Heating Planning

Good day forum community, my wife and I are planning an extension at home and are now considering the possible heating options. Just to explain, we are building onto an existing house (built in 1975), my parents-in-law live downstairs and we live upstairs. The existing heating (storage heaters) and hot water supply in the entire building are to be renewed. It is also certain that we will not install underfloor heating, and the heating and hot water are to be produced centrally. Windows and insulation are all up to date!!! One possibility would be a gas boiler, but the question is how expensive the gas connection is. For a heat pump, only an air source heat pump would be an option since we do not have enough land for geothermal. I am somewhat skeptical about air source heat pumps; if I understand correctly, heating is not a problem and hot water for showering or similar up to 40°C should still be energetically okay, but what about the kitchen sink?? Because there you sometimes need really hot water, so that the entire hot water system does not have to constantly heat all the water to 60°C just for the kitchen sink, I had the idea of simply installing an instantaneous water heater under the kitchen sink connected to the hot water line coming from the heating. This way, you would have hot water immediately without delay, and since the water is preheated to 40°C, the instantaneous water heater would only have to raise the temperature by 20°C from 40 to 60°C. Would that work???? How sensible would that be??? Do you have other ideas??? Many thanks, regards Peter

Why do you need 60 degrees in the sink? We manage well with 40 degrees – most of it goes into the dishwasher anyway. If you have an instantaneous water heater, you have to install a circulation line in front of it to get hot water immediately. That hardly makes sense with a heat pump. In other words: When you turn it on, cold water is initially in the pipe until the warm water comes. Then the instantaneous water heater has to heat it from 10 degrees to 60. But again: What's the point of all this?

It is recommended (keyword: Legionella).

There are various recommendations. Legionella would only develop in pipe lengths over 20 meters or in the tank, etc.
My heating system automatically runs a Legionella program every month and heats the tank to kill them all.
BUT: Why exactly do you need 60 degrees in the sink? Legionella can develop in any pipe (also in the bathroom),....
Then all heat pump owners would have Legionella in their houses,....

Have you ever looked into Legionella more closely? You don't get Legionella from drinking or washing your hands. You get it when you inhale water vapor. So most likely in the shower. Therefore, having an instantaneous water heater in the kitchen makes no sense at all.

It was not about the legionella .... but only about rinsing off grease .....