The main reason these will not happen is because they are a bit mad, the other is the expense of it all. Is it possible that these crazy homes are in need of holiday home insurance?
Underwater Homes
It has been done many times before so it can be done, the record is actually 63 days. Even though you could easily fit your vacation into this time it is very unlikely this will ever happen even though it has been shown in many movies? A nice holiday like this could really relax someone. Although sitting under the sea is easy for the Characters of bednobs and broomsticks, it is not likely to happen as it would be very expensive.
Moon Landings
With the recent bomb placed on the moon to see if there is in fact anyway we could build a station on there as a half way point from there to Mars it could well be possible that few people could be spending months if not years sat on the moon monitoring probes, the real question is how much is the second homes insurance likely to be. Very expensive is the answer to that.
Underground
How about building a home with a nice pool underground. This one is easily the one that really could happen simply because there are already underground caves that exist so in theory you could build yourself a nice home there to spend the summer vacation in. The bat cave and this idea seems to be very similar?
If however, you are a normal person then you would be better off going off to somewhere like France and having a loo kfor a holiday home, make sure you do not forget your French house insurance.