OK, I just watched the Gameplay trailer again. This the quote where they say real estate and show the Dynasty 8 website.
"You decide what to spend your money on, anything from sensible investments like tattoos and fast cars, to risky ventures like stocks and Los Santos real estate. And then there's Grand Theft Auto Online, which we will show you properly soon."
From this it makes me believe that real estate is purchasable in SP, not Online, because the way they describe it, it's separate. But, we know how things can change, since in the first previews properties weren't in at all! Now they've come out with this comment, maybe they made real estate Online only, since they want it to be dynamic, and Online will be our world, whereas in SP the safehouses are related to M, F & T's storyline. If that makes sense.