I don't know whether it defeats the point but the compromise I can think of (which might manage to not tick either box) could be having the separate building for the purposes of sound but it being close with a bridge making trip to the bathroom based in the home/house as short and simple as possible.It is..but can be done to a decent degree, especially if you can spare some inches in the room, soundproof panels make the room slightly smaller. There is a lot that can be done but you would need a professional to advise, and would cost but perhaps not more than building an outside room.
I don't know whether this would just be used during the day whilst others are in and what planning says about a pathway vs a new entrance (eg near a loo) or even covered bridge etc but each could add to security I guess, certainly if you were there when dark and alone it might be a boon to have a second door with a secure route back into the home.
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