House construction project - Reality check

Nope.
Only the heating rises somewhat linearly. Electricity, waste fees depend on "consumption," not on the living area. The insurance also does not increase linearly with the m², because the greatest risk is initially the property itself. Whether it has 100 or 200m² increases the potential damage amount... but not the risk of a damage occurring.
Maintenance of the property, possibly street cleaning, property tax... all these only relate to the living area of the house to a minor extent.

This is true for all insurances, also car insurance. Whether I drive 5,000km or 50,000km makes rather little difference in the end.

But I have to strongly disagree with you on almost all your examples.
With 100sqm you have 15 lamps, with 200sqm 25, ergo more electricity, thus higher electricity costs.
Whether you have to repaint 600sqm of walls or 1200sqm makes the material cost for wall paint alone twice as high.
My insurance wanted to know the exact living area, and this is explicitly stated in the contract. More living area = higher potential damage amount = higher premium.
Also, the kilometers driven with the car play a role in the premium, and not just 1 or 2%, but in your extreme example there's easily a 15%-20% premium difference depending on the insurer.
Compare, sand, sowing, for example, 500sqm in relation to 100sqm lawn. 2kg of high-quality grass seed costs 30€ and is enough for 80sqm.
Only the waste collection will probably remain about the same because you won't necessarily need a bigger bin. The rest scales more or less with the size of the house/property.

Only the garbage collection will probably remain roughly the same because a larger bin is not necessarily needed. The rest is more or less scalable with the size of the house/property.

Not quite. Electricity actually changes only slightly due to some new lamps - thanks to LED. The electricity drivers TV, refrigerator, waterbed, stove/oven, etc. remain the same. Water/sewage is initially the same, since you don't hold more meetings or shower more often in the house. The garden can be a driver if watered with city water. Otherwise from the cistern or the well (yes, the water pump also needs some electricity). Street cleaning will increase, insurance too. Property tax can be interesting. I pay less in the big house than before in the 75 sqm apartment. So it is individual. Garden, renovations, repairs and similar things are of course more expensive, since they were not present before or smaller or the landlord’s responsibility. Heating costs tend to decrease with new construction compared to the apartment (provided you do not live in a fairly new apartment there as well). But the pure ancillary costs compared to an apartment do not increase linearly overall. I actually have to agree with .

It's okay with me, of course everyone is entitled to their own opinion. After some consideration, one probably has to distinguish more precisely between "Nebenkosten" and "Unterhaltskosten" and which incurred costs are allocated to which category. The Nebenkosten are probably somewhat less linear than the Unterhaltskosten. I think with increasing living space, for example, (not always) other/more/higher-quality technology is often found in the house. Be that as it may, the OP ultimately has to know that, I am the last person to dissuade someone from daring projects, just to sharpen the view, that doesn't hurt.

Hello,

thank you very much for the many contributions!
To briefly bring the topic back to financing:
Is the approach and planned repayment within under 30 years reasonable for my age (33), or would it also be reasonable to stretch this until retirement with the currently known figures? (~35 years loan repayment)
So, assuming €2k/month as fixed and disregarding special repayments. Is this still an acceptable duration for the repayment of a construction loan?
Regarding the purchase of a young existing property: Yes, that would certainly be the best solution. The question there is availability with the corresponding parameters. In the last 12 months of observing the local market, there was at least one existing property that would have suited me. Unfortunately, I was just too late.

Regards

At my age (33), is the approach and planned repayment within under 30 years reasonable, or would it also be reasonable to stretch this until retirement with the currently known figures? (~35 years loan repayment)

Everything is reasonable. Why not go for long terms? Who forces you to be done in 30 years? Every tenant at 97 still has to pay their full rent, so as an owner you can also have a remaining debt in retirement (moderately high). A 40-year term is not uncommon. You can also work with a long term = initially low repayment and the option to change the repayment. For example, if you get 15% more salary in five years through annual salary increases and/or changing employers, you can simply increase the repayment. The reverse is also possible as a safeguard (often used by couples during the time when someone stays home for childcare – as soon as the stay-at-home person goes back to work, the rate is increased again). It all depends on your situation. However, with your initial desire, you wouldn’t get a loan from us. That has to be said clearly.

So let’s take 2k€/month as fixed

That is a 4.8% annuity. Towards the end it already becomes difficult to find a bank that will accept full financing with only one percent repayment with poor creditworthiness. Mathematically it would be possible. For me it’s too much.

There the question of availability arises with the corresponding parameters.

Adjust parameters to your own financial parameters.

Hello Nordanney,

thank you for the prompt response.
I am grateful for every point of view.

For me, the suggestions are sufficient for now. Many thanks to everyone!