New construction financing single-family house 150 - 160 sqm

A question about the new kfw conditions: We will probably build a KFW 55 EE house using solid construction methods. We achieve the EE class through the district heating connection. There is a total of 26k€ funding and a possible loan of 120k€. however, only a maximum of 10 years fixed interest period and no special repayments allowed. That means after the 10 years expire, a substantial remaining debt remains and must be refinanced. I am considering choosing only the 26k€ grant option but without the kfw loan. The loan (amounting to about 420k€) will then be taken out entirely through the bank. Or does the 120k€ kfw loan actually make sense?

So first of all, good equity, good income, and the most important thing: The plans are not wildly exaggerated but the planned budget fits you. So maintain cost discipline during construction, but otherwise proceed with a clear conscience *make it*. Better a fixed price with a general contractor than the risk of cost increases with an architect.

current "rent": 500€ (a house belongs to me and my siblings, therefore low rent)

You own part of a house? What do you plan to do with it? Rent it out? Sell your share to your siblings?

- during that period we will probably get married, which will move me into a better tax class and thus leave me with about €500 more net

Every tax class calculator I have tried so far spits out the same annual household net income for tax classes 3/5 and 4/4 (=1/1). Am I calculating wrong, or is the switch to 3/5 actually irrelevant over the course of a year? (excluding the advantage of higher parental allowance due to the previously higher net income)

is the change to 3/5 really irrelevant on an annual basis?

That's exactly right. With joint assessment, there would simply be a slight tax refund without the change.

You own a part of a house? What do you plan to do with it? Rent it out? Sell your share to your siblings?

It will probably be rented out temporarily and is expected to be sold within the next 5 years.
I want to invest the majority of the sales amount (estimated €100k) as retirement provision / for the costs of a possible children’s studies in 20 years and not put it into the financing.
That is why I do not include this amount in the financing calculation. If it ever gets tight, you can still make extra payments.

Exactly. With joint assessment, there would simply be a bit of a tax refund without switching.

Does this also apply if the main earner makes about 80% and the wife works part-time?