Adding a bathroom in the attic afterwards - costs?

I am currently facing the decision of whether to opt for a fully finished (standard) bathroom including its own underfloor heating, including tiles etc., but without a bathtub, in the attic (underfloor heating is present) in a new build. The costs would be about 15,000 euros. If I save this cost initially and then decide after a few years to retrofit a complete bathroom (I would have it fully done including tiles and all sanitary facilities), would such a retrofit probably be similarly expensive or even more expensive? All pipes, hot water, sewage, electricity, everything would have to be completely retrofitted. I received the tip from a close circle to leave it out at first and then have it built later for significantly less money. Somehow, however, I doubt this statement. I simply have zero experience and cannot at all assess whether such a retrofit would be feasible for 6,000 or 12,000 or even much more. Thanks in advance!

For me, it would be much more interesting to know whether I will ever actually need the additional bathroom in the attic.

All pipes, hot water, wastewater, electricity, everything has to be completely retrofitted.

You could have the lines laid already. And then decide later what happens. What is the plan if you don’t install a bathroom there? How will the room be used then?

If the bathroom is located above a bathroom on the floor below, then I would have all the pipes pulled up and capped. If this requires going across the house, then I would leave it.

So far, the plan was that if a bathroom is in the attic (which is then located almost directly above the bathroom on the upper floor), then the rest of the attic (a total of about 60 sqm) would be used as a bedroom. And on the upper floor, no bedroom but rather several hobby/work rooms. But if the bedroom were to be on the upper floor, then the attic would get, for example, a study or fitness room + another room, or if a child is about 20% likely to come along later, it could be used there as a teenager and wouldn't always have to go down a floor with its own bathroom. But it's already a good idea to have the pipes pre-installed; I'll mention that. I just wanted to get an assessment. It could also be that retrofitting a bathroom would be really complicated and one would definitely recommend having it done directly with the new build.

then put a bedroom in the rest of the attic (approx. 60 sqm in total). and on the upper floor no bedroom but only several hobby/work rooms. But if the bedroom were to go to the upper floor, then the attic would have, for example, a study or fitness room + another room,

So a big and expensive house or proper creditworthiness. Then finish it directly as it is intended later.

So a large and expensive house or proper creditworthiness. Then finish it directly as it is intended later.

or there is only an unclear idea of room layouts. :)