Here's a link to a 2019 Congressional Report that provides nine detailed scenarios for deducting mortgage interest. https://crsreports.congress.gov/product/pdf/IF/IF11111/6
Example 8, repeated below, is relevant for your scenario.
Example 8
Mortgage Origination date: February 2015.
Current mortgage balance: $800,000
Home equity loan Origination date: November 2018
Home equity loan balance: $90,000
Used for: Home remodel
The couple would be allowed to deduct all mortgage interest paid because they are below the applicable loan limit ($1 million) for mortgages originated before December 16, 2017. They would not be allowed to deduct the interest paid on the home equity loan because debt incurred before December 16, 2017, counts toward the $750,000 limit for debt incurred on or after December 16, 2017.
