Good morning,
... If you naturally have such "extravagant" wishes, it gets expensive!
Your wishes are not "extravagant"—although this adjective seems misplaced to me in this context—but they are expensive, especially regarding the basement.
What are the usual additional construction costs?
Here you can find the corresponding info:
So we definitely need TEUR 400 in total, right? That's about TEUR 50 more than planned or available...
Yes, and with this amount, the basement is still not finished, but just a simple utility basement.
Where can one best save money? Or should we plan completely differently? A different house type or another roof?
My husband lost a foot in a motorcycle accident and has been wearing a prosthesis ever since... But unfortunately, it has often happened that he had to switch to crutches, and you never know if a wheelchair will be needed someday... Therefore, we want to build barrier-free and have extra-wide doors, etc... Unfortunately, we also know that this does not make things any cheaper!
A bungalow—if it is even permitted on the plot?—I consider sensible; disabled-accessible doors or appropriate clearance in front of sanitary objects also don’t cost the earth. The problem—if you want to implement your space program entirely on one level—is the required area on the ground floor. Then the foundation slab must be larger, and the roof area also increases significantly; this explains why building a bungalow is generally more expensive than a classic single-story house of comparable total floor area (distributed on ground floor + upper floor). You should discuss with your husband whether an alternative might be that you both live on the ground floor and the child(ren) and visitors live upstairs.
The desire for a basement does not really make sense to me given your husband’s disability; the plan is to make life easier for your husband—at least at home? And if the basement is "only" supposed to serve as a storage room, that’s a very expensive junk room; there are more affordable solutions.
Or should we plan completely differently?
Looking at prefab or solid houses, or getting feedback from acquaintances and friends, is a first step among many on the way to a single-family home. In my opinion, since your situation is not everyday due to your husband’s limitations, you should try to have a constructive consultation with an architect or a seller you trust; on request, most architects offer such consultations free of charge (explicit request is important, otherwise you’ve signed a contract). This conversation should list your must-haves—disabled accessible equipment—and nice-to-haves (the rest of your room program), also taking into account your daily life rhythm and budget.
Building fairs can also give a first overview, or at least sensitize your gut feeling about the other party; to clarify where the journey should go. By the way, such a fair is taking place on Sunday in Hilden; I will accompany my partner, who is an exhibitor there, and if you think a personal conversation could help you, I’m happy to take some time for you in the morning. I always wear the HBF logo around my neck at fairs, so you can identify me. It has proven in the past that some questions from users are easier to ask in person than to write; the same often applies to their answers.
Here in the forum or on the internet in general, you can find many construction diaries; it’s always worth a look. Often it also helps to roughly limit the choice of possible building partners. If construction projects are described from your area and you generally like the described process, you can also ask the respective author if you can visit their "dream house" or talk to them about their experiences.
Walking through new development areas has also proven to be effective; there are more than enough around Heinsberg. Do yourself a favor and only go to these areas on weekends. Then the builders are alone on site, no craftsman or seller nearby, and the chance is greater than ever to get an honest answer.
I think you first have to develop a feel for what you are really getting into; slowly get familiar with the subject of house building. The above tips and certainly the contributions here in the forum can help you with that. When you then feel more confident (a slight uneasy feeling usually remains because you are a layperson), the next steps become easier. One thing you should never lose sight of: even if you are a layperson, your gut always notices whether you are on the right path or maybe need to make a detour. Evolution—thank Mother Earth—has not been able to take this warning instrument away from us.
Rhenish regards