Improve floor plan single-family house 1 1/2 stories

200000 including incidental construction costs such as outdoor facilities, surveying, etc. see the pinned thread will not work without much own effort.

Holiday apartment. Does it pay off? Every time final cleaning, key handover, laundry, guests mostly when you want to go on vacation yourself, repairs (tenants often do not treat carefully), tax return, etc.
How high would the occupancy rate be?
50%?

Holiday apartment for two in a house with a child still playing in the garden.

What does your relatives actually say about the apartment having no direct garden access?

If you like a small bungalow, talk to the building authority and ask for an exception.

Honestly... wait a few more years. Construction prices will not rise to infinity. Eventually, they will stabilize or there will be a crash.

Additionally, building a house can be a tough ordeal if you are not regularly on site. Because of your job, you can only inadequately monitor construction progress.
Otherwise, the house builds itself for you. Even if you travel a lot, a little house is nice.

So, to be clear again: You want to build a house for your cousin. But he can't pay enough rent, so a holiday apartment is to be included for rental. After 10 years, the cousin is supposed to move out and you move in. And you are doing all this because money is so tight. Honestly, just don't do it. Nobody knows what will happen in 10 years.

200000 including ancillary building costs such as outdoor facilities, surveying, etc. see the pinned thread will not work without much personal contribution.

200000 without ancillary building costs. But that is still tight because apart from maybe painting inside we cannot provide personal contribution.

Holiday apartment. Is it worth it? Every time final cleaning, key handover, laundry, guests mostly, then when you want a holiday yourself, repairs (tenants often are not careful), tax declaration etc. How high would the occupancy rate be? 50%?

Cleaning etc. could be done by a provider from the area. Occupancy is conservatively estimated at 20% even though it is a tourist area. But mainly in summer.

Holiday apartment for two in a house with a child who is still playing in the garden.

That is difficult. Yes.

What does your relatives say about the fact that the apartment has no direct garden access?

The housing shortage is so great that it does not matter.

If you like a small bungalow, talk to the building authority and ask for an exception.

The people I spoke to were very skeptical, but they were kind enough not to rule out anything in principle.

Does anything remain at 20% and an external service provider?
My aunt had two holiday apartments with about 50% occupancy. After deducting costs including capital service, tax advisor, etc., only a tip for cleaning and laundry was left.

Either wait and then build as you need, or build now as it will probably suit you in 10 years and rent at market rates.

This way you are twisting the floor plan now, only lazy compromises. Build on a tight budget, get little rent, and to compensate, try it with a holiday apartment. Later take money in hand and try to shape the house more badly than rightly for yourselves.

here first of all the requested single-family house version of the first draft,

Ah yes, thank you. At least this is how I understand your remark that it could be remodeled well / easily. But I had actually (in vain) hoped that the later state would be clearer / more linear than the initial state. Now, however, it seems to me that both phases are hopelessly original botched and messed up. Highly interesting for aspiring technical draftsmen who want to train their spatial imagination – but too convoluted to build it in reality. Meanwhile, you have explained your reasons in sufficient detail to be able to receive constructive advice. Take them, and file the drawings under "quirky ideas."