How high are the additional house costs?

So are 400 euros besides the loan costs already realistic?

We pay just under 300€ per month for ancillary costs at our current 126 sqm house for electricity, gas, water, sewage, residential building insurance, property tax, waste fees, chimney sweep, heating maintenance (contract) and GEZ, for 3 people and with frequent multi-day overnight guests, distributed over the month!

We have not saved a maintenance reserve; instead, we paid directly from our ongoing salary when something arose. Our house was 9 years old when we moved in.

500 €, but everything would be included

I would say you should definitely expect around 500 euros. Basically, almost everything is increasing. The state and its agencies are relentless when it comes to collecting money. So, steadily rising, if not even exploding costs are to be expected here as well.

Electricity and gas are of course the first. Property tax is also likely to increase significantly. Insurance, trash, water, and sewage anyway.
I usually had heating costs between 60 and 80 euros with oil heating. Now it’s 100 - 120 euros.

And with 3 children, I would even estimate higher. Just the washing machine alone is likely to be used significantly more than the average citizen.

Our house has about 140 sqm of floor space and we are 2 adults + 3 children (4 to 12 years old) and also from Brandenburg.

We built according to the Energy Saving Ordinance and do not meet any special standard. The heating runs on gas without solar etc.

Our electricity costs amounted to 79 euros per month last year (consumption approx. 2,800 kwh). Heating costs were 72 euros per month (consumption 11,400 kwh / more than in previous years, as it was very cold until April). Water/sewage are about 72 euros per month as the washing machine runs daily, as does the dishwasher.

Building insurance costs about 29 euros per month. Additionally, there is liability insurance for the inventory at a similar amount. GEZ is the normal 17.50 euros per month. Telephone and internet currently cost 23 euros per month. Netflix and Prime another 14 euros extra.

Other monthly costs I can think of:

Waste fees: 9 euros
Property tax/street cleaning: 18 euros
Heating maintenance/chimney sweep: 15 euros

Adding it all up, it comes to about 380 euros per month.

However, reserves for maintenance have to be built up at some point. This usually starts around year 5-10 after moving in.

We have been doing this since moving in, but initially with smaller installments of €250/month. When the house is 5 years old, we want to increase that. I wouldn’t sleep peacefully at night without building reserves. Having takes precedence over needing. :)

Otherwise, our ancillary costs have been similar to those in the old rental house. Energy consumption and insurance are similar. In addition, there was a residential building insurance of about €40 per month, and the fees to the municipality of around €80 (but that includes sewage, which we had already paid before).

I also "offer" at least €500.
Indirectly, RLV, BU, etc. are also included.