General objectives of the stage: Mansion ground floor is fourth and also most important part Personal Contract mission. The main goal of this stage is to obtain a keycard granting you an access to mansion. The keycard is in possession of the head of the security. This goal can be achieved of course in different ways.
First, let's talk about two vital issues. Your current target is the head of the security, shown on the above screen (he wears a black suit and sunglasses). This character stays in the main mansion room by default and your goal is to obtain a keycard he has. Unfortunately you can't rely on pickpocketing so you have to somehow stun or kill the head of the security.
Another important issue is the initial disguise. If, according to my suggestions, you've taken mansion interior guard disguise from the security room, you can continue on immediately. Otherwise you have to think, whether you want to risk with no disguise initially or get rid of one of nearby guards. If you choose the second option, I recommend neutralizing the guard standing on the upper balcony (screen above).
Mansion interior guard disguise is not the only available option, because you can also try to obtain chef disguise. You'd have to wait until the guard staying in the kitchen will be thrown out of it and then get rid of the cook. Depending on your preferences you can knock out this person (drop some sleeping pills into the soup pot - a place where to get them from is shown later in the text), stun or kill. Do not forget about hiding the body in the closet. The advantage of having chef disguise is that you can easily move around the ground floor.
Let's go to details. I strongly advice not to get into firefight with guards inside the mansion, because that would worsen your score and you could also be surrounded by them. Instead of that, it would be good to neutralize your main target in one of two ways. First method is to stun or kill the head of the security after he walks away from his subordinates. In order to implement this method, go to the upper floor, preferably using the stairs located to the left of the starting point (screen above).
Note a single guard patrolling the upper mansion balcony. If you're going to get rid of him, it is best to wait until he goes to the bathroom and stun him there or kill him, hiding his body in the box. If you want to only distract his attention, use any throwable item (e.g. statue bust on the table), forcing him to move away from the railing.
The reason why you want to get closer to the railing is the possibility to use the nearby window blinds system. Opening it will alarm the head of the security, forcing him to check the source of problem and going to the upper floor.
It is beast to get rid of the head of the security when he walks the stairs (screen above), but try to attack him so that he won't be able to defend himself. Depending on how many guards are still in the area, you can leave his body on the stairs or move it to the bathroom on the floor. Do not forget to take the keycard which will appear next to his body.
Another method of neutralizing the head of the security is to use sleeping pills. The first step in this case is of course to obtain them. Use the previously mentioned stairs, getting to the vicinity of the upper balcony. Go to the bathroom, finding sleeping pills as shown on the above screen.
The next step is to apply sleeping pills into a coffee, which the head of the security is drinking from time to time. A coffee mug can be found on the counter located on the ground floor, next to the mansion main room (screen above). The safest way is to approach it disguised as a chef. But if you're wearing guard disguise, approach the mansion main area from the kitchen and use Instinct or try to distract guards (e.g. with poker or radio taken from the kitchen), while you put sleeping pills into a coffee.
After that action you only have to wait until the head of the security drinks the coffee. He will then go to get some rest, sitting and falling asleep on the chair located next to the kitchen passage. Examine his body, looking for the keycard and then hide the body in a nearby closet.
Before you leave this place, you might look around for new evidence, located on the mansion ground floor, nearby the stairs more distant from the starting point. In order to use the keycard, you have to get to the door on the upper floor, shown on the above screen. If the local guard was not eliminated before, you might distract his attention somehow or wait until he goes to the bathroom. Use the keycard on the door scanner to load the next, last stage of this mission.