The Problem with Gas Heating
Unfortunately, the days when you could install a modern condensing gas boiler in your house are over. Today, you need at least an additional solar system for domestic hot water preparation and heating support. This results partly from the Energy Saving Ordinance (Energieeinsparverordnung) and partly from the Renewable Energies Heat Act. The latter regulation aims to help conserve fossil energy resources and at the same time reduce dependence on gas, oil, and coal imports. The Energy Saving Ordinance, in turn, "forces" you to build an energy-efficient house.
This now creates the following "problem" for you:
When the thermal protection certificate is calculated as part of the building planning, all heat-transferring components must be specified with regard to their geographic orientation, surface area, and materials used. These are the floor slab, walls, windows, shutters, and doors as well as pitched roofs or top floor ceilings.
Next follows the technical system part, namely the type of heating including hot water preparation, type of distribution pipes, circulations, and some other details. All points refer to the newly constructed residential building. In the background of this calculation, the so-called reference building is calculated in parallel. That is an identical house in terms of volume, but with fixed values (according to the Energy Saving Ordinance) regarding the components and system technology. The resulting outcome for transmission = components and primary energy = system technology thus represent the minimum requirements for your "new" building.
The problem is, if you enter "gas heating + solar system" here, that the reference building is fundamentally calculated with an exhaust air system and thus consumes less energy. You could compensate this with even more insulation and a chimney for the later fireplace connection. The chimney works because the planner can add a decentralized wood stove with up to 10% heat yield. This, together with a bit more insulation, might barely be enough to meet the standards of the aforementioned regulations at all. When we now consider all the mentioned efforts, economically we end up in the price range of a reasonable heat pump, which makes installing a gas boiler completely pointless. That gas heating is no longer desired is also shown by the fact that the heat pump is rated better from the outset in the calculation.
So, the options left are heat pumps, electric heating, and combined heat and power plants.
The combined heat and power plant is (still very expensive). Then we have the choice between heat pumps.
I myself would install a BAFA-listed air-to-water heat pump today. On the one hand, I come off cheaper than with a gas heating + solar system + gas connection, and on the other hand, this is probably currently the most economical heating system on the market.