The answer to your question would be that the bolded words stand for a noun clause.
"Which house has more space" is a subject complement following the linking verb "to be". A subject complement is a word, phrase, or clause that follows a copular verb and refers back to modify, describe, or complete the grammatical subject of the sentence. What is more, if you change the order of words you can see that the phrase in question becomes the subject of the sentence: Which house has more space was one consideration.
