Is building/buying a house feasible?

The apartment still counts as equity, but only with the portion that has already been financed. Of course, the loan amount for the house is then higher, but so is the repayment rate.

The condominium, which has nothing to do with the new construction, does not count as equity, does it? It may increase the security for the bank, but it cannot be considered as capital investment.
That means the loan amount is higher and two financing arrangements have to be managed (of which part will "maybe" be covered by rental income).
This is already quite a tight situation. With such enormous risk increases, this will be a suicide mission.

With a value of 220,000 you already have a 4,400€ allowance through depreciation + other possibilities to deduct something. There shouldn't be that much tax involved.

Depreciation is only on the building value and not on the land. However, in the region, the land is likely to make up a significant part of the total value.
The marginal tax rate should be around 35%.

And once again. 2% collective agreement increase. That is inflation. Yes, the loan is fixed, but everything else is getting more expensive. So calm down and think about changing jobs, unless you are comfortable.

Increase income despite collective agreement, no problem. I found a 450€ job, sacrifice anywhere between 2-4 weekend days as I please, and look forward every 15th of the month when between 240€ - 450€ additional income hits the account. Of course, you don't have to do it, but I've been doing it for 1.5 years now and will continue to do so until March 2019. Regarding financing here, I can only share my personal opinion: Certainly possible, but I would definitely not do it. Even with a rate of ~1300€ + ~400€ additional costs, that’s 1700€. The salary is good but not excessively high, and personally, I wouldn't put myself through the 1700€.

How do you come up with €400 additional costs for a modern house? That seems a bit high to me...

Heating costs for a KfW 70 house with gas heating (I didn’t find the type of heating here, only controlled residential ventilation with heat recovery) are under €50/month. It should actually be a bit lower here. In a house, you don’t have additional costs like a caretaker, etc. €350 for electricity, water, insurance, garbage, I consider that very high.

We are financing in roughly the same range, but with somewhat higher equity. A 20-year fixed interest period with special repayments and relatively low amortization (just over 2%) results in a burden of €1650/month for us. Therefore, I consider the €1700 entirely realistic. However, as I said, I would set the monthly additional costs somewhat lower. Doable, but big leaps are not possible. A second child with a loss of income will be tough. Maybe you can find financing where changing the repayment rate is possible, and you can switch to lower amortization for the time one person is at home.

I would also like to point out again that the suggestion to "look further out" definitely has its justification. I would also like to build on a quiet, centrally located plot at the Isar in Munich

Help help: you are a banker yourself and advise changing jobs before financing? Really??? With an employment contract during the probation period, at least here with the banks, you’re not necessarily the favorite customer… just saying.

So I actually don't find 400€ to be set too high but also not too low.

Property tax: 66€
Electricity: 70€
Water: 36€
Wastewater: 10€
District heating: 107€
Internet: 25€
Trash + street cleaning + rainwater: 24€
Insurance: 60€
GEZ: 18€

That’s already over 400€. I have to say that I chose the highest possible installment for district heating + electricity + water. So there’s surely a few euros coming back.

A reserve fund is not included yet, well my house is only about 3 years old anyway but a reserve fund makes sense and with 100€ or 200€ you’re not exactly taking the sausage off the plate.

Honestly, additional costs + reserve fund then go towards 700€ + then there’s still the mortgage payment.