Costs for shell construction and roof covering/insulation - fixed price offer okay?

I am trying to check whether the price offered to me for a house construction is reasonable.
Model: An architect builds houses at a fixed price through his company. He buys a plot of land, demolishes the existing building, and constructs two semi-detached houses on it. I am offered the plot + demolition of the existing building + shell construction + roof covering/insulation/gutters + garage at a fixed price (notary basis). The interior finishing is contracted individually by me; I have offer prices by trade, executing company, and service description from the architect. The architect does not earn anything more on the interior finishing (I pay only a €5,000 flat fee for his site manager), i.e., his profit is completely included in the "notary basis."
After deducting the market price of the plot, passed-on incidental purchase costs + 0.5 demolition costs of the existing building (including cellar) + subdivision, there remain:
About €400,000 as a fixed price for planning + shell construction + roof covering/-insulation + garage + profit:
Here is my estimated cost breakdown based on my research (I don’t have expertise myself) with the request to check whether this roughly fits:
Details: 1.5-story semi-detached house, 8x13 m, attic developed as living space, waterproof concrete cellar, construction according to the 2016 Energy Saving Ordinance, approx. 1000 m³ enclosed volume (my calculation based on section).
1) Earthworks + shell construction: approx. €150,000?
(Cellar: 36.5 cm masonry or concrete, partition wall neighbor made of sand-lime brick; masonry: load-bearing: sand-lime brick + 120 mm core insulation + facing bricks (€0.5 per brick), non-load-bearing: sand-lime brick/brick/plaster walls; ceilings: reinforced concrete, stairs from cellar to attic: reinforced concrete)
2) Masonry + facing bricks garage 9x5 m: approx. €30,000?
3) Roof covering + insulation + gutters: approx. €50,000?
(approx. 160 m² roof area (my calculation based on section), roof truss made of softwood, quality concrete tiles on underlay, gutters + downpipes in zinc, insulation meets the requirement for living space attic and attic floor)
4) Architect and engineering services: approx. €35,000?
Subtotal costs: €265,000
Remaining amount (€400,000 - €265,000): €135,000
- thereof VAT: €22,000
- thereof profit/margin: €113,000

a) Does the allocation of the fixed price appear realistic?
b) Is the margin/profit market-typical – considering that there is no additional profit from the interior finishing?

Thank you very much for any input in advance!!!

That is extremely expensive. The profit margin is very, very high. We are only talking about the shell construction without windows, plaster, or interior fittings.

You have to calculate about 250,000 euros more for the rest. Are you aware of that?

You still have to calculate about €250,000 for the rest. Are you aware of that?

I didn't want to overload the thread with the topic: For the rest, I have a calculation of €150,000 broken down by trade and company. That also seems very low to me. The construction service description partly has a rather lower standard. I have sent change requests with a request for pricing to the architect. Nevertheless, he must have extremely good prices if he wants to purchase the rest for €150,000. For example, the design has considered very large window areas as well as high ceilings (ground floor 2.90m clear height) with correspondingly high window heights and exclusively wooden windows.

Yes, you see, that will never work. Your price calculation above is correct and shows how high the developer's offer is. 150,000 for the rest is therefore more than unrealistic.

50 heating/plumbing, 20 electrical, 40 plaster, 5 painter, 30 windows, 10 shading, 30 floors, 5 garage doors, 4 front doors, 7 interior doors, 7 screed, 10 stairs, 20 extras like stove, ventilation, roofing etc.

Profit in property development should not fall below 15%, but rather be around 20-22%. Since you have no idea of the actual costs, you cannot make any reliable statement about the price or the architect's profit here.

This means that with production costs of T€ 550 (estimated by you), the house should be "purchased" at around T€ 450 so that the architect makes a reasonable profit.

I will now simply reproduce the calculation from a preliminary discussion with a [GU]: 1,000 m3 x 400€ = 400,000€ One turnkey house plus fees, outdoor facilities, garage, special requests, etc. Ready-to-build plots of land are currently being sold by us at about 50-100% above the standard land value. :-o Now you can calculate it yourself again.