Is it sensible to retrofit a decentralized ventilation system?

Hello,

we have a semi-detached house 150 sqm + basement as a new build from the developer

I wanted to ask if a decentralized ventilation system makes sense to retrofit afterwards?

The semi-detached house is KFW 55 ventilation is to be ensured by opening windows (this is currently planned)

installing a central ventilation system is not possible the decentralized one would have to be installed by us after handover

is there also a decentralized ventilation system with built-in air conditioning (for the bedroom)

I wanted to ask if it makes sense to retrofit a decentralized ventilation system?

These things need electricity. And a core drilling. The latter is quite easy to accomplish. The former will be a problem. Then the electrician should already lay additional cables. Is there a chimney?

is there also a decentralized ventilation system with built-in air conditioning (for the bedroom)

As far as I know, no. I also wouldn't know how that should work.

What does "meaningful" mean? What kind of purpose do you want to have?

I have just retrofitted the bathroom and bedroom with decentralized devices because a) the humidity in the bathroom is not easy to control, and b) the CO2 level in the air was significantly too high in the bedroom with the door and windows closed.

Even with decentralized devices, one should think about the ventilation concept. It can make a big difference whether someone (or how many) is at home during the day or not...

Having the electricity installed would be a cost-effective preparation...

And: To my knowledge, there is no decentralized device with a built-in air conditioning system either. But even here: something can be achieved through bypass functions. As I said, this also needs to be planned...

the decentralized system would have to be installed by us after handover

In a new building, I wouldn’t add anything, as you should still be able to include any additional desired features during construction. However, based on your thread history, I get the overall impression that you are actually an individual builder who has gotten involved in a developer project, or that this is at most plan B for you and you’re now trying to sweeten the bitter pill. Then my suggestion would be: kindly consider the alternative of accepting some "optimality flaws" and resolving the conflict by planning from the start to move into a dream house in ten years.

Ventilation is to be ensured by opening windows (this is how it is currently planned)

That is rather wishful thinking. Or rather, since we have had Corona, this has finally been possible again. However, if the residents are outside the house most of the time due to work, school, etc., ventilating by windows is more of a theoretical possibility. The most comfortable way is to have the devices installed from the start. Or to have the core drillings/recesses planned right away and electricity laid at the desired locations. One cannot avoid a ventilation concept no matter how it is done. Simply buying something and installing it is botched. That will go wrong. (Even if it apparently works). Air conditioning (active cooling) and ventilation are two fundamentally different things. The only overlap is air that is moved and goes somewhere. If you want active cooling in the bedroom (which is very sensible with today's construction methods), then have it installed right away or at least prepare for it. Here too, you need a core drilling (50mm is enough). Electricity and condensate drainage for the indoor unit.

The most comfortable way is to have the devices installed from the start. Or to have the core drillings/recesses planned right away and electrical wiring laid at the desired locations.

Actually yes - but actually I suspect the difficulty here is that a BT says "njet" when you make the improper request ;-) to build not two dozen identical houses but only 23 identical ones and one with fairy dust and chrome trims, just because a princess (m/f) got lost in a Vonderstange project. In this respect, I would reduce the proposal here (as an alternative) to pure pre-equipment as before. However, my "royal way" would be, as mentioned, to make compromises (even if you are not a pharmacy technician in the gastronomy, *SCNR*) and to focus on considering the disliked sub-optimum as an intermediate house.