I wouldn't know how the entrance from outside could be designed now. Since we have such a small property, we didn't want to have another stairway outside. And since we also have to get into the basement, this solution seemed right for us.
I have to discuss the budget with the construction company again. They offer everything from a single source and we have a contract with them for all services at a fixed price. I'll clarify this again
I understand the problem that you don't want to take even more away from the garden. But as far as my lay knowledge goes, a basement access is allowed in the setback areas. That means I would consider a stairway from which you can get from outside into the hallway of the granny flat. On the upper floor, as already said, possibly a landing stairway at the place where the stairs to the granny flat are now. This stairway once down to the basement, into the hallway of the granny flat (on the ground floor you could provide a door under the stairs to the basement entrance, so it would be closed downward and you can still go down if needed) and once up to the upper floor.
Then remove the straight staircase. The landing stairway is shorter, so the kitchen would gain in width. Rotate the guest toilet by 90 degrees and place the wardrobe in front of it. This would make the entire ground floor open.
Or then place the toilet on the right side in front of the kitchen.
Just roughly put into words, of course some things would have to be rearranged accordingly.
But tell me, how do you want to proceed?