Gas without solar possible for new construction?

For me, it is understandable that one wants to compare. Especially currently, one cannot look into the future. I would also be in favor of preparing for a heat pump and then installing it as a monoblock myself. The prices called for are simply very cheeky. However, I am not sure if the total requirement of 4000 kWh/a is correct. That is already low, especially with a view to the basement.

My plan is to start with gas first but have everything prepared for a heat pump (underfloor heating designed for low flow temperature).

This is exactly how we are building as well, but we have chosen the standard solution "gas with solar thermal". Photovoltaics is prepared and will follow once we have more money again and the storage systems become cheaper or one can use the electric cars as storage. :)

Six years ago, when building our house, I already said from the start that it “must” be an air-to-water heat pump. Even without a cost comparison. I knew that the heat pump was more expensive, but just the idea of not being dependent on fossil fuels was worth it to me. And today, with the photovoltaic system and the knowledge of how things currently look in the world, I am glad that I made that decision back then.
And today a gas heating system would be even less of an option for me.
Of course, it is understandable to do a cost comparison, but for me that would be secondary nowadays. I find other points like “dependency” much more important than costs today.
But €36,800 is of course quite a sum. We didn’t even pay that for the entire heating + plumbing trade six years ago. The heat pump alone, I think, was just under €14,000 back then (Stiebel Eltron LWZ 304)

Photovoltaics are prepared and will follow when we have more money again

So never. :D You would be the first builder who has more money after the construction than before. (Inheritance, 6aus49 etc. excepted…)

Do you mean 4000 kWh gas (and thus 1000 kWh electricity)?

I mean heat. A modern gas boiler achieves an efficiency just under one hundred percent, so roughly it's the same. A KfW40 house has a calculated heating demand of 25 kWh/sqm, which results in the 4000 kWh. What you make of that is your business. With the basement, it can of course be somewhat more, but I consider 12,000 kWh to be just as exaggerated as the surcharges of your construction companies. That would also be the point where I would start and look for another company. To build a dead technology into a state-of-the-art house strikes me as complete nonsense. As a second-best solution, I would then reduce from 40 to 55, and as a third-best solution, I would award the heating installation myself.

I see it similarly. I would do EVERYTHING to avoid having gas installed. However, I would also do A LOT to avoid having a heat pump installed for 25,000,- (excluding [Kontrollierte-Wohnraumlüftung], plus boiler + connection) that you can get in the same quality for 5,000,-. The ideal solution would be to build passively and save a total of about 100,000,- € in operating costs and heating renewal costs over the next few decades. But unfortunately, not every homeowner can do that yet...