Joan had told Sherlock that he wouldn't have to do anything if she had a child, that it would be her responsibility alone. That said, it seems likely he would have been somewhat involved.

"Oh! Yes. Sorry, I should have explained." The Brownstone is such a central part of her life in New York that it seems like a matter-of-course. "It's a row house in Brooklyn. Technically it belongs to Sherlock's father. It's where we live, and where we do a lot of our work. Which means it's not exactly your usual home." She slips her hand into his and gives it a squeeze. "I would love to show it to you someday."