and one more question: would it really be possible for you to overcome the stairs with a stairlift? how complicated is that? could you really imagine using the stairlift every day?
So you are planning a house for four! people - not for three. Put the two children's rooms plus a small bathroom under the roof, the rest on the ground floor.
Have a look at nullbarriere.de. I got a lot of information there 3 years ago. I would build it so that it is comfortable for me. If it is already difficult for you to use a stairlift now, it will probably be impossible in old age. A stair platform lift would probably be an option for you. You need a lot of traffic space before and after the stairs for that. I think the tip to get an exemption from the development plan is good.
Unfortunately, the development plan cannot be changed on this property! Due to subsidies, we are also in negotiations. We were at the home builder today and it looks like it will be a bungalow with a 45-degree roof.