air heat pump in combination with solar, optional gas connection

And googling for sources of errors... do you have typical keywords that I can search for more specifically?

No buffer storage, no solar thermal system, no water-bearing fireplace, individual room control could already be left out during construction (not allowed without exemption), .. you will definitely find something.

The important thing is the design of the underfloor heating for the lowest possible flow temperature (keyword: room-specific heating load according to DIN).

How elaborate and expensive is a photovoltaic system? Would a retrofitting be unrealistic and not well thought-out due to the already installed technology?

Costs: it pays off by itself after about 10-15 years. Costs around 10k€. If not installed during construction, then on a large meter cabinet, no second meter for the heat pump and pay attention to the power cables for photovoltaic to the roof. You will pay for scaffolding twice then.

The combo air-water heat pump + photovoltaic + fireplace also sounds interesting....

I would categorize the fireplace under "comfort" – it does not pay off for heating and if integrated into the hydraulic system of a heat pump, it leads to disadvantages for the heat pump.

I would classify a fireplace under "comfort" - it is not economical for heating and if integrated into the hydraulics of a heat pump, it leads to disadvantages for the heat pump.

It depends on the perspective whether it is a comfort feature. The first thought is actually what we do in the transition period and in winter when the air-to-water heat pump fails! The second thought is that the fireplace is a visual highlight in our living area.

By the way, I would consider controlled residential ventilation, we have that too, even with a ground heat exchanger.

It depends on the point of view whether it is a comfort item. The first thought is actually, what do we do in the transition period and in winter when the air-to-water heat pump fails! The second thought is that the fireplace is a visual highlight in our living area.

I agree with you on the highlight.

Transition period: that’s when the air-to-water heat pump actually runs best. As a backup for the heat pump, electric heaters are unbeatable in terms of cost (fan + heating element in the storage tank). If the power goes out for a long time, the fireplace without a water jacket is the highlight, also for the neighbors ;-)

I’m not familiar with fireplaces – we only have the DVD – but the most sensible option in a new build is a fireplace with low output and high storage capacity.

I would only consider a heat pump or a heat pump with photovoltaics. Saving a lot by using more technology is probably rather difficult. After one year, we have about 60-70€ heating costs per month with our air-to-water heat pump at 23-24 degrees in the house.
Due to a defect, our heat pump was out of order for a week once, then the device ran in emergency mode (heating element) at -10 degrees outside. No problem at all, the approximately 250kWh additional consumption does not really count as a backup. And a complete failure is probably a very rare case. The service would have been there faster than in a week anyway.