After everyone separated after New York, Thor back to Asgard, Bruce to the unknown, Steve to explore the new world, and Clint and Natasha back to their mysterious spy ways, Tony Stark was left with a LOT of free time on his hands. So what does a billionaire playboy philanthropist do in his free time? Create things. Many don't know it, but he's a mechanic as well.
He concentrated mostly on the renovation of Stark Tower, which had been completely destroyed during the battle of New York. Pepper helped occasionally, but mostly left him to his own devices; she was busy dealing with the press. Tony and his robots worked day and night, fixing up the tower to its previous glory, and then some. Eventually he hired a crew of people to help rebuild the building, but everything had to be cleared by Tony first.
A nagging feeling was growing inside of him, but he couldn't place it. It intensified every time he looked at the giant 'A' still attached to the building.
If he was honest with himself, he missed his teammates. He had only known them for a few days, the bird brain even less, but they had been outstanding people, and just the type of people he wanted to be around. Of course, he wasn't going to admit that to anyone, not even to himself, but it was true.
About halfway through the renovation, the significance of the still hanging 'A' hit him, and he remembered an old, insane, and wonderful, idea. He called in the lead people on the project, and pulled out a scroll of plans he had created the evening Thor and Loki returned to Asgard. They were plans for floors for each of the Avengers.
Tony let the hired workers do most of the basic layout of the rooms, but he himself did the personal modifications. He added in specific touches that he thought would suit each Avenger's needs.
Now all he needed was a plan.