Construction costs are currently skyrocketing

I based the slab on 120 EUR /m², which is supposedly still the price according to the web. But let's say 150, then rounded up you're at 13,000 EUR.
I had a total of 9k for earthworks and I had to replace 1.2m, which is not exactly easy. There are soils where the 30cm often included in [GU contracts] is enough to remove.

But for my sake, let's add 20 TEUR.
Then we're at 2300 EUR/m².
That's already much cheaper than the average otherwise. Requires a lot of personal effort and it's still much more expensive than some projects here with their 1700 EUR/m² or whatever there was sometimes.
So you can definitely notice the cost increases even in the DIY sector.

Yes, because even if you do a lot yourself - you cannot avoid the significantly higher building material prices. Compared to "having it done," however, it is surely still interesting for some. I would also prefer to do it myself before giving up the dream. We have always done it that way with our renovation so far.

Self-performance at Massa Haus is already feasible. The entire insulation and cladding of walls and ceilings can also be done without much DIY experience. Instructions are easy to understand. It gets more difficult with the installation work. But that can also be outsourced. However, the coordination of the individual trades should not be underestimated when some parts are done in self-performance and other parts by professionals. Furthermore, the time factor should not be underestimated. We renovated back then with three people. Always in the late afternoon until late at night. Six days a week. It took almost 10 months. Electrical and heating installation was outsourced.

One must also consider that during the DIY work, the loan is already being serviced as a rule, but one still lives in a rental.
That means, in addition to a month of DIY work, there are also 800, 1,000 or 1,500 EUR rent + additional costs to pay.

My tiler has tiled the kitchen (17m²), hallway (12m²), upstairs bathroom (10m², barrier-free shower) and the ground floor WC (2.5m²) in 2 weeks and is still not finished. Grouting, silicone and bathtub still need to be done. He has worked 10 days so far, 8-9 hours each day.
For an amateur, you can roughly estimate double (semi-professional, who has tiled a few times before) to four times (absolute beginner, mixing tile adhesive for the first time) the time. Besides work and family, I can manage to free up at best 25-30 hours per week, most of them on Saturdays and Sundays.
So for the tiling alone, I would have needed at least 10 weeks. That would have been 2.5 months’ rent just in my case, so the tiler would almost be "a bargain"...

DIY saves money especially when you do it while already living in the house or when you live rent-free.
In all other cases, you have to calculate very carefully; as a rule, it is not worthwhile or you save much less than you think.

Above all, one should not underestimate how much it takes out of you to work on the interior finishing every spare second next to your job for months or even years. After 1-2 rooms of drywall, you simply get fed up... but there are still 7 more rooms to go ;)

That's all true, but you can already sacrifice (approximately) your annual vacation two or three times for personal work. You can then work hard for days or weeks. Quite a lot can be done then. You work long days, the ratio of output to setup time improves, you are in the flow, etc. You (hopefully) don't go on vacation during or shortly after the house construction anyway.

My tiler has now been working for 2 weeks and is still not finished. Slope screed, shower waterproofing, facing walls done, sanding the floor, primers,... are taking quite a while. The actual tiling, I believe, is the smaller part. Although, of course, you can avoid some problems here. If you install it yourself, then there is just a shower with a low tray. That is less work and above all, there is less chance of making mistakes.