Ok I just heard that there ARE in fact residential properties that you can buy AND enter but they are not what you would call a safehouse. They are separate from safehouses and businesses. Businesses you can buy because you can just go up to them and it lets you purchase them, residential properties are bought via the Dynasty 8 site.
The reason no one can buy them is because Social Club is overloaded and as a result of it you cannot use SC-related features in game (most people anyways, since I see some people are able to upload screens here and there). This is the reason Bawsaq is down and Dynasty 8 (the site you use to buy these properties) and thus you cannot do this.
Once they get it fixed and SC is working properly in-game, we should be able to purchase residential properties via Dynasty 8 and they are enter-able but again....the game seems to treat them differently from actual safehouses which the three characters start with and then acquire throughout the story; Michael only ever has the house he starts with as a safehouse, Franklin has his starter home with his aunt and the one you get through the storyline and Trevor has his trailer and 1-2 more safehouses he acquires throughout the story.
To back this up somewhat:
OOk so this is the Dynasty 8 site that no one can access at the moment because of SC being overloaded. It was shown in the gameplay video and don't give me that BS about how it was talking about Online. It wasn't. It was discussing what you could spend your in-game money in, in SP and after that sequence we heard "and then there is Grand Theft Auto Online, which we will be showing you properly soon".
In addition to this, in the manual you come across this:
Which heavily implies and in my own opinion basically confirms you can own residential properties, they just aren't regarded as safehouses and I suppose we'll find out when Dynasty 8 is back up once SC is fixed.