Sarah Hyland and BF Welcome New Addition to Family

Sarah Hyland and her BF are taking it to the next level -- 'cause nothing says you're serious like committing to grabbing piles of dog crap together.

Sarah and Dominic Sherwood -- who've been together less than a year -- adopted a lab/shepherd mix puppy from Wylder's Holistic Pet Center in Studio City, CA last weekend ... according to the shop's owner.

The couple was looking for a tiny companion for the "Modern Family" star’s 5-year-old dog ... which she had before Dominic came on the scene. So, this is their first "child" together.

We're told they named her Boo, and immediately took her on a bitchin’ shopping spree, dropping more than a grand on fancy bowls, decorated collars, and yes ... Halloween costumes.

Welcome to the good life, Boo.