Planning guest WC in new construction - How big should it be? (DIN?)

We also had a mini guest toilet in the first self-built house. About 1.20 x 1.80, the door opened outwards. It's easy to manage.

Thank you for the explanation of terms 11ant. A general contractor is practically found. We will probably decide on one of two offers next week.

We are planning the semi-detached house together with our neighbors. However, they are planning a shower bathroom on the ground floor and therefore need more space anyway. We will forgo the shower and would be satisfied with the smallest guest WC in the world.

The plans are not yet professionally drawn, and I actually wanted to spare us all my amateur Roomsketcher draft here. Just for orientation, here is the draft for the hallway corner + guest WC. Measurements are only available for money with Roomsketcher.

What you see should be taken with caution:
- Guest WC with 1.95 x 1.00 m, door 76 cm, position of WC and sink not final
- Staircase 2.13 m wide
- Entrance door on the right
- Door to the living area on the left
- Door to the utility room at the top
- The swing directions of the doors are not final and have not yet been discussed with professionals
- Size of hallway, bathroom, and utility room are not final
- Lower wall is the wall to the neighbors

Fundamental: The conditions on the property are complicated. The house will be small (105-110 m²). My main concern here is still to find out whether a bathroom MUST have a minimum width.

The question is only whether one can freely decide on the room dimensions in a new building here or if one has to conform to some DIN standard.

There is no DIN specifying how big a room should be; there are guidelines for distances. For example, 60 cm in front of a toilet, 25/30 cm next to one, 20 cm next to washbasins.

we do not want to keep non-living rooms small and rather give the gained space to the living rooms.

The relation is missing! I agree with you if you reduce a 3 sqm toilet to make a rather small living-dining room (or whatever) more furniture-friendly. But if, for example, 50 sqm are available for the living area anyway, then you can also take away a sqm from it to make a 2 sqm small toilet more comfortable.
Therefore, no measurements can be specified across the board now.

Conclusion: 1 meter width and 2 meters length are more than enough.

90 x 130 cm is enough, too.
But these are finished construction dimensions. In a drawing, it is about raw construction dimensions.

Then there is also the difference whether a slim young man is standing peeing and does not need any room width next to the toilet or an overweight mother-in-law comes to visit very often and then simply needs a bit more space beside her to maintain hygiene.
Some people are fine with a small hand washbasin; others cannot manage with that at all.
Do you wish for many children washing hands together, or are you only two yourselves and mostly alone on the toilet?

Is one not allowed to plan a 1-meter-wide guest WC in a new building?

Yes, see above.

In our case, the door would be on the short wall, the toilet opposite the door, and the washbasin on the long wall.

It would be better to have the door on the wider side.

As you can see: the room size issue is very complex; already a slight omission is enough so that one is reluctant to use the 2 meters. I would always plan something like this as a whole, one floor as part of a house.
If you discuss your entire design here, you will have added value ;)

What you see should be taken with caution: - Guest WC with 1.95 x 1.00m, door 76cm, position of toilet and washbasin not final

You don't have many other options here. However, you will have the washbasin right in your face while sitting and there is no freedom of movement when standing up. The room as such (toilet) is very poorly usable.

- Staircase with 2.13m width - Entrance door on the right

Apparently the other semi-detached house is at the bottom of the plan. The planning of the upper floor is interesting with this staircase location!

The WC door must definitely open outwards and the washbasin further towards the top of the plan. Otherwise, using the WC is really impractical.

When I see this type of hallway, I see about 5 to 6 sqm of walking space without any other use. No proper space for cabinets, etc., and I also recommend posting the entire floor plan. However, do so in the area with consideration of

In my opinion, it would be better wider and shorter instead. We have 1.25x1.40m, toilet on the short side. With a reasonably decent sink, 1m width would be too tight for me...

Moreover, as the electrician nicely said to me: "You have 3 women at home, so this will be your bathroom..."