Seamus' mother is an Ill Girl (or simply very elderly), and he is looking after her.
Seamus' house or apartment burned down, and he had to move back in with his mom...but only until he finds a new place to live.
Seamus is careful with his money, and high house prices/rents mean he can't justify moving out.
Seamus O'Taku moved to an apartment of his own on his twelfth birthday, bought with money he made on the stock market in the name of a fake identity he created for himself. This was all in protest to his parents refusing to give him a raise in his pocket money. When his dad loses his job, they're forced to live in his basement. Hilarity Ensues.
Seamus' mom moves into his house.
Gender Inverted: Alex has yet to move out of her family's house, even though she's through with higher education, has a relationship and a job.
Seamus is first introduced living in his mother's basement. It's later revealed that he's getting his own penthouse apartment knocked through into the flat below and had to move out while the builders were working; he came to his mother's house to try one more time to persuade her to move into the mansion he bought her, but she won't budge.
Seamus saves up enough money to move into a low-rent apartment downtown.
...And then he's bankrupted, and has to move back home for real.
But finds he can't make it on his own when times get tough, and has to move back home.
Defied: Seamus doesn't want to move back with his mom. He instead lives off his emergency fund as long as he can while looking for a job, or crashes at a friend's apartment.
Discussed: "I hear Seamus still lives with his mom!"
Conversed: "It's just not right for a man his age to live with his mother..."
Implied: Mike and Alex, who are about to marry, have moved in together away from their respective parents, and the audience gets to see their apartment in detail ... not so for their less fortunate Drop-In Character friend, Seamus. A lot of Seamus' anecdotes seem to involve his parents, as well, but whenever he's asked about his living arrangements he changes the subject.
Deconstructed: This is going to be inconvenient for Seamus (who may have a hard time being taken seriously and/or getting a girlfriend) and for his mother (who may still have to provide for him and may not be able to do things she wants to do because he still depends on her).
Reconstructed: Seamus is saving money, and he also pays rent and otherwise earns his keep there. He is not a burden on his mother, and eventually leaves the nest. He gains skills he needs to survive 'Out There'.