Attic expansion - how to supply hot water?

Hello,
my parents-in-law and my wife and I have taken on the big project of dividing a terraced house into one unit for the seniors in the basement and one unit for the juniors with expected offspring on the raised ground floor and attic. The conservatory, guest toilet, laundry room, workshop, heating (in the basement top right), and sauna are used communally.

The attic has a floor area of 74 sqm with a base height of 1.02 m and a roof pitch of 23°, so not much area with a height over 1.8 m results. Access is from the conservatory via an open staircase.

So far, only the supply and wastewater pipes as well as the heating pipes run in the area where I have drawn in the toilet. Except for the chimney, the room is completely open.
Where the pipes come from the basement, we plan our bathroom (our bed is to stand in the bottom right and a work area is to be created on the left). Under the skylight, a bathtub (my wife bathes every other day) is to be installed, and a double washbasin is to be installed on the recessed wall on the right.

This is now supposed to be specifically about the hot water supply.
The easiest way would be to connect the hot water from the basement to the bathtub and washbasin. However, since no circulation pipe is installed, it will probably take forever for warm water to reach the washbasin (the pipe would be about 20 m long).

Therefore, our idea would be to install an electric instantaneous water heater directly on the back of the washbasin (theoretically a small instantaneous water heater would be enough), but continue to take warm water from the basement for the bathtub.

What do you think, which type of hot water supply is most sensible for our purpose?

Best regards from
Babenhausen

Is a circulation pipe from the basement really that complicated? I would prefer that. For a tankless water heater, you need a separate supply line. That doesn't make it any better either. Alternatively, at most a boiler (hot water tank).

Hello,
thanks for your reply.
It's just the question of what makes more sense.
I asked a plumber. The circulation line with pump would cost around 600 €.
But we probably only need the bathroom in the morning to freshen up and in the evening to get ready for bed, and the pump would run all day, or the water would be kept at temperature (hot water tank).
What do you mean by a separate supply line to the instantaneous water heater?

An E— instantaneous water heater also needs its own supply line and must be registered with the utility provider, depending on the power. Then you can also take care of the entire water supply upstairs with it. A circulation pump can also be time-controlled via a timer. I would not install both. Either — Or.

Instead of a timer switch, you can also use a staircase timer.
Push button at the sink in combination with a socket.
This way, the pump can circulate warm water at any time for a self-defined period whenever desired.

(Für die Schotten)

The continuous flow water heater is not getting any cheaper either. So the circulation variant is actually not so bad at all.