Floor plan of an accessible bungalow

We are foregoing the attic conversion, so no stairs, no knee walls, no concrete ceiling. [...] I want the roof pitch to be 35 or 40 degrees,

Either/or: with attic conversion, 35° is a good measure. Without use, however, that is already clearly too much, and even for classic attic use like Karsten's, a "proper" staircase is not a luxury.

Honestly ... you urgently need to decide what you want to build. Either a house for your child (but then really properly barrier-free - have a professional advise you on that) or a house for the entire future family (then your son will have to live with the others disappearing upstairs). Please don’t get so fixated on a bungalow. Also consider whether a classic single-family house might be an alternative. Because this option gives you the space you need/will need without the costs becoming too high. In addition, you can create a nice domain downstairs for your son.

Otherwise: your son will probably rarely be in the kitchen with you, right? So I would limit it to the necessary size and that’s that. Better to have more space in the living room and dining area. Then don’t underestimate the dimensions of an adult wheelchair. They really need space to maneuver. Your hallway is not sufficient! Your son will probably not have a small sporty one, but rather the large bulky semi-reclining variant. The same applies to the bathroom. I can speak from experience. The father of a friend from school was in a wheelchair, although the sporty variant. Even he needed plenty of space so that he didn’t have to crawl through every door at a snail’s pace. Their doors were wide enough for him to turn on the "threshold".

Then say goodbye to the idea that a house saves you money. The opposite is true. You will definitely pay more monthly for the fun. We’re finding that out right now.

And for God’s sake, don’t let yourselves be rushed! That one can take his 3% increase and shove it. They may increase 3%, but then they give more room for negotiation on the price. If it really fits by November, that’s nice. But it shouldn’t be a must for you! If necessary, I would change the construction company. Or are you building with a developer???

My experience with a 1.5 m wide hallway is that driving straight through with an electric wheelchair works, driving forward out of a room also works, backwards requires a good, fit driver. The hallway, as in the larger bungalow, is more comfortable

Much has already been said, I will not repeat it.

Just a small remark: as has been noted several times, you would probably be able to accommodate your space requirements a bit more cost-effectively on 2 floors.
If the wheelchair-bound son absolutely must/have to have the possibility to get to the second floor, then a so-called home lift could possibly really be an option. Space requirement is about 1.2x1m and it can initially just be set up as an option (i.e. planned breakthrough in the ceiling). This is not exorbitantly expensive and, unlike the classic stairlift, very well suited for wheelchair users.

What I also find a little lacking is the consideration whether a care service might possibly be needed at some point. Then one would also have to think about how to possibly accommodate it. Then I find the concept over two floors very sensible again. Downstairs a disabled-accessible bathroom with a disabled room plus a caregiver’s room (which can then be used as a bedroom or as a care service room), upstairs three/four rooms for any use (children, guests, bedroom, office) and a normal family bathroom.
As I said, I don’t know your situation exactly now, whether a care service room will ever be necessary or not. But keep in mind that your son will grow bigger and heavier, that other children will also want to be cared for, and at some point the time may come when you simply can no longer manage on your own. I have this situation in the wider circle of acquaintances (son a severely spastic but mentally completely fit), there one also always thought it would go without, but just wait till puberty comes, then the cards will be reshuffled.

Either/or: with attic conversion, 35° is a good measure. Without use, however, it is already significantly too much, and even for classic attic use like Karsten's, a "proper" staircase is not a luxury.

I want 35 or 40 degrees only because of the look and of course it has another advantage that is already known. However, it depends on what the fun costs!

I want to go back to Yvonne’s statement once more because it contains a bit of truth that everyone who is not a civil engineer or something similar by profession experiences. At first, you think: a house isn’t that expensive, why on earth do people pay so much for used properties? Then in conversations with the general contractor / construction company, you realize that there is still extra to come and that turn-key doesn’t mean finished. And then... now imagination should come into play... then it dawns on you that a house is really only finished when the driveway, paths, outdoor lights, bushes, etc. are done, when all fees and connection costs are paid, the house is surveyed... and unfortunately, in our case, a constant topic, the furniture has been adapted to the new space, although I often say about that: it’s okay. She says: it’s not okay. Yes, with some discipline you can also achieve something beautiful with fewer means. But it’s always easier to reach for the top shelf, which many also do: it only costs 1200 more. 10 times 1200 is also 120000,- Karsten