ᐅ Semi-detached house, upscale fittings: Are the additional costs accurate?

Hello,

Yesterday, we met with an architect who showed us a plot of land for a semi-detached house. The other half is already built.
The house has 140 sqm (1507 sqft) of living space, although a large part of this already includes the basement, as it is usable living space (with a window).

The house will be finished to a high standard (I will copy the real estate listing at the end here).

The architect will act as the general contractor and work with local craftsmen.
For additional work such as extra electrical outlets, different bathroom fixtures, etc., I will need to contact his respective craftsmen, who will then provide me with the costs.

Now to the main questions:

The house comes only with a gas heating system and radiators in the rooms.
He said if I want underfloor heating, it will be operated by an air-source heat pump.
The extra cost for this system plus underfloor heating is about €15,000 (about $16,200).
This sounded reasonable to me as I didn’t have any idea before. On this forum, some people were shocked when someone had to pay €8,000 (about $8,600) more.
What do you think about my price?
Also, he kept saying that this air-source heat pump is a form of solar technology, because it extracts heat from the air.

Furthermore:
There are additional costs of about €18,000 (about $19,400) for earthworks, sewer connections, and utility connections.
Is this reasonable?
He said this includes excavation and laying the connections from the street to the house (electricity, water, etc.).

Another extra:
We could install a heat recovery ventilation system with air filters for about €15,000 (about $16,200), similar to what you find in passive prefab houses.
He estimates this could improve the energy efficiency rating from KfW 70 (the usual) to 55.
Is it worth the extra cost, also considering the health benefits?
I am allergic to pollen and suffer from extreme eye itching/burning, throat itching, etc.

These were my main questions so far.

Additional points:
I forgot to ask about the cost for electric shutters. What would you estimate?
And approximately how much would a garage cost?

Is a bedroom with a walk-in closet sized at 15 sqm (160 sqft) big enough? It seems a bit small to me.

Finally:
The plot is on a slight slope. This means roughly half of the basement is below ground level, or put differently, from the terrace there are a few steps down to the garden.
On the left side is the other semi-detached house, on the right side there is a stream, and behind the garden, directly adjacent without any gap, the forest slope goes up.
There is occasional wildlife visiting (according to the neighbor: mice, two rats, a marten’s nest somewhere nearby, a young deer, wild boar).
How would you assess this kind of plot? We are basically city people.

That’s all from me for now. Thanks for your help.

Here is the listing:

+++ NO COMMISSION +++ “BAUHAUS STYLE” IDYLL BY A STREAM – with Basement

Purchase Price:
289,820.00 EUR

Rooms:
5.0

Living Area Approx.:
140.91 sqm (1,517 sqft)

Plot Size Approx.:
420.00 sqm (4,500 sqft)

Main Features
House type:
Semi-detached house
Usable area approx.:
16 sqm (172 sqft)
Number of floors:
3
Bedrooms:
4
Bathrooms:
1
Guest toilet:
Basement:
TV / Internet / Telephone:
Electricity consumption: from 2,500 kWh*

Costs
Purchase price:
289,820.00 EUR
Buyer’s commission:
None

Building Quality
Construction phase:
House in planning (designed)
Standard of finishes:
High-quality
Heating type:
Central heating
Energy certificate:
Not yet available

Property Description
Idyllically situated by a stream, secluded at the edge of the forest, a planned, attractive detached family home with a basement will be built here, turnkey and solidly constructed. This spacious and modern energy-efficient house can be customized with the architect to meet your personal needs, including changes to the floor plan and finishes for your “dream home.” - the open living/dining area is spacious and bright with lots of glazing overlooking the garden and terrace - room layout changes possible - master bedroom with walk-in closet and two children’s rooms - daylight-filled comfort bathroom - generous living/work room in the garden-level basement

Features
“Masterful craftsmanship by local specialized tradesmen” - finished basement - energy-saving, breathable natural brickwork with additional external insulation for energy-conscious comfort - heating system with efficient gas condensing technology, optionally with solar collectors and underfloor heating - family-friendly daylight bathroom with tub and shower - ceramic sanitary fixtures by “Villeroy & Boch” - bathroom and guest toilet tiled in granite - smoke detectors in all bedrooms - partially glass interior doors with decorative balcony railings on the upper floor - interior window sills in “Carrara Bianco” marble - solid wood veneer interior doors with stainless steel handles - electrical switches by “Gira” “Everything is possible, nothing is mandatory... let your wishes run free.”

Additional
Availability: approx. 16 weeks construction time after start
Floor plans and elevations may include optional features at extra cost.
Turnkey means in this offer:
- including plot
- including finished basement without plot-related costs such as earthworks, sewer and utility connections
Floor coverings and painting/wallpapering work done by buyer