House construction with a granny flat - extension?

I noticed:

with a small extension for a granny flat. (for the parents later, possibly offered on Airbnb in the meantime

The ongoing additional costs would be split 50/50, so for us it would be ‘only’ 250€ per month.

With whom do you want to share the additional costs if the parents are only supposed to move in later?
Besides, it is questionable whether they will feel comfortable in 50 sqm when they get older. I find the size a bit borderline. My parents need 50 sqm each.
Well, everyone as they are used to... but something to think about

According to my research, I have calculated additional construction costs of 50-60k.

The additional construction costs are cheaper here in the north. Calculate about 30,000 if the soil survey turns out neutral.

I am rather calculating with 140-150 sqm. The houses only serve as examples in the setup.

Then the ratio of the main building to the extension will look different.
If you put the extension into the main house, it means: additional load-bearing and separating walls. An inexpensive model house is out of the question - you then enter architect-built territory, and with that fee you are in a completely different league.

Think simpler and without the extension, then you can achieve quite a bit here in the north with EL.
Whether the 3000 are enough, I don’t want and can’t say.

Again about the house size.

How large should your living unit and the [Eingliederwohnung] be?

If it is to be rented out to others, I can only recommend the bungalow.
I don't want a stranger above me.
With the bungalow, you need an appropriate plot of land, 800 m2 +

I think an apartment ownership is still better than paying rent, and if you perhaps find something on the ground floor with a garden and garage, that also represents a good alternative.

The problem is the rental prices here. You really need extreme luck or live in blocks. At the moment we pay about €1100 warm rent and save just over 1k monthly. Who knows what it will look like here in 10 years. It's no use if the prices then rise to 500k-600k and you have saved the difference amount and then end up in the same situation as now. We might sell the condominium then and use it as a down payment on a house. That would also be an idea...

We did that too, only we didn’t sell the condominium but rented it out. That increases your income side!

You cannot fit a granny flat into 140 - 150 sqm.

Besides, the budget is tight. So why do you want to spend more on the house than what would be necessary for you to live in? That is contradictory...

We are flexible and want to start looking for a plot only when we have a somewhat fixed house price.

At this stage, I expect the "spanner in the works" when you find an affordable plot of sufficient size only for personal use (i.e. without space for the rental apartment). I would move away from the classic granny flat in addition to the full family living unit, and towards an apartment rented out until the children need it. This means building a children's bathroom and two children's rooms without a partition wall yet. When the personal need arises, the kitchenette will be removed, a partition wall inserted, and an access installed under a door lintel already prepared for it.

Exactly how this might look will still depend on the plot. In my opinion, granny flats are generally most suitable as "temporary living space" (when weighing a basement apartment against a partial cellar; or converting the attic now even though it will only be occupied later), or as a reserve for use as a mother-in-law module. For a main focus on income through rental, a single (especially small) unit is probably not the big hit.