Construction costs per sqm - "fixed and variable"?

Hello everyone,

I have been following the forum attentively for the past four years and have learned a lot. Our construction project is completed - a simple Flair 125 on a perfectly level plot ;-) I was never tempted to create my own thread about floor plans/financing or similar because I was able to absorb a lot here. Out of interest, I have stayed connected to the forum and always enjoy reading here.

Therefore, I have decided to write something that I often wonder about: generally, construction costs are roughly calculated with a value per square meter - the exact value, whether 2000, 2500 - or wherever the range goes - is not really important. When it is then discussed to reduce a floor plan, the justified objection arises that costs do not decrease proportionally because not all costs, such as heating, are reduced equally. But then I can understand the forum participants who are somewhat confused: "On the one hand, I am supposed to calculate with this value, but on the other hand, I am not allowed to continue calculating with it?"

Therefore, my question of interest: would it be possible to include a constant in the formula for such approximate calculations - for example, in the order of magnitude of 120-150 sqm fixed amount 100,000 EUR, order of magnitude 150-180 sqm fixed amount 130,000 EUR, and then a square meter price that reflects the "somewhat" proportional costs?

Construction costs are always individual, and the prices per square meter are only a rough guideline. In theory, some things become cheaper with a larger area since only one staircase, one kitchen, etc. is needed. However, I believe that this balances out quite well in practice. Those who build 200 sqm will install a larger and therefore more expensive kitchen. Those who build large will rarely use the €30 tiles. And so on. I think it balances out again. Besides, ultimately it is not decisive how the costs arise but how they are invoiced ;)

The rules of thumb are the modest attempt to press something complex and individual into a constant in order to make it somehow tangible despite all uncertainty. They only help prospective builders, who otherwise have never had anything to do with construction and current construction prices, to dispel the illusion. They can also be used to roughly estimate a project. But nothing more. There is no need to tinker with it further.

Since most single-family homes (EFHs) that are built are of similar size, the mentioned values (approx. 2500€/m2) are quite accurate. Villas usually have different features and therefore higher m2 prices.

When there is a discussion about reducing a floor plan, the legitimate objection arises that costs do not decrease proportionally because not all costs, such as heating, are reduced equally. But then I can understand the forum members who are a bit confused: "on the one hand I am supposed to calculate with this value, but then I am not allowed to continue calculating with it?"

I answered that shortly after your question – though in a different thread:

Actually, all prospective builders who want to scale the size of a specifically offered example house for price reasons could write this as a principle in their poetry album: More square meters are added at the sales price, fewer square meters are only deducted at the purchase price. And of course, with less house size, the "allocation" of the necessary basic equipment per area increases. That’s why many general contractors say the price only remains the same if the external wall frame remains the same (this is admittedly said imprecisely, but it is translated into the understanding world of laypersons).

Hence my question of interest: couldn’t one incorporate a constant into the formula for such rough calculations – for example, in the range of 120–150 sqm a fixed amount of EUR 100,000, in the range of 150–180 sqm a fixed amount of EUR 130,000, and then an sqm price that reflects the "somewhat" proportional costs?

As soon as I have found this rule of thumb – which may take a while because my microscope has too limited precision for that – I will gladly pass it on at popular hourly rates. In the meantime, you can manage with the fact that your question almost suggests the direction itself: namely, to understand "proportional" not narrowly as "linear." For example, you could continue researching by taking the cube root of the reduction factor (e.g., 1.1 for a 162 instead of 180 sqm house) or something like that. Report back on the results of your test series ;-) (but don’t forget: no matter how methodically one proceeds with empirical determination, the name "rule of thumb" is not by chance, even a harpsichord played with fists no longer sounds delicate!)

Thank you for the answers – especially the "More square meters are added to the sale price, fewer square meters are only deducted at the purchase price," which I find frequently worth quoting in the construction cost threads with sqm prices.

That way, I also got to enjoy a poetic 11ant response. :-)

Since you just joined my thread, one of your mantra statements has always left me somewhat puzzled: houses are often planned here from the perspective of "barrier-free / low-barrier, in case something happens in old age." I see it the same way as the majority of forum members, that it is better to build for the foreseeable future and not for all eventualities in the future. You then like to argue that a not insignificant portion of today’s homeowners would build again at such a stage of life.
I always wonder WHERE? Even if they then, in their mid to late 50s, start following your Barthel tips – I cannot quite imagine that there would be enough free spaces where senior-friendly bungalows (or similarly suitable building types) can easily be placed. There might be a handful more areas with buildings, but not unlimited, and you and others rightly ask the very justified question whether the existing stock is not worth preserving.

So, I also often see moving in later life as the sensible planning option. But once again, bluntly asked: WHERE do you think people in their prime years will build their second house (the one for the friend, as you so like to say)?