Do we have realistic chances with the bank?

Yeah, the property value would be interesting.
Nevertheless, liquidity is getting tight.
The idea of improving your financing situation with a granny flat simply doesn’t work at the moment. Construction costs are way too high; you won’t recoup that with rent.
So if you want to build right now, limit yourselves to the house without the granny flat.
A 300k loan is feasible with that income, maybe a little more. The plus your liquid equity is available for the house, nothing more.
That can be enough for 130-140m² without a basement in a cheap area in RLP. More is currently not possible.

Important for the bank is also that your employment contracts are permanent. There are some banks that also grant loans with fixed-term contracts, but they then have a significantly higher interest rate.

Hello Daniel150488

So the acquaintance at the bank can only offer what his bank offers. That means if his bank wouldn’t support it, it doesn’t mean it would look similar at other banks.

1. Income
with 13th salary = 2800€ on average
rental income: 500€
parental allowance? How much? That would be important

Equity:
70,000€ + property (even if it is valued low) I assume here a total value of at least 100,000€ (property at least 30,000€)

So 400,000€ financing = 80% of the total costs (400+100 equity)

So even if your wife only receives 300€ parental allowance here, not all banks would finance this, but quite a few in Germany would!

so the acquaintance at the bank can only offer what his bank offers. That means if his bank wouldn't support it, it doesn't mean that it would look similar at other banks.

Yes, of course – in a pinch, there is a consumer loan as equity, etc., etc. A bank for that will be found, the question is only – at what price?

And this is exactly where the real problem comes into play – the credit limit of €400,000 is not enough for a house with a granny flat. Building & buying has unfortunately become so expensive now that after the real estate agents, the lenders’ businesses are drying up – the money is so cheap that hardly anyone can afford it anymore. ;)

Yes, of course – if necessary, there is a consumer loan as equity, etc., etc. A bank for that will be found, the only question is – at what price?

And that’s exactly where the real problem comes into play – the credit limit of 400,000€ is not enough for a house with a granny flat. Building & buying has unfortunately become so expensive that after the real estate agents, now the lenders are drying up the business – the money is so cheap that hardly anyone can afford it anymore. ;)

Well, you can build quite a lot for 400,000€. We built a KfW-55 house with an architect and developer for 375,000€ (170 sqm living space), and that was rather one of the more expensive ones in this region. So.
The land is the most expensive part, and they have that.
They can also build with Town & Country, a two-family house with 180 sqm for about 300,000€ (ok, with hardly any fittings), but then with 400,000€ including ancillary building costs and somewhat more fittings it should be doable.
Of course, they must not expect 250 sqm top fittings here.

The idea of improving one's financial situation through a [Einliegerwohnung] simply does not work at the moment. Construction costs are far too high; you cannot recoup them with rent.

The rental yield on real estate is at 3% and thus higher than the interest on debt. With well-structured contracts and moral flexibility, it can be arranged so that the tax office helps with the repayment.