Is building a semi-detached house sensible despite low equity with a long loan term?

Had danixf overlooked his post before sending mine, can only agree with that!

So, we have now organized all finances over the past few weeks and also included our few special requests in the house price. The equity we want to contribute has changed minimally. Perhaps this assessment will be easier for you.

Construction costs (incl. land): 643,000 euros
Additional construction costs: 73,500 euros
Equity (incl. own services): 77,500 euros
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Loan amount 639,000 euros

For outdoor facilities, additional furniture and kitchen, we have set aside another approx. 35,000 euros (calculated conservatively), which we have already saved or can still save until the time we need the money. Some of the kitchen appliances and many pieces of furniture will be taken from our apartment (they are only a few years old). The outdoor facilities will be done on our own (as mentioned, my father and I are quite skilled with crafts and have already paved several terraces, balconies, or paths, as well as built our own carports).

Of this, 100,000 euros through KFW153 and 539,000 euros as an annuity loan through a bank. The commitment-free period would be 18 months, we have 5% special repayment per year (+ KFW431 for the energy consultant) with a fixed interest rate for 30 years. Currently the loan offered to us is at 1.82% (2,180 euros/month), since the interest rates (at least for KFW153) have been revised downwards, our bank wants to send us an updated offer. Let’s see what changes here.

You can combine KFW 124 with 153, right, or are the conditions bad?

You can combine KfW 124 with 153, right, or are the conditions bad?

For us rather uninteresting, since we are already taking KfW153 and do not want to take more from KfW. It would initially lower the interest rate, but the fixed interest period is only 10 years; who knows what interest rates will be imposed on us afterward. The remaining amount from KfW153 would, under current conditions, theoretically be payable after 10 years with a one-time payment of the residual debt.

Break down the construction costs a bit more precisely. What does the land cost now, and what does the house cost? Do you already include "more expensive" special requests in the house, like additional windows, roller shutters everywhere, additional shower, etc.?
Otherwise, I find the 35k somewhat critical. Just roughly estimated.
5k for outdoor facilities in EL standard
10k kitchen with a few new appliances
5k for new furniture

That leaves 15k for the sample selection. And the money really goes quickly there. In the bathroom, you easily spend a few thousand. Especially washbasins and base cabinets cost a lot of money. Of course, you can also do that later.
Electrician – the standard equipment is basically a joke.
Socket on its own circuit (for example if stove and oven are separate) 80-100€
Normal socket 30€
LAN socket 125-150€
Multimedia channels 150-200€
Each LED spot 150-300€

So a moderate four-figure amount goes quickly. Rather even five-figure if you take some extras.

Windows/front door/patio doors – different handles, low thresholds, security packages, or other locking mechanisms.
That can also easily add up to 2-3k.

Stairs – usually the standard is light beech. Oiled, stained, or other posts also cost quite a bit. We paid 3k per staircase and still held back. It can go a lot higher.

Quick question for further discussions, do you want help, discussions, and support regarding construction costs and the construction project, etc.?