New single-family house construction in 2025 possible on a low budget?

Better to pay taxes on income than to have no income. Otherwise, you just go part-time.

Better to tax income than to have no income. Otherwise, you just go part-time.

But I assume (at that price) that the return after taxes is significantly below the interest for the new loan. The maintenance reserve also adds an additional burden. So the OP pays out of pocket every month. And the sharply increasing CO2 tax can also really cost the landlord a lot of money. Of course, you can speculate on appreciation… … or get annoyed when, for example, a major maintenance measure comes up that you have to pay for in addition (heating from gas to heat pump, etc.). I don’t know anything about the apartment to be able to say more.

An apartment with a possible monthly rent of 1000€ and a value of 250,000€ appears at first glance (without knowing the year of construction and the condition) to be a good investment that one should not sell. Even with 4 rooms, it will probably be an attractive layout. However, whether it pays off again with the longer and higher interest rates must be carefully examined.

1,000€ cold at market value T€ 250 does not fit. But that is another matter.

1,000€ cold less 150€ maintenance reserve less 35€ administration = 850€ liquidity inflow = € 10,200 p.a.
Depreciation let's say T€ 4 p.a.
Then approximately T€ 8 is taxable, with a marginal tax rate of about 36% = approx. 2,500€ taxes
From the aforementioned 10,200€ the taxes are then deducted, so effectively about 7,700€ remain in the account.
Thus the rent shrinks from 1,000€ to 642€ monthly (possibly additional costs such as CO2 surcharge, tenant change costs etc. not considered).

That is just 3% return after taxes on T€ 250 after regular maintenance (if the apartment is older, then rather more house money, younger = less house money).

If the TE now sells, he can take a tax-free increase in value and - assuming repayment of private loan - has to finance T€ 180 less.
The financing of T€ 180 costs nearly 1,000€ at 2% repayment, of which 675€ interest and he saves another 70€ interest for the repaid private loan.
From a liquidity perspective it makes no sense to rent out the apartment.

And no, it is also not a good investment, but a bad one because designed with poor return. And if the apartment is worth more, the return further shrinks or the possible interest savings from a sale become ever greater.

Possible value increases always left aside.

I would really be interested now, the original poster probably too. Let's take the Bodensee 129, it costs 270,000 EUR in a mid-priced region like Hannover, turnkey. That makes 2,100 EUR/m². What is still missing to reach 3,000 EUR/m²? Or even beyond that. Please explain.

That makes 2,100 EUR/m². What else is missing to reach 3,000 EUR/m²?

No idea from whom you expect an answer, since the OP has already given it. Surcharge on country house 142 plus upscaling = 2,800 €/m² plus services to be provided on site (especially floors/painters).