With BU, costs are constructed based on an example

You are completely right! Although I really wonder why that is, when private customers must always be quoted the final price and even no final price with the note "plus VAT" is allowed.

No idea ... with us they always talked about the surcharge, in the end the VAT was added. Always said that's how it is in construction.

With the 57k ancillary costs, one should be on the safe side. And by ancillary costs I really only mean items directly related to the construction: authorities, notary, appraiser, earthworks and disposal, water/sewage, electricity, cable/FTTH, backfill material, construction site setup, surcharges for electrical work. No outdoor facilities, no garage, no furniture/kitchen/lamps.

However, I cannot understand how four people can be properly fed with 450 euros per month. Almost every slice of toast must be counted and the mineral water rationed...

How to properly feed four people on 450 euros per month, however, is beyond me. You would have to count almost every slice of toast and ration the mineral water...

Depends on how old the two little ones are :p our "big one" is 3, the little one is 3 months old (only eats indirectly, but does cause weekly costs), we get vegetables, eggs, and also meat from a community-supported agriculture (so rather high-priced and now in winter of course not the quantities as in summer, with the same monthly fee - so we have to buy more at the supermarket) and at the moment we are at about 450 a month for CSA contribution, drugstore (without diapers), supermarket and beverage store. We buy plenty of fruit and certainly sweets and such, but drink almost exclusively mineral water and apple juice and make an ultra-nerdy meal plan and weekly shopping. Because of this, we waste almost nothing, which saves a lot of money :) So it works, but admittedly it gets difficult with teenagers.