Realistic costs for a single-family house?

Good day,

after having read along here for some time, I would also like to participate in the discussions, as we want to build soon; the plot is already available and paid for.
So far, we have designed our own floor plan, which is kept very simple, so no dormers, bay windows, etc.
The entire house should be between 130 and 140 m²; I have not included the roof slopes according to DIN.
The ground floor has about 70-75 m², and the attic about 65 m², judging from other house plans I have seen.
The house will have a basement, and the garage will be in the basement. Additional rooms might include an office, the technical room, and a small storage room.
On the ground floor, a living-dining room with an open kitchen as large as possible is planned; this should be about 50-55 m², plus a shower bathroom and the hallway.
On the attic floor, 3 bedrooms and the main bathroom with tub and shower are planned.
Regarding the fittings:

The heating will probably be an air-to-water heat pump and (of course) underfloor heating.
We are not sure about the ventilation system yet, but probably yes. (Does anyone have a tip on which one is recommended?)
Tiles should be laid everywhere in the basement and also in the ground floor. I know many find that not very cozy, but we have seen some nice wood-look tiles that will go into the living area.
In the attic, we want to lay laminate (in EL), except in the bathroom, of course.
Regarding the electrical installation, some spotlights are to be installed (kitchen, hallway, bathroom), enough sockets, LAN and TV connections in every room, but no KNX or other luxury.
The bathrooms will be neither the cheapest nor the most expensive. However, glass partitions, stainless steel strips, etc. should definitely be included, as well as level-access, tiled showers.
There should be rather ample windows, especially in the living area.
In the attic, one (floor-to-ceiling?) window per room is sufficient for us.

Can one roughly estimate how much the whole thing would cost?
We still have about 50k€ in equity but an income of about €3700, which seems rather low when I read here.
If any further information is needed, please just ask.

We built in 2013 according to the 2009 Energy Saving Ordinance (KfW70) quite similarly, about 135 m² plus a guest room in the basement (about 11 m²) and a garage in the house, with a heat pump and ventilation (Zehnder). Including our own labor (that is, the craftsmen's fees for what we had done added up) we paid about between 240 and 250 k€ including painting work and flooring, in other words, that would have been the "ready-to-move-in" but not turn-key price. Oh yes, the Zehnder is, as far as we can judge, quite okay, only the control via the standard control unit is somewhat tricky and the unit should not be in immediate proximity to other cables, which initially caused disturbances for us, but this may also be because the shielding was initially not connected to the ground. For current cost estimates, maybe a building expert can help further???

So, neighbors, friends, fellow countrymen...
In recent weeks, we have been looking at some model houses and obtaining offers, both solid houses (our preference) and prefabricated houses.
So far, we have found that prefabricated house manufacturers are reluctant to change their model houses or only do so at very high surcharges.
The solid house builders appear to be significantly more flexible in this regard.
Smaller companies are also more willing, compared to, for example, Massa Haus, Heinz von Heiden, or Town & Country.
For our plan, we now have prices ranging from about €240,000 to well over €300,000, with prefabricated houses mostly at the higher end.
Who has had similar experiences or can assess what is realistic or what are bait offers? @Peanuts also built 3 years ago; prices must have risen somewhat since then?

My layman's opinion: You are building in 2016... and not according to the 2009 Energy Saving Ordinance. Ventilation system, floor-level shower, floor-to-ceiling windows, etc..... --> With a basement, there will definitely be a 3 at the front (if it goes "badly," even a 4... depending on the equipment variants). Planning under 300k would be naive.

The entire house should be between 130 and 140 m² ... The house is to have a basement and the garage will be in the basement. As additional rooms possibly an office, the technical room and a small storage room. On the ground floor there is to be a as large as possible living-dining room with open kitchen, which should be about 50 - 55 m², a shower bathroom and, of course, the hallway. On the upper floor there are 3 bedrooms and the main bathroom with tub and shower planned. The heating will probably be an air-water heat pump and (of course) underfloor heating. Ventilation system we do not know yet, but rather yes. (Does anyone have a tip on which one would be recommended?) Can one roughly say how much the whole thing would cost?

The single-family house with 130 sqm according to energy saving ordinance; TEUR 208 Partial basement garage/rest of the basement living spaces: TEUR 65 Painting/flooring in EL: TEUR 20 Incidental building costs: TEUR 30 Reserve foundation additional costs concerning slope location: TEUR 15 (earthworks/exposed masonry etc.) Outdoor facilities in EL: TEUR 10 (only the essentials) Reserve Extras: TEUR 10 All in estimated: TEUR 358 Recommendation: throw the cheapest and the most expensive offer into the folder "P" and then look at what the BBs of the remaining providers differ in Rheinische greetings

Recommendation: throw the cheapest and the most expensive offer into the folder "P" and then see how the BB´s of the remaining providers differ

Rhenish greetings

I don’t know from which time this recommendation comes. You can have good experiences but also really mess up.

Look at all offers. Which are realistic, what can you work with. This way you can get a good feel and, if time allows, also have 2-3 conversations. Unfortunately, you also have to expect that not many offers will come back. If you eliminate 2, then suddenly it’s bare. Surely it’s more difficult with single contracts, but even the ready-made providers have quite a demand. And if you annoy too much or the margin doesn’t fit, then you also quickly get careless offers—