All single garage homes in this community have a backyard means that every single garage home has one backyard.
All single garage homes in this community have backyards. Technically, this could mean that the homes have several backyards which wouldn’t make much sense.
Both sentences are grammatically correct.[YSaerTTEW443543]
All houses have backyards. (This can mean that every house has one backyard or that some of the houses have several backyards.
Every house has a backyard. (This means every house has one backyard.)[YSaerTTEW443543]