Unrenovated single-family house from 1973 - Renovation or new construction?

It does not surprise me that they are cheaper to renovate per m².

There is not much difference compared to private renovators. The professionals have, for example, much higher ancillary construction costs including financing costs than you do. You have the advantage that you can take the small local craftsman around the corner.

Only the interior walls that we no longer wanted have been removed. Those that fit can be left standing.

A question for understanding: When you talk about gutting, do you actually mean removing interior walls? Why would you do that in a house with good substance whose floor plan essentially suits you?

Technically speaking, gutting is actually the wrong word. Of course, as many walls as possible remain standing, including the staircase. But it is very common to create an opening through a load-bearing wall between the kitchen and dining room, and other smaller changes in the layout to make space for a second bathroom or something like that. Also, you no longer make walk-through rooms, or maybe you do again for a dressing room. If you like the floor plan, that's great.
Personally, I also had the opening, plus a new guest WC on the living floor that was not there before, WC walls removed in the basement, and the entire attic had to have the walls removed. In addition, completely new interior plaster. I think I can rightly speak of a core renovation.

Thank you also for the further responses. A question for understanding: When you talk about gutting, do you actually mean removing interior walls? Why would one do that in a house with good condition, whose floor plan basically suits one? The (if I have calculated correctly) 158k from don’t seem so far off to me compared to the estimated 270k for us. The two years of own work definitely have their value, architects don’t work for free, and unfortunately, we hardly know any craftsmen personally. Complexes with 150 apartments as described by are obviously a different situation than a single-family house. It doesn’t surprise me that they are cheaper to renovate per m². : Unfortunately, I don’t have a comprehensive list, only the total estimate of 270k (incl. tax + architect) and a few individual items:

[*]Windows+Doors 25k [*]Scaffolding (for the following items): 3k [*]Roof covering: 30k [*]Roof insulation: 15k [*]ETICS (EPS): 35k (incl. balcony)

We removed all ceilings because they were partially sagging, and otherwise, it is difficult to carry out a complete new installation of the electrical system. Furthermore, we completely removed 4 walls and rebuilt one or sealed up doors with bricks. In the remaining walls, a lot of chasing for electrical installation takes place. We were able to keep the base plaster on the 2 floors. Only in the "basement" the plaster had to be completely chipped off. Also very expensive and laborious was the installation of underfloor heating across all 3 floors. Unfortunately, this causes extremely high costs and effort (removal of floorboards, leveling the floor, floor insulation, screed, moving all lintels upward, numerous core drillings for pipes etc.). Consequently, I consider 270k for a renovation with an architect depending on the scope of services to be quite realistic. It really depends on how extensive the renovation is.

We took down all the ceilings because they were partially sagging, and otherwise it is difficult to carry out a complete new installation of the electrical system.

Old shack? The OP's house is from '73 – so reinforced concrete ceilings. It’s hardly comparable to your house. Plus, only two instead of three floors need renovation.

Old shack? The OP’s house is from ’73 – so reinforced concrete ceilings. Not really comparable to your house. Plus only two floors to renovate instead of three.

I count more than two floors:

The house is from 1973, 2 full floors, large attic, fully basemented, has about 160 m² of living space

The legal building consideration might be two floors, but basement, ground floor, first floor, and attic make four levels. That is also my reality, reflected in renovation costs for the stairs, stairwell, etc. being correspondingly high.