The question is a subtle one and the answer is not simple.
It appears to depend largely on the positioning of except in the sentence, whether a noun precedes or follows it, and how it's being used (preposition, conjunction,).
Grammar Today says: Both "except" and "except for" are correct after a noun:
Another source quotes The Advanced Learner's Dictionary of Current English: "Except for" is required when what is excluded is different from what is included.
Similar guidance comes from Quora (below)
In your example, Except precedes the noun and the main house is different from the three cottages.
So, according to these rules, "Except for" the main house..... is required.
This also seems to me to be the idiomatic way of phrasing the sentence.
Numerous longer explanations and examples are given in the sources.