New energy saving regulation from 2016 -> What to build?

Ok, that reassures me. I also had the feeling that the KfW loan 153 offered from April 2016 is really interesting. Assuming an interest rate of 1% compared to 2.1% at a bank and a term of 20 years, as well as a repayment subsidy of €2,500, I come to an advantage of about €15,000 when choosing the KfW loan. And this can probably cover a good part of the additional costs (KfW55 vs. KfW70).

KfW has also adjusted its interest rates (also during the year) to some extent according to the general interest rate development in the past. I see the forecast you described as extremely optimistic because:
1. Currently, KfW70 is already being subsidized less than KfW55, and I suspect that with tightening of the energy standard, KfW70 might no longer be subsidized at all and KfW55 less than before. For example, the repayment subsidy could be eliminated or the lowest interest rates offered only for KfW40 or lower.
2. The interest rate of 0.75% or 1% currently applies for 10 years. If KfW allows a fixed interest rate period of 20 years, then the interest rate (as with all other banks) will be higher. If you currently get about 1.5% for 10 years fixed interest, then for 20 years it is already 2.2%. Therefore, under otherwise equal conditions, KfW loans with a 20-year fixed interest rate would probably be around 1.5-2%.
3. There is still time until April 2016 for interest rates to jump 1-3% upwards. If then KfW sets its new conditions, the 1% will be outdated.

In short: As a counterexample to your optimistic scenario, it could also happen that you cannot even remotely offset the additional costs for KfW70/55 by the then 3% interest rate over 20 years (without repayment subsidy).

The truth probably lies somewhere in between.

According to the current status, KFW 70 is no longer subsidized under the Energy Saving Ordinance 2016. That the interest rates then rise to up to 3% in the case of longer terms for KFW 55 / 40 models would indeed not be particularly good. Should it still come to that, I think it will be clear that these funding methods will only be relatively interesting anymore. But as so often... there is no need to get worked up, because no one can predict the future. It will come as it comes.

Currently, the KfW interest rates at 0.75% are about 50% below the market interest rate. I assume that the interest rate will remain the same for 10 years and that the interest rate for 20 years will possibly be between 1 - 1.25%. That is my tip.

We are also planning with the new conditions from 1.4.2016. A single-family house with 3 [WEH] is planned. So we are already more than curious about the final KfW 55 conditions.

What might be interesting: The primary energy factor for electricity will drop to 1.8 at the turn of the year. With a heat pump, KFW40 should then actually come into reach. The difference to KFW55 lies only in the lower primary energy consumption (times factor), if I understand correctly.

..., you also no longer need to worry that the exhaust air heat pump will cost you a fortune, nor that the KfW 55 efficiency house standard will not be demonstrably achieved. He (Viebrockhaus) has - crystal ball mode on - probably learned from past mistakes - crystal ball mode off - and now already offers a small photovoltaic system as standard.

This "super-mini" (very small) photovoltaic system (SunPower) is just cosmetics! Viebrockhaus just wants to generate additional revenue with it, since this system on large roofs is a joke. Builders are burdened with additional costs for "useful" systems.

In winter, when the "exhaust" air heat pump draws electricity, the mini system is useless.

I am curious when the conditions for 20 years and KFW55 will really be fixed, and how long they will probably last until the next change.