Can we say: "My house is 86m² big"?
People ask, "How big is your house?"
The answer does not use the word big, but uses the unit of measurement. "About 1200 sq. feet."
That's how we'd say it in the US, except we'd use "feet" of course.
They just moved into a new 5,000-square-foot house.
Wow! My own house is only 1600 square feet!
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
I'm not sure I understand what BobK wrote...
Can we say :"My house is 86 square metres"?
Yup - but you'd be more likely to use a different verb, 'measures' maybe. Alternatively, call it a 'decent-sized [or whatever - a vague description] house - about 86 square metres altogether'.