Building description for a single-family house with a basement

Kfw 40+ would give a €75,000 subsidy

The point is, however, that KfW40+ is quite expensive. You need the appropriate building envelope, which is no joke with a basement. Then there’s also photovoltaics and a battery, which also cost money. If you calculate that against energy savings ordinance houses – purely from the subsidy aspect – KfW40+ is usually a subsidy business that significantly increases your financing needs. The whole thing is actually paid for by the builder first and then there is the subsidy. 40+ is especially interesting if you would build the photovoltaic system on a KfW55 house anyway and reach the KfW40 standard with little additional insulation (i.e., costs under €6,000). Then you get the battery highly subsidized. The same applies if you build a granny flat in the basement. The basement is practically gone, meaning compared to a house on a slab foundation, you usually only gain the utility room in the basement. It takes ages to recoup the extra costs for the basement through rent, and you also always have other people in the single-family house.

The basement would be converted into a granny flat. According to the construction and service description, the basement expansion (room height, connections, bathroom, etc.) costs about €26,000. It is supposed to become the refuge for the mother-in-law once she retires in the next 3-4 years.

From the Energy Saving Ordinance to KFW 55, it costs me according to the construction and service description €7,500 (42.5cm bricks on 49cm and a bit more insulation on top and bottom). I would hope that the jump from KFW 55 to KFW 40 also lies within this range. My layman's calculation would therefore be €7,500 for KFW 55 + €7,500 for KFW 40 + €18,000 (gross) for photovoltaics with storage (I was offered a 9.75kwp + 7.5kwh storage) = €33,000 more than for the Energy Saving Ordinance. This would mean the subsidy would be €75,000 - €33,000 = €42,000 profit compared to the Energy Saving Ordinance. Which in turn would cover the entire basement expansion, etc.

I have a meeting with the general contractor on Wednesday and I will have everything calculated. Whether this works like that or if I am completely off track with my calculations.

Thank you very much for your many suggestions, I will discuss them all.

It is supposed to become the mother-in-law’s refuge once she retires in the next 3-4 years. [...] I would hope that the leap from KFW 55 to KFW 40 also falls within this range. My layman’s calculation would therefore have been

... at this point, in my opinion, nothing more needs to be added to the sentence: the hope that the jump from 101 to 102% is not noticeably steeper than from 100 to 101 is rightly often considered naive (it's rumored). Housing seniors not on the ground floor is also said not to be the right thing ...

According to the construction and performance description, going from the Energy Saving Ordinance to KFW 55 costs me €7,500 (42.5cm stones to 49cm and a bit more insulation on top and around) I would hope that the jump from KFW 55 to KFW 40 is also within this range.

That would be exactly the point to ask about before making plans. The next level in energy standards is usually significantly more expensive than the jump from the last to the current one. Simply making the wall even thicker will be difficult, 49cm plus plaster inside and outside is already quite a lot. You will probably have to work with filled bricks or ETICS to achieve KfW40.

So today there was another appointment with the general contractor. The basement is now painted... it already hurts... but in the end it is simply too expensive. If the mother-in-law wants to come in a few years, a solution will surely be found. We have now decided to build the way that suits us, without a crystal ball about what might possibly happen in the future. We enlarged the house by 12 sqm to also have enough space in the utility room for a drying rack. We are still waiting for the offer, but the rough estimate was 315,000€ for a KFW 40+ cabin. Photovoltaics with storage were wanted anyway, since we also drive an electric car. The equity capital then covers the incidental costs and the kitchen.

It sounds at least more realistic. But please explain to me, how can the GU or you say in the first posts that 150sqm with a basement costs 315k€ and now 162sqm without a basement costs 315k€?

We once had a TE here who fiercely defended the prices. In the end, it turned out that she was just repeating what her husband, who had no clue or only half-heartedly researched, said and the numbers were not valid.

I mean, who said what and if it was the GU, how credible are the statements when the discrepancy is already so obvious.