Bathroom accessible from 2 sides - What kind of locking system?

I also think the idea is good, but I wouldn't invest in technology to prevent both doors from being opened. If the pool is being used outside, the interior access is simply locked. If nothing is happening outside, then the terrace door to the outside is closed. Quite simple.

You are probably right... In the past, there were bathrooms in hospitals that were used by two rooms at the same time.. I just thought such a variant might now exist nicely for private use. But thanks for your opinions. When I am further along in the planning, I am really looking forward to sharing my floor plan with you :)

That will certainly happen. However, the special feature with you is that the locks must not run bluntly in sync, otherwise you constantly have an open exterior door.

When the pool outside is being used, the indoor access is simply locked.

Why?
Well, I really don't see the problem: if the garden is being used and also the pool, then the guest bathroom doesn't necessarily have to be reserved exclusively for guests inside the house. Or does half the family have garden day while other family members have Tupperware day, card night, or welcome an insurance agent? I don't understand the point of just leaving door door and terrace access terrace access... With us, accesses are often open in two rooms... and in the evening you ask: Have you already closed door so-and-so?
It's just a single-family house, right?

I don't understand the whole question. The toilet is of course located in the [Poolhaus]?!?