Are is correct.
Okay, there's more to the sentence?
These houses are a blend of commerical and residential units that SERVE(S?) as the office as well.
Should it be SERVE or SERVES?
These houses (plural subject) are (plural verb to match) a blend of commercial and residential units (plural) that (pronoun referring to the plural noun "units") serve (plural verb to match) as the office as well.
I find a problem at the end of that sentence, though: How can "commercial and residential units" (plural) serve as "the office" (singular)???
I don't know, my dad wrote it. I'm trying to fix his grammar.
Just ask him exactly what he means by the last part of that sentence. If he clarifies it for you, you should be able to make it make sense.
I have to do that with one of my grandsons all the time. He assumes that others will automatically know what he means! I have to keep questioning him ... and he's getting better ... slowly.
These houses are a blend of commerical and residential units that may also serve as offices.
^Perhaps that is what he meant.
...serve as an* office~
"serve as offices" makes more sense, yes.
Yeah, I see your point. Actually, I think only one of the houses is the office. So, I don't know, I'll ask him to claify. Thanks guys!