You can only enter Hawke's Manse after buying it from Lyle in Internal Affairs. The 8000 coins is a rather steep price, but I consider it to be worth it just in the items that can be found, not to mention the spells and having a free place to rest.
The first room you enter is the entryway, and it's here that you'll first meet Marjorie. The rest of the time you'll find Marjorie in the dining room, which is to the north of the entry hall.
Marjorie will talk to you a maximum of four times during the game. These are shown in pop up boxes, not in the dialogue box, so pay attention to pop ups while in Hawke's Manse.
The first talk will be to tell you there's Basilisks in the pantry. The pantry is located two rooms to the west of the dining room.
The second talk will be to tell you that there's Rats in the storeroom, which is located at the end of the northern hallway.
The third talk will be to tell you that "the Master" is about again. This means that you can find Hawke in the rear coutyard, which is two rooms west of the entry hall.
The fourth and final talk is to tell you that Hawke is roaming the halls again. This means that he can now be found in the far southeastern room of Hawke's Manse. He'll direct you to a statue in the northeast corner of the room. Looking at this statue after this will give you the Alien Blade.
The room just to the west of the entry hall is the library. There's one book on each of the bookshelves, all of which teach Level 3 spells. Two of them teach Slow Lv.3, one teaches Lightning Spray Lv.3 and the last one teaches Capture Soul Lv.3.
The rooms to the south of the Manse are both interactive rooms. The eastern room lets you store characters, the same as the Character Storage Room in Fort Emergence. This room also contains a secret passage along the western wall, which leads to another secret passage which finally leads to Hawke's treasure trove.
The southwestern room lets you rest, which heals your party and restores spell points.