Once you have the key you can open all the doors that were previously locked. It will greatly speed up moving through the manor and it will unlock completely new areas.
Find other clues related to safe combination
Open the door on the left from the chest and the ones at the bottom of the library, on the right side of Sir Reginald's armour. Unlock both passages, but use the one below. In front of the door you will find a letter from Winston. Walk through the door at the end of the corridor and you will reach the ground floor of the Grand Hall. Climb upstairs and turn to the left twice. There you will find a chest you previously missed and another relict inside. Now you should have 31 relics, 20 documents and both available murals.
Now go back to the closed door on the right. The part of the manor that wasn't visited by anyone since a long time is ruined. Walk through the room with a collapsed tree hanging from the window and open the drawer on the corridor - behind the door on the left there is another recording of Lord Croft. Pick it up and open the door at the end of the corridor.
You will reach Lara's parents' bedroom. Next to the door there is Amelia's journal on the pouf. Once you collect it, open the cupboard on the left and, similarly as in the corridor, you will find an audiolog behind the left door. Then open the cupboard under the wall and pick up the item you find there. Amelia's wedding ring is located on the bed. Walk around the bed and take the necklace from the commode. Examine it thoroughly because there is an important clue on it. Under the window there is Lara's mother's photo and a few steps further, on the toiler, there are perfumes. Under the window in the corner of the room there is father's bust and on the commode on the right there is a watch which is the last item in this room. You should have 37 relics and 24 documents by now.
When returning, turn left in the room with the tree - you will enter a new corridor. At its end there is a cupboard with Winston's letter inside. Open the door on the left and you will enter Lara's mother's atelier. Directly on the left from the door there is Amelia's journal. Once you get it, pick up the photo on the working table behind you. On the desk under the wall there is another photo of Amelia and Lara. Under the window you will find another journal. It contains an important advice related to the safe combination. In the niche on the left side of the room there is Lara's mother's piano. Walking towards it will trigger a memory. Go back to the entrance to the atelier and use the ladder on the left from the door. On the left side you will find a chest with collection of Amelia's works and on the right side there is another journal. Collect the last item and go back to the safe.
Open Lord Croft's safe
After entering the office Lara will reminiscence her conversation with her father. Walk towards the safe and Lara will notice that she found all the clues.
On the Egyptian crown there is a symbol that matches number five on the hieroglyphs list. Lord Croft's favorite painting is the one with red rectangles - there are four of them on the painting. Lara's parents' wedding anniversary takes place four days before her mother's birthday which occur at 13th October - which means that the anniversary is at 9th October. So, the safe combination is 549 and that is the combination you must enter in order to open the lock. Inside it you will find a clay imprint of Lara's hand, an audiolog, continuation of the book about immortality and a blank page.
Find a way to read the blank page
Go back to the lower floor of Grand Hall. There is a fireplace there. Start the fire with the lighter and use its warmth to reveal the clues written with invisible ink. There is a clue at the back of the page. By comparing the hieroglyphs you will reveal a number combination - 1-4-2.