Construction costs per square meter - how to calculate?

Who offers such prices?

Der.Büdelsdorfer. It has also been discussed here many times. Prices are listed on the homepage.

A friend wants to have a new bathroom installed and has cost estimates from 4 companies ranging between €18,000 and €32,000! For the same bathroom. Something must be wrong there, right?

As long as business is going well, there will be one or the other defensive offer. And whether the same is really included in the offer is questionable. As an amateur, it is difficult to assess, so it should be put out to tender by a professional. Although there can always be surprises in an old building.

Bungalow is more expensive than 1.5, for whatever reason.

Small is always more expensive than large (calculated per sqm).

Der.Büdelsdorfer. Has also been discussed here many times. Prices are listed on the homepage.

Oh those - whose houses, despite the "solid" label, are not always solid and whose final prices are exorbitantly high compared to the "from" price because every wish is priced higher than elsewhere. Apart from that, many customers are not satisfied at all... But yes, the "from" prices are cheaper. Unfortunately, not the final prices.

There, apples are regularly compared to pears.

Sorry, but at the moment it is _you_ who is comparing apples to pears. In addition, factors or price calculators must also be applied correspondingly carefully. And if you can’t manage to convert relatively small houses with the factor… others can.
The flat factor of 3000€/sqm is an average and applies to single-family houses in the average size and standard segment. Nothing more and nothing less.

there the 84 sqm bungalow costs 2,200 euros per sqm. The 150 sqm 1.5-story costs 1,500 euros per sqm.

So what? Everyone who has been reading here for a few years or is involved with house building knows that catalog prices are far from the final price. And if you don’t know that, then you should quietly read along in this thread, but not assume it would be otherwise and that you would know better.

Conclusion: smaller houses are more expensive than larger ones. Bungalows are more expensive than 1.5-story houses,

see above.

, these announcements are worthless if you do not also state the year of completion.

Year of construction 2022, move-in summer 2023, Rhine-Main area, KfW55, no own work, no smart home, no fireplace, no photovoltaics, no special architecture (protrusions etc.), gable roof.

What I would describe here as "deviating from the standard" besides KfW55 is:

[*]central ventilation system
[*]increase of living space height on the ground floor to approx. 262cm upper edge of finished floor
[*]basement as living space
[*]more electrical wiring, i.e., sockets and LAN ports
[*]floor covering above the frequently offered €40/sqm by general contractors
[*]2 bathrooms or 1 full bath and 1 shower bath, but here we were able to buy ourselves at a large online retailer, which saved us several thousand € compared to an all-in offer from the plumber
[*]partly sunshades

If you go into the race with €3,000 in the north/east and €3,500 in the south/west or near large cities, you should have a good first (!) orientation value for a standard house.

A friend is currently planning to have a new bathroom installed and has cost estimates from 4 companies ranging between €18,000 and €32,000! For the same bathroom. Something must be wrong there, right?

As long as business is good, there will be one or another defensive offer. And whether the same is really included in the offer is questionable. As an amateur, it is hard to judge that, so it should be tendered through a professional. Although with old buildings there can always be surprises.

Please convey my heartfelt thanks to your friend for illustratively exemplifying one of the other reasons for this enormous range: namely the huge difference between a proper tender and an amateur request for bids. Exactly described qualities, amounts, and quantities vs. guesswork.

I am really afraid of being taken advantage of and am therefore trying to inform myself as well as possible about prices and services in advance.

In this sense, I have just advised you for good reason in to already operate with the preliminary draft when submitting the preliminary inquiry. My house-building roadmap – which I hopefully do not have to mention where to find anymore – applies roughly the same for extension and remodeling projects. Because here too, it is necessary to ensure cost control in the second half.
Because basically your question is not about how to statistically index out-of-control costs afterward, but about controlling the costs for the purpose of manageability – and that only works actively.
(The pathologist knows everything, just unfortunately too late).