Emily had kept her relationship with Mike more intellectual/emotional than physical. She wanted him to love her, not lust after her. She wanted them to be friends first, lovers second.
Now that he was at the Agency, now that he needed a place to live, she would take the step.
Occupying a large flat owned by her Aunt, paying a pittance, having two bedrooms and two bathrooms, plus a large living area and kitchen, she could easily take a room mate. She had thought about it over the year but decided that she would be better alone than in a forced friendship.
To live with Mike, (having spent time with him at the Hotel) was prime choice. She believed he really liked her as she liked him.
Emily believed that love was there. She had capped her heart so as not to waste time and give herself a bad experience. Now, with him working at the same firm, living in the same flat, she could open her heart. And she wanted to.