[James2001]

Must have been added very late in the show's run, it wasn't there in the late 90s.

Edit: A quick look and it was there on the 1999 title sequence, must have been added earlier than I thought... it wasn't there in the days when the Simpsons were living there though.

Looks like the garage got attatched to the house some point earlier during the 90s though, before the upstairs extension was added around 1998/99-ish, though unlike Billy Corkhill's garage on number 10 there wasn't a garden and tree directly in front of it!

Edit: A bit more digging and it looks like the garage
on number 9 had been there since 1985! Very early on, while Heather was still there and before the Corkhills arrived and added one to number 10. The upstairs extension over it appeared in 1998.