Planning extension (foundation/load-bearing capacity/alternatives)

Hello,
I am Peter and new here.
Currently, my 8-family house is troubling me.
The house was built in 1957. Actually, it is 2 adjacent 4-family houses with 2 apartments next to each other and 2 above each other in each. All apartments are about 57 sqm each.
The previous owners renovated quite a bit over the decades, but not the roof, and it is worn out.
Since the stairwell leads up to the attic, the idea came up to add another floor with 4 more apartments (side by side).
The city building authority has also approved it. Unfortunately, there are no old plans, not even in the city archive, so the structural engineer instructed to expose the foundations of the load-bearing exterior walls and the central wall in one piece.
And here it ends. All foundations are only as wide as the load-bearing walls (30 cm) and the statics are not sufficient according to today’s standards, even for the existing structure.
For an additional floor, all load-bearing walls/foundations would have to be reinforced.
However, for the building length, the reinforcement would allegedly cost no less than €100,000 just for the foundation strengthening.
I also do not want to impose this intervention on the old house.
Currently, neither the architect nor the structural engineer offers a solution.
For example, the idea occurred to me to build the apartments almost self-supporting with steel columns/steel beams over the existing building at a height of 5.5 m. Visually, I find steel beams along the existing structure appealing and thus would decouple the new apartments from the weight. Everyone still looks at me incredulously, but I am looking for solutions and the architect as well as the structural engineer seem somewhat conservative to me at the moment.

In this forum, I hope for national and many suggestions.

Thank you very much.

Peter

Everyone is still looking at me incredulously, but I am looking for solutions and architects like structural engineers seem a bit conservative to me right now.

I am certainly conservative as well, and I wholeheartedly share the incredulous looks. With a property of this age with apartments all of the same size (= homogeneous tenant mix), you have completely different problems than placing a penthouse floor on stilts above it. 30 cm exterior walls, even if made of pumice, are no longer a big deal on the market. If there are no plans left from the house, good luck with the renovation of the risers of the (probably still lead, if not even cast iron) water pipes, and the extension of the heating system upwards. Also expect additional chimneys to be retrofitted. In Voreifel / NRW, you will find here in the forum the nearest expert (however for realistic renovations / modernizations) in Münstereifel.

The problems you described are known and not the issue. The problem remains an alternative due to the statics. A new roof only costs money but does not bring in income, which is why so many houses are simply extended upwards to compensate for the high costs with rental income. Installations, etc. have been considered. If the statics had been as the architect suspected, everything would already be profitable. Now the question arises whether it can still be profitable to extend upwards given the tight housing situation, and here I only see a load transfer that either does not act or only acts to the extent of the old roof on the building. Why should I retrofit chimneys? I have enough dead chimneys since renovations have already taken place in the last decades. I almost believe I am better off with a system hall builder. I can only create profitable affordable housing if I can still add something on the existing plot.

Now the question arises whether, with scarce living space, it is still profitable to add extra floors [...] I almost think I am better off with a system hall builder.

I didn’t know that joke yet – but I did know their prices

OK, then I don't understand your joke. I am seriously looking for a solution, but you also seem to know no alternative to the classic foundation reinforcement. Maybe others will still respond.

OK, then I don't understand your joke.

The joke was yours and consists in wishing for a special solution to be more economical than a traditional one.

but you also don't seem to know any alternative to classic foundation reinforcement.

Putting the house into a system hall without walls would not be called an alternative here – at least not economically to simply renovate the roof without any gain in living space.