Perimeter insulation / floor slab insulation / brick width / window

I have a developer's offer from a renowned company in the neighboring town. For budget reasons, I asked this company to calculate the cheapest possible house that complies with the current energy saving ordinance but does not have to achieve KFW 55 or better. Based on my requirements, they calculated accordingly. I then gave the offer to an acquaintance who works in the field of construction planning/construction supervision and also collaborates with other construction companies. He also gave me a cost estimate for what it would cost if he did it.

His main points of criticism:

- Developer only builds with 30cm Ytongg with WLZ 0.09 --> my acquaintance thinks it should definitely be 36.5 cm Ytongg with WLZ 0.08
- Developer has only 5 cm perimeter insulation in the basement --> my acquaintance thinks it should definitely be 10 to 12 cm
- Windows UG value 0.7 (he recommends 0.5)
- Foundation slab is only insulated with 50 mm, but should have 100 mm.

How do you assess this here?

I don't quite understand. You wanted cheap and now they have offered cheap. Now your acquaintance suggests more insulation because it is better. Of course, more insulation is better. But that was not your objective.

The point is: Is it worth investing the additional cost of at least 10T in better insulation because the costs will amortize within the next 5 to 10 years?

Whether it is sufficient is calculated individually, the calculation is part of the building permit. If the contractor calculates that this is sufficient, he has perfectly fulfilled your requirement. Where is the problem? If the opinion of the acquaintance is the most important to you, then do it with him. Otherwise, you will go crazy, because in construction there are simply too many ways to Rome (and of course everyone believes theirs is the best).

No. It is not worth it. If you save 500 per year on heating through better insulation, and that would be extremely much, in reality it is significantly less, you have to heat without interest or anything else for 20 years to break even on the insulation. Since we live in a house like the one you offered cheaply, I'll tell you, it is warm and pleasant to live in. My gas supplier estimates me at around 800,- per year. Karsten

Thank you for your assessments, that already helps me a lot. I really have to save a lot, but I was just unsure if the low insulation wouldn't cause problems for me later.