Adding a bathroom in the attic afterwards - costs?

Also big and expensive house or decent creditworthiness. Then just finish it directly as it is intended later.

Oh, no, I expressed myself incorrectly. In total, the attic is 60-65 sqm (including sloping walls), the rest is the bedroom minus a bathroom of about 10 sqm. Then about 22 sqm remain for the bedroom if you also subtract the stair area. It's only a semi-detached house.

or there is only an unclear idea of space programs. :)

it basically comes down to whether I should take out 15,000 more in credit and then have everything finished, or save that (including a few thousand more in interest costs saved) and then later invest 15,000-20,000 for the rebuild, or whether it is significantly less or even significantly more. Everything is new to me right now. I think if I have it done directly, I have to take out the 15,000 additional credit, I don't think the developer will allow me to put the bathroom in later from my own reserves within a year.

If you don't need it urgently, I wouldn't have it built now. Developers are expensive. Have the riser pipes installed, nothing more. Later, you buy all the sanitary items yourself and include that in the tender. Then it will be significantly cheaper in my opinion.

or there is only an unclear idea of spatial programs. :)

... or uncertainty about how long one wants to stay in the non-purely better-earning residential area. If the Benz remains unscratched, the bathroom will be retrofitted; otherwise, the house will be put back on the market with the saved second bathroom investment (?)

Aside from that, the factual question can generally be answered as follows:
1. Bathrooms and kitchens become outdated in taste faster than technically. Delaying bathroom construction is all the more sensible the more trends will pass by until then. The fashion-related depreciation of the advanced investment thus opposes the tied-up liquidity or increased financing effort.
2. Standing water is a risk for pipes and the health of the pipe system users. Blind branches are best avoided altogether, i.e., even a T-piece for a later branch should only be exchanged shortly before retrofitting.

I have certainly already expressed skepticism about acquiring unused capacity when buying a home for the time being.

We faced the same question with our first house back then. The attic was initially unused, later a bathroom was to be installed. We had all the pipes for the bathroom, electrical wiring, and underfloor heating installed and then just laid cheap carpet flooring ourselves over it. 15 years later the bathroom was finally realized, and the amount of tradesmen and dirt that had to go through the entire house each time was enough for me….

We faced the same question with our first house back then. The attic was initially unused, later a bathroom was to be installed...

Apart from the craftsmen and the mess, did everything else work out? Was the decision to build it only 15 years later a good one?

... or uncertainty about how long one wants to stay in that not entirely pure higher-income residential area. If the Benz remains unscratched, the bathroom will be retrofitted; otherwise, the house will be put back on the market with the saved second bathroom investment (?) ..().....
I have certainly expressed skepticism about acquiring capacity not immediately needed for a home purchase.

For me, the situation is not easy; the costs are already very high, and wherever you can save smartly, it definitely makes sense. Therefore, every tip here is valuable. I'm not the type driven by luxury; I want everything with the best cost-performance ratio, weighing what makes sense in the long term. Of course, still a kind of luxury—not living in a rental apartment where you can be thrown out and it’s not even worth doing much or changing anything at all. Hence, I don’t need a Benz but rather an e-car + photovoltaic system in the next few years, with which I can then save in the long run since I drive very many kilometers.
Thanks for the tips on depreciation and standing water; I will definitely take those into account!

If you don’t need it anytime soon, I wouldn’t have it built now. Developers are expensive. Have the risers installed, nothing more. Later, you buy all sanitary items yourself and put that in the tender. Then it will be significantly cheaper in my opinion.

What I haven’t thought about at all: For building a bathroom, a complete partition wall would also have to be installed in the attic. I don’t even know if this wall has to comply with any regulations; in the drawing where the bathroom is included, some really thick walls are drawn, and all sanitary facilities are attached exactly to this wall.
Regardless of that, I take away that it’s probably still best to retrofit the bathroom sometime later and have the pipes laid up, avoiding standing water.
So somehow I even benefit twice financially: The bathroom doesn’t have to be financed with expensive interest, and the land transfer/notary fees are also omitted then. And I still have to research what this has to do with the tender; it sounds like one could deduct something tax-wise… I have absolutely zero knowledge about taxes.
Furthermore, it would even make sense to initially forgo the garage and photovoltaic system and install them afterward for the same reasons. Only pre-laying the corresponding electrical lines/pipes to the garage/wallbox, etc.

All these are very helpful tips for which I am grateful; my question here has at least brought me a lot :)