A bit puzzled about the heating question, the agony of choice

....Keyword holistic and integral planning means nothing should be strictly excluded and the planning should not only involve the architect!

Correct. First of all, everything is open.

.... also include an MEP planner, but many project managers (architects) still shy away from this due to the supposedly "additional" costs incurred by the client.

The costs are comparatively low. An architect, since he has to bill according to HOAI himself, assumes HOAI. In the single-family house sector, MEP planning according to HOAI is not necessarily required. Here it is completely sufficient to focus on the critical points (individual details). The costs for this for a "normal" single-family house are about 1.1 to 1.6 thousand euros, depending on the agreed scope of services and degree of difficulty. A complete MEP planning, in my opinion, is dispensable and costs 3 to 4 times as much without significant additional benefit for the client. Moreover, with a focus planning it is much easier to find a suitable system builder, since he can also bring in his own experience here, which is not the case with HOAI planning because here virtually every screw and clamp is prescribed. Whoever does not want to afford the focus planning will have to live with a less economical system in the long run.

....The energy consultant and/or MEP planner support the development of a long-term concept, at the beginning of which everything should be open.

The most sensible orientation is the personal union of MEP planner and energy consultant. Some universities of applied sciences have now recognized this and offer corresponding degree programs, unfortunately still the exception. Historically low interest rates favor higher initial investment with subsequently low operating costs, while high interest rates rather point to a initially low investment with subsequently higher operating costs. It is certain, however, that the specific costs of all energy carriers will increase progressively in the future; to what extent no one knows, not even the speculators who contribute their share to price driving. Energy will probably become a luxury good; everyone will be well served who, for example, has realized a concept of independence in the annual balance, i.e., heating and hot water almost at zero cost regarding operating expenses.

best regards

NB: Future energy costs for heating and hot water are already determined by the site selection, the orientation of the building as well as the construction and geometry of the building body!

The costs for this amount to approximately €1,100 to €1,600 for a "normal" single-family house, depending on the agreed scope of services and degree of difficulty. A complete TGA planning, in my opinion unnecessary, costs three to four times as much without any significant additional benefit for the builder. In addition, with a focused planning approach, it is much easier to find a suitable system constructor, as they can also contribute their own experience here, which is not the case with HOAI planning, since virtually every screw and clamp is prescribed there.
Those who cannot afford the focused planning will have to live with a less economical system in the long term.

Do you also include your own costs in the economic calculations?
With an additional €1,600 of fees to be financed, the builder could instead consume 10€ more gas every month for 20 years, without you being better off with any other heating type. Now surely the argument comes that energy costs are rising and therefore much less gas can be bought for the €10 in the future. I am quite aware of that.
But how do you explain to the builder this worst-case scenario for him – if at the end of your calculations it turns out that the standard heating system gas + solar thermal system proposed by the general contractor is the most effective for his single-family house and he has wasted the €1,600?

A legitimate question...

Do you also include your own costs in the economic calculations?

Of course, otherwise it would hardly make any economic sense for the client ;-)

With €1600 additional fee to be financed, the client could instead use €10 more gas each month for 20 years without you doing any better with any other heating system.

Hardly credible, where does this knowledge come from? ;-)

...Now the argument will surely come that energy costs are rising and therefore much less gas will be available for the €10 in the future. I am well aware of that.

Regarding speculation about future energy prices, I gladly leave precedence to the client; here he can influence according to his own taste (interactive consulting, planning), I naturally advise according to my knowledge and experience.

...But how do you explain the worst-case scenario to the client – if in the end of your calculations it turns out that the standard heating system gas + solar thermal system proposed by the general contractor is the most effective for his single-family house and he has wasted the €1,600?

99.9% unlikely, simply due to the fact that solar thermal systems in the single-family house sector are anything but economical ;-) Regardless of that, anyone who thinks in the case of, for example, gas condensing boilers, for whatever reasons, that exact planning is dispensable, should look at botched practical installations.
Furthermore, take a look at the contract documents of a general contractor! Is there contractually secured any clause referring to the expected energy costs? Hardly :-) At best, the efficiency status to be realized on paper is stated, but nothing more. Unfortunately, the majority of general contractor/general contractor dealings fail to reach this status in practice, mainly due to lack of planning! And now – lawsuits with high effort and uncertain outcome, although the construction budget is exhausted or overextended?
This can be avoided in advance.

Best regards