In the sentence:

Tu sais, je crois que les maisons spacieuses comme ça, ce n’est pas pour moi !

the subject of the verb est is ce. Ce is a demonstrative pronoun, it is neutral and singular. It is impersonal but we can say it represents the idea expressed in the preceding sentence, but it does not stand for the word maisons itself.

If you wanted a verb in the plural you would have to have a subject in the plural, such as:

Tu sais, je crois que les maisons spacieuses comme ça, elles ne sont pas pour moi !

Ce is a singular pronoun so the verb is expected to be at the singular too:

Ce n’est pas pour moi.

However, as you learnt somewhere, ce can be followed by être at the plural (ce sont) when referring to a plural noun but this is only valid if that noun is itself following the verb and is therefore the actual subject of être. Instead of a noun, that can also be a collection of singular and/or plural nouns, or a third person plural pronoun, e.g.:

Ce sont des maisons pour moi.

Ce sont une maison, un appartement et un immeuble.

Ce sont eux (C’est eux is however frequently used in casual French.)

C’est nous, c’est vous (plural, but not third person.)

The last part of your sentence might then be rewritten that way:

Ce ne sont pas des maisons faites pour moi !