Extension of a residential house in bungalow style with a 30-degree hip roof

Hello, I had written that above in the text:

Sorry, I missed the information about the heating.

So far there is a foldable loft ladder, but we have already planned a larger opening for a proper staircase in the reinforced concrete ceiling (it is now covered up).
Since I am not an expert, I had hoped to find some hints and tips here.

Then feed the experts here (with the completed questionnaire, floor plan, and sections of the existing house) and express your wishes or motivation: are you becoming frail and want to create space for a caregiver, or quite the opposite, have you been unexpectedly blessed with triplets?
By the way, I can basically very well imagine it makes sense to only build up the main leg of the L. However, my first choice would probably be a change of property; a bungalow built in 2016 with "so far completely sufficient" 170 sqm is popular on the market.

Give me some data from the soil report (soil composition). The foundation is only on 30 cm of gravel? I haven't read anything about strip footings? As it reads, the soil bearing pressures might not be sufficient.

Please provide some data from the soil survey (soil structure).
Is the foundation only on 30 cm of gravel? I haven't read anything about strip foundations?
As it reads, the soil bearing pressures might not be sufficient.

It is a complete slab foundation (no strip foundation).

Gravel filter layer d=30; gravel filter layer under the slab foundation, supply, install and compact.

Slab foundation C 20/25 reinforced d=20
Slab foundation made of reinforced concrete C 20/25
reinforced 2 x Q257 + required supplements
on existing gravel subbase including edge formwork share and foil; supply and manufacture.

The soil survey contains these data:

Soil type (DIN 1054): organic soil up to 0.70 m below ground level (GOK), thereafter coarse-grained soil, non-cohesive
Soil class (DIN 18300): 1, 3, 4
Ground and/or layer water: At the time of drilling, groundwater was encountered at 1.60 m depth below GOK.
Density of deposition (DIN 1054): There is a dense deposition (0.5 ≤ D < 1.0).
Allowable average soil bearing pressure at 1.00 m & 0.50 m foundation width (DIN 1054 Tab.1): 270 kN/m²
Design value σR,d of the bearing capacity of the base plate at 1.00 m embedment depth & 0.50 m foundation width (DIN 1054:2010-12 Tab A 6.1): 380 kN/m²
Stiffness modulus cal ES (EAU): 25 MN/m²
Subgrade reaction coefficient ks: 20,000 kN/m³
kf value: 1.3·10⁻⁴ m/s to 1.0·10⁻⁵ m/s

The residential building is to be constructed without a basement.
Groundwater was encountered at a depth of 1.60 m below GOK at the time of drilling. It can rise during floods in the river up to 0.90/1.00 m below GOK (Fe++ deposits).
The encountered subsoil conditions on site are ideal for the construction of a single-family house without a basement. The existing gravel sand to medium sand, gravelly to strongly gravelly, lies dense to very dense and was only drilled through up to a maximum of 3.00 m.

The expert report does require a foundation up to 1 m deep. So that the organic layers up to 0.7 m are completely penetrated. I think as an alternative, a soil replacement of at least 0.7 m should have taken place. So 0.3 m is clearly too little. Would also explain the cracks in the existing structure. So if it was really founded like that, then it is already too little for the existing structure. An extension should then not be done...

Is it really possible for the structural engineer to significantly underestimate a foundation with the anxiety surcharges encountered everywhere nowadays? Then also to ignore the subsequent trade (shell construction)? I really don't want to imagine that properly. The potential damage is huge, and someone has to take responsibility for it... Do you have pictures from the construction phase, especially of the excavation pit and BP etc.?