Floor plan, building with tree, granny flat, and existing buildings

You plan with a corridor to have access without weather influences in case of care needs and at the same time you reduce the structural requirements in your father's room. In Allkauf Haus with multigeneration there are also examples of granny flats that do not have a full floor area. But these are apartments and not rooms with bathrooms. You are living very cramped now if the 61 sqm is correct. I find the distribution of living space unbalanced. You grant your father 1 room for sleeping/eating/cooking/living that is about the size of the children's rooms. A few more sqm would not be a luxury even for a fit and healthy person.

How do you actually want to proceed now? Is there already a plan?

There is no Hamburg bonus. This "bonus" is hidden in the land prices, but the land is there. Hamburg is even cheaper than BaWü for the house price itself.

But even if you calculate with 3,500 EUR, for which you definitely get a house with upscale equipment, you end up with "only" 630,000 EUR for 180m². That leaves 170,000 EUR for demolition, incidental costs; you can certainly do without a basement for 180m².

450,000 EUR equity is available, plus land. Honestly, I don't see a problem there and wonder where it should get tight.
House connections are also available, so the expensive road excavation is not necessary.

So the plan is that we continue to do research - last week we had a few appointments with tree care specialists, and the unanimous consensus was that the tree can be pruned by a maximum of 2 meters, but not in height. But everyone also said that in Hamburg it is "well received" if you take various aspects into account already in the building application and don't act surprised later on. Therefore, as also suggested by , we are considering starting the house at the western building boundary. The current idea is 8.5m wide and 12.5-13m deep, I am sketching it out right now and when my idea is finished, I will gladly present it here for critical review ;) We are also still looking for an architect...

At the same time, we are starting to get demolition quotes in order to get a price and are continuing to clarify our budget - our equity is solid, but the monthly income is "only" average. The new KFW program should therefore suit us quite well since we planned KFW40 and timber construction without fossil heating anyway. The granny flat will probably have about 30m² in the end, I have further looked into @haydees links for that - but that's also why we still want to have a basement...

So, I have sketched something with everything in mind that I have read here over the past few years. And hopefully remembered. The site plan and aerial photo can still be found in the first post of the thread, I haven’t sketched the basement yet, the pantry has already been crossed out.

I'm still concerned about the space consumption of the staircase and hallways, but I have not found another solution that allows for a shared entrance area with basement access, from which both the granny flat and the main apartment can be reached, and where the staircase to the upper floor is not directly behind the apartment door. Also, you should be able to reach the converted attic with a fairly normal staircase.

And now I’m curious what I might have forgotten ;)

Actually, I started similarly, but thought I would wait for your plan. Now I really have to ask, how high is your knee wall? It's hard to have less than 2m with the stairs. With the development plan "ein Vollgeschoss" it probably won’t work that way.