Is only part of the building permit enforceable? How long is the building permit valid?

Some people already live in their own world. Please do not take this personally or negatively, it is simply a statement.

If you’re already putting up the entire shell: But at least the roof and windows would be needed, right?? And the insulation of the roof? I mean, even if you don’t finish all the rooms, you have to close the thermal envelope, right? And then you heat the whole place with just the two rooms?

It sounds pretty unattractive, sorry...

"Slave to the bank," when I hear that... I know someone who saw it that way too. Every euro goes to repayment so you can shake off the bondage. Now the loan is paid off and he has realized that you still need a job because the rest of life has to be paid for too...
I always have to come up with the money for rent too! Of course, the finances have to add up overall.

Unless I have overlooked the proposal somewhere, how about splitting the property and a classic (small) semi-detached house. Then the 2nd land register would still be free for the financing of the 2nd semi-detached house.

If I haven't overlooked the suggestion somewhere, how about dividing the plot and creating a classic (small) semi-detached house. Then the second land register would still be free for the financing of the second semi-detached house.

That sounds exciting! There are even two plots that were actually supposed to be combined. Semi-detached houses don't have to be exactly the same, but just share the wall/floor slab? I'll look into that - thanks!

Unless I have overlooked the proposal somewhere,
how about dividing the property and having a classic (small) semi-detached house.
Then the 2nd land register would still be available for financing the 2nd semi-detached house.

But then much less land will count as equity for the financing, and various costs will occur twice.

1) Is it possible to have architects apply for and get approval for the complete dream house, but then build only the granny flat (which is a separate house, just not the large dream house)? And build the rest of the house at a later time or never?

In our case, that would be possible. The construction site must not remain inactive for longer than one year. We got approval for the house and carport. We are taking our time with the carport. If we lay the foundations for the carport within 12 months after the house inspection, we have continued the construction work and get another 12 months. This can go on up to 4 years, so not indefinitely.

2) Assuming 1) is possible, we plan to have the basement built directly for the entire house. So about 40 to 50 sqm under the granny flat and another 100 sqm for the planned house. This 100 sqm basement section could therefore stand alone for years or forever without a house on top. Would something like that be approved and does the ceiling have to be specially sealed?

Yes, you can build the basement right away and sealing it is an extra effort. Talk about the plans and background with the building authority in good time instead of trying to trick them from the start. They are not enemies but usually extremely helpful service providers - if you ask politely and early enough.