Just because someone finds something "super" does not necessarily make it the most suitable solution for every house.

I am not green enough to pay the extra cost just for my conscience.

That is refreshingly honest. Thumbs up!

Will the 18k additional costs for KFW40 pay off over 30 years or rather not?

That depends on the assumptions you make.

Unfortunately, quite certainly not.
Of course, it depends on the costs you pay for the local heating network.

That’s what I mean by assumptions: If you base it on today's costs, then probably not. If you calculate with a price increase, then possibly yes. You can argue it well or badly with good arguments either way.

Office people regularly overestimate how reliably they can complete their own work fitting the scheduling of the tasks. Especially many of the preparatory and auxiliary tasks are time-critical.

Yes, the difference between theory and practice is precisely this: theoretically there is none.
I still wonder why house construction is not thought from the end result but always only from the first step. There is still a lot of room for improvement and productivity. I would love to spend 2 years going through a prefabricated house provider with a Kaizen team.

I still wonder why house construction is not thought of starting from the final result, but always only from the first step. There is still a lot of room for improvement and productivity. I would like to spend 2 years with a Kaizen team going through a prefabricated house provider.

Oh, believe me, the big manufacturers all have their Lean/OpEx teams.

So we should hand over performance phases 1-8 to an architect if at all. What will something like that cost? I read that with some you can also agree on a flat rate?!

Yes, I recommend commissioning the architect continuously from performance phase 1 to 8, in three phases. I explained the modules in the post "A house-building schedule, also for you: the phase model of the HOAI!" And yes, flat rates are possible.

We are not sitting in the office. We always have full days off, not just an hour after work... Still not feasible?

The closer your own activity is to the actual work done by oneself, the smaller the misjudgments naturally are.

Yes, I recommend commissioning the architect continuously from service phases 1 to 8, in three stages. I explained the modules in the post "Ein Hausbau-Fahrplan, auch für Sie: das Phasenmodell der HOAI!" And yes, flat rates are possible.

Here in the forum? The forum search with the title did not lead me to the goal

The forum search with the title did not lead me to the goal

Then try the Google search ;-)