Gas boiler in the attic or in the utility room on the ground floor?

I’ll just jump in briefly.

We are currently also in the planning phase with our architect and have already thought about this (gas condensing boiler in the attic). We are building a townhouse style (approx. 22° hipped roof) and also including controlled residential ventilation, which, according to the architect, should also go into the utility room. If I can of course put both in the attic, then that sounds very tempting. What are the additional costs for insulating the attic? Does the upper floor ceiling need to be designed differently (structural engineering)?

What are the additional costs for insulating the attic? Does the upper floor ceiling need to be designed differently (structural engineering)?

This cannot be answered in a general way. Theoretically, it is sufficient to only insulate the area where the equipment is located. However, I prefer "all or nothing."

In our case, about 150 sqm of space in the loft was insulated with 200mm rock wool WLG035, vapor barrier, battens, and cladding with OSB – I believe the material costs were about 2,300 EUR including small items like screws, staples, tape/cartridges for the vapor barrier, plus 30 hours of labor.

Whether the ceiling needs to be designed differently I cannot answer; the size of your house or the roof area to be insulated in the attic greatly influences the costs.

Has anyone gained experience by now on what it is like to live with heating and/or ventilation under the roof? Are noises audible on the upper floor? Is a wooden or concrete ceiling installed? How is the device decoupled and the room insulated?

No noises present. Ventilation, boiler, and hot water tank are virtually above the bedroom. Wooden beam ceiling and attic insulated.

Ditto. Also above the bedroom and also a wooden beam ceiling. When the heating was not properly vented once, you could hear gurgling. This was fixed accordingly. The ventilation unit is mounted hanging on the gable wall and decoupled within the housing. The only thing you hear, which applies to every room, is a slight noise from the airflow when you turn up the ventilation system.

Mine is currently working with night cooling, meaning it passes the fresh supply air past the heat exchanger in summer without warming it. For this, the ventilation runs automatically at a low level during the day and then ramps up between 10 pm and 6 am to push fresh air into the house. It works well, I am satisfied.