The TARDIS, being dimensionally transcendent as she was, had a fluctuating number of rooms at any given time. Some were all purpose, and so almost always there. Some where reserved solely for when they were needed and would spend the remaining time in that pocket of space that only the TARDIS could understand. Some were kept on her matrix, but never ever used. And then there were a handful of rooms that were always there, but never to be found by a wandering companion.
These were the rooms that the TARDIS held closest to her heart, the rooms that not even the Doctor would look in to. Oh, he'd wander by the doors from time to time. Sometimes he'd pause to lay his hand upon the door. But then the moment would pass and he'd be off again, down another corridor and pretending like the past had no hold over him.
These few precious rooms that the TARDIS couldn't bare to be without were guarded fiercely. One room looked like nothing more than a junk heap, mementos and souvenirs of times past that not even the Doctor knew she had kept. A few of the rooms at one point or another had belonged to companions long gone, those who had called the TARDIS home and whom she had claimed as her own in return.
One such room had belonged to a simple London shop girl. With too much pink and in a state of disarray that screamed 'I'll clean it soon,' the room looked as if the owner would return at any moment. This room was unique from all the others the TARDIS had kept, in that it had been changed. There was now an attached door that had never existed when the room had been lived in. This door, which could easily have been mistaken for a closet, led to an exact replica of a simple London Council flat. The artificial light of the TARDIS warmed the living space, perfect in every detail just as it had been when the Doctor had left it after transferring everything from the original dwelling. It had been the last time he'd been in these rooms, but the TARDIS kept them well, and she knew that knowledge comforted him.
His first home may no longer exist, but the last home he'd had, the last place that he'd truly felt a part of a family, was there if ever he needed it.