In theory, any of the millions of homeowners holding a assumable low-rate mortgage have a valuable perk to sell with their home. Still, real estate agents say it can be hard in practice to transfer them. For instance, homeowners who transfer a V.A.-backed mortgage can lose their ability to get another similar loan unless they can find a V.A.-eligible buyer to take their original mortgage.

Or consider a homeowner who has a low-rate mortgage but has paid a chunk of it down: To assume the loan, a buyer would have to come up with a large down payment to account for the seller’s equity — something that very few people can do.

Craig O’Boyle is hoping to create a business making assumptions faster and easier. Mr. O’Boyle is a real estate agent who has been selling homes in Colorado for three decades, long enough that he remembers having to read through the door-stopper contracts that buyers and sellers now just click through on DocuSign. Reading over the lines about certain loans being assumable, he said, he had long thought that if rates ever spiked those owners would suddenly discover that their debts had value.

“And then here comes this shift in the interest rate market,” Mr. O’Boyle said.

Last year, he and a partner started Assumption Solutions, a consulting firm that, for a $1,100-per-deal processing fee, helps real estate agents navigate transferring mortgages between sellers and buyers. In his pitch to agents, Mr. O’Boyle argues that they push sub-3 percent rates as they do marble countertops or a view of the mountains.

“You market this, and let’s say you’re competing against the house next door, your house should sell either faster or for more money,” he said.