Construction project, estimate construction costs?

Hello!
The house planning is slowly approaching, as the house comes before the children planning. I would appreciate the assessment of people who have already gone through everything.

Here is the financial situation:

Current expenses:
Rent, electricity, phone & internet, GEZ 475 €
Food and daily necessities 300 €
Car A 1% rule -- €
Car B 280 €
Fixed costs 1124 €

Net income A 1330 €
Net income B 2050 €
Total income 3380 €
(Both still tax declaration. 1)

Surplus 2256 €

Then there is still what you need for living otherwise, occasionally eating out, clothes, etc.

Currently, we save between 1500€ and 2000€ per month.

We plan to have 2 children (not yet existing)

The income will increase by about 2000€ (net) in about 2 years.

Then construction should also start roughly at that time.

The house should be between 150 and 180 m² in size. Without a basement, without frills.
We don’t need a garage either, a carport is enough.
The garden should also be simple, so that the lawn mower robot can get through well!

Plots cost here in the countryside between 50 and 100k€.

Attached is a plan (approx. 160sqm).
I have taken a house with a floor plan that we like and altered it a bit with Photoshop. It can also simply be a cube; the floor plan does not have to be offset. Please don’t pay too much attention to the details; that still has to mature. But the layout of the rooms should be roughly like this. Guest room = fitness room.
We don’t need the balcony.
The house is in Bauhaus style, as I am quite tall and don’t like sloped ceilings that much.
KFW 55 would be desirable, also residential ventilation with heat recovery.
And underfloor heating! Mandatory!

The house should be built largely without own work, painting, laying floors, etc. I can do myself.

A relatively low rate of about 1200€ per month is planned. We prefer to save the remaining money on the side to have a financial buffer. If everything goes well, special repayments should be made with it, or used in the follow-up financing after ~15 years. The term may be 30-35 years.
It is to be expected that the interest rates will be very low, as all business accounts are also run through the same bank.

Unfortunately, I can hardly estimate what a house of this size costs.
I have roughly estimated:
House 300,000€
Plot 80,000€
Additional costs: 35,000€

Is this a realistic plan?

What stands out:

Cost of living with EUR 300.- calculated too tight?
Car costs A not specified.
What about the second income when the children arrive? Still EUR 2000.- p.m. increase?
What do the calculated incidental costs of 35 TEUR include? Does this only refer to construction or are acquisition incidental costs included?
If acquisition from the general contractor: Be aware that the land transfer tax might apply to the total volume (including construction costs).
With the currently low interest rates, banks prefer higher repayment rates. This also affects the conditions. Therefore, calculate exactly what should go into repayment and what into the "reserve".

Hello! The 300€ are not living expenses. They are food and daily purchases, or rather the average over the last 7 months. We were both students and know how to manage efficiently. We keep records and do monthly accounting, so we know this exactly. However, these are current figures, and of course, they can increase. Car A is, as mentioned, a company car (1% rule) and is therefore already deducted from the net salary.

The main salary remains full-time, the second one decreases (but with a high home office proportion).

For the cost calculation, I honestly took the construction costs of a good friend and scaled them proportionally. I have no idea about that!

Hello! The €300 are not living expenses. They are groceries and daily purchases, or the average over the last 7 months. We were both students and know how to manage efficiently. We keep records and calculate monthly, so we know exactly. However, these are current figures, it could of course increase. Car A is, as mentioned, a company car (1% rule) and is therefore already deducted from the net salary.

The higher salary remains full-time, the second decreases. (but with a high home office proportion).

For the cost calculation, to be honest, I took the construction costs of a good friend and scaled them proportionally. I have no idea about that!

OK, it comes down to the same in the end, but as mentioned, it seems relatively tight either way, especially if children are involved.

Salary: The question remains how realistic the increase forecast of EUR 2000.- p.m. is if Salary A disappears. Does Salary B then increase by EUR 3,330.- p.m. (without considering child/parental allowance)?

Cost calculation: Secondary purchase costs including property transfer tax can generally be calculated at 10%. Of course, the question remains whether the property transfer tax is calculated only on the land purchase or also on the construction costs, see my previous remark.

And above all not to forget: equity? Of course, with appropriate conditions, one can also aim for 100% financing (or even more), but usually with significantly higher repayments and not quite as favorable terms as, for example, with 20% or 40% equity.

Yes, I don't disagree with you there either! That was just the listing of the actual costs, not the calculation for the future. The 2000€ refers to my salary. In 2 years, I will take over my parents' company. My girlfriend's income will surely increase as well, but that is not reliably calculable. Besides, she will definitely switch to part-time as long as it makes sense. At the start of construction, there will be about 100,000€ in equity, assuming no major unplanned expenses occur.

That means if I have the house built in a "package" by a company, do I pay the real estate transfer tax on the entire amount?

That means you will be self-employed at the start of construction and your wife will be on maternity leave?