General assumptions: The next step, after exploration of area around the residential building, is getting inside. The game offers in this case several possibilities, however the most important thing is to have the best possible disguise.
You can get inside the residential building in three main ways. The first variant is to use the main entrance door, the one located near the mission starting point.
As mentioned earlier, the lobby is restricted for unauthorized persons and you can't count on sneaking past the guards, so you need some disguise. Personally I recommend using plumber disguise or Blackwater custodian disguise. In case of having one of guards' disguises you would have to keep using Instinct to hide your identity.
The second method is using the side entrance door, located at the building's back. As for reminder, this door is located near the parking lot guarded by two or three enemies (screen above).
After using the mentioned door, you'll find yourself in the corridor inside the building's basement (screen above). In this case it would be best to use plumber disguise, because later you may encounter both Blackwater guards and janitors on your way.
The third possibility to get inside the building is crossing the ventilation shaft. The entrance is also located at the building's back. You have to get to the guardhouse located near the partially flooded underground parking lot (screen above).
The shaft entrance can be found in one of rooms adjacent to the guardhouse and is shown on the above screen. If you're using Blackwater custodian disguise (or you do not have any), then you have to watch out, because there are many janitors in the area.
After crossing the shaft you'll also find yourself in the basement, but behind the counter shown on the above screen. As result, you can try to sneak to the stairs leading to the building's lobby (unless you have plumber disguise, because in this case you do not have to sneak) or attack a guard patrolling this area to obtain Blackwater Park interior guard disguise.