I was at that mansion just recently at the end of my game, and I had a bad ending. I really don't care if I see that place again. You're not there very long anyway.
EDIT: I have to say to the OP that I did watch this video, and I have to say 'well done!' for your effort despite my response. I doubt that I could do that. But I now know what that mansion means regarding the progress of the game, and I really don't want to see that place again. Apologies if I came of sounding upset. Just taking a quick look around I did not see an elevator so I don't think there is any other access to it other then flying in, or 'climbing' to it. Maybe others can give a better description than I.
Its part of the "secret ending". I feel it should be made available after you're done with the story, just what I think. Otherwise, having such a huge asset on the map just seems weird when its being unused for like 99% of the game.
I have explored it and returned back to it more than once, fabulous location and design and amazing views, the pool is extremely fun. This is far more like the idea I had that you could become successful and buy better apartments, cars clothes, weapons etc... it does have an elevator with a control panel you can turn on, but thereafter it does not work, if it's in the ending I presume the elevator might work but from what people say, you don't get to keep it. However if you can find a way to make it up there easier during gameplay, you can save game then log off there and will reappear there on logging in, and there is at least one mod in progress that will soon let you teleport to it. Will add some photos on another post soon. I've been careful not to take any of the objects or guns from the racks yet, but the most annoying thing (like everywhere else in the game) is you literally cannot sit down anywhere, no chairs, seats, bed, nothing is useable... no interaction at all - you can walk around and that's it, except for swimming in the pool. But watching the flying cars, taxis and aerial advert floaters crossing the sky from the balcony and seeing sunset/sunrise is a great visual immersive experience.
There is a door to it in V's Mansion but there is no content behind it. There is a control panel which activates the hand pointer but obviously does nothing if you click. In the second (empty) apartment above V's in the building there is no elevator door (not finished) and you can see that the shaft is just empty space.
you also can soft lock your game in the rouge ending... like I did just for trying - if u get the hover boots in her ending u can jump down from the roof without dying. but the game is kinda restricted that way u can walk wherever u want but most of the story npcs are just vanished if u try to visit them. also u can't draw a weapon plus - you can't enter the afterlife from the front door to finish the ending quest: the door is closed what's kinda funny if we consider how V is called in that ending... but can't access the afterlife on foot lol... - so yeah just another bug/thought RED didn't consider. only because the hover boots are marked as quest item and won't be removed from your inventory after u wake up in the mansion...