Renovate Basement with Concrete Slab on Grade

[Er Be]

Hello,
I have just moved into a house built in '73 where we have a basement/split-level. The plaster on the basement wall where the shower stands is flaking and "bulging" out.
The wall itself (see attached image) is about 70% below ground, and we want to have a fully tiled bathroom there. There's no mention of drainage, and as far as I know, none of the neighbors have drained, so I have no more information about that.

According to the inspection report, it was noted:
**Discoloration and cracks occur on the wall in the shower zone, no elevated moisture indications were noted at the time of inspection.
**There is no moisture barrier on the wall in the shower zone; water splashing against the wall should be avoided if possible, and installing a shower cabin is recommended.

My thought is to start removing the existing plaster on the wall myself and apply a new one. I need tips and advice on how to think about this and what type of plaster I should use for the future bathroom.

Questions for the forum:
What type of plaster is best suited for a basement wall in a wet room (lime-cement plaster, pure lime plaster, or other)?

Should one reinforce with primer/mortar before applying new plaster?

Should a moisture barrier/sealing layer be applied directly on plaster or should an inner wall with waterproof board be built?

Experiences in renovating a basement bathroom from the 70s – pitfalls to avoid?

 

[Er Be]

I can add that right where it bulges out, there is a downpipe on the outside going into the ground..🤷‍♂️

 

[jonaserik]

I can add that exactly where it bulges out, there is a downpipe on the outside going into the ground..🤷‍♂️

And then what. Via pipes in the ground away from the house, do you know that. Roof water should be taken 4 meters away from the house and go out at the lowest point.

 

[Er Be]

And then. Through pipes in the ground away from the house, do you know that? Roof water should be 4 meters away from the house and go out at the lowest point.

How do I know if it really does that? Where the downpipe goes is the highest point (entrance level). Could cracks or bad pipes with leaks make it try to penetrate the plaster? Trying to understand why the plaster has a "belly" on the inside.

 

[jonaserik]

How do I know if it really does that? The highest point is where the downpipe goes (entrance level). Could a crack or faulty pipes with a leak cause it to try to seep through the plaster? I'm trying to understand why the plaster has developed a "belly" on the inside.

You'll need to dig about 40-50 cm deep; normally, downpipes are connected/inserted into pipes in the ground, and the ground pipe should be about 5-10 cm above ground level to prevent soil from entering.
If you find a pipe, you should be able to remove the downpipe properly, then pour a bucket of water in and see what happens. Does it come out somewhere, or does it (municipal stormwater if available) go to a stone well or ditch? Normally, they don't crack/separate in the ground. If you have trees around the house, needles/leaves might go down into the pipes and become a blockage.
But start by pouring water into the pipes.