A mortgage taken out after October 13, 1987, to buy, build, or improve your home (called home acquisition debt) but only if throughout the year these mortgages plus any grandfathered debt totaled $1 million or less. The limit is $500,000 if you're married filing separately. For homes acquired after December 15, 2017, the debt limitation is $750,000, or $375,000 if you're married filing separately.

Your client should receive a 1098 for any loan secured by his home, and if it meets the rules its deductible.