This is what confidence in the nation’s reviving housing market looks like: a $100 million office building in West Des Moines with a commons area the size of a football field.

Wells Fargo & Co. broke ground Friday on the newest addition to its Wells Fargo Home Mortgage operations in the Des Moines area, the national base for the bank’s storied mortgage operation.

It’s the fourth office building at the bank’s Jordan Creek Campus in West Des Moines and includes a training center, parking garage and enough room to eventually house about 1,800 workers. Construction should be done in two years.

The expansion comes as the nation continues mending from the foreclosure disaster, and as housing experts forecast a decline in the burst of mortgage refinance activity that has powered the earnings of several big banks recently. It’s anybody’s guess how quickly the appetite for new mortgages generated by home purchases starts making up for the refi decline for lenders.

In its earnings call this past week, Wells Fargo CEO John Stumpf said he expects steady gains in home sales, building and appreciation in the near term. Mortgage revenue slowed in the first quarter, but the bank’s volume of mortgages for buying homes, as opposed to refinancing them, was up 31 percent from last year.

Wells Fargo, the country’s largest mortgage originator, funds about one in four new mortgages in the country, more than twice the volume of any other lender, according to Inside Mortgage Finance numbers.

“Wells has managed its mortgage operations — and profits — as well or better than any other big bank or lender. So, it’s hard to fault them on being too optimistic — or not realistic — about the housing market going forward,” said Inside Mortgage Finance publisher Guy Cecala.

“Their mortgage emphasis in 2013 is on the home purchase market, and given the expected decline in refi activity and the growth in home sales, that seems to be a good bet.”

Wells Fargo Home Mortgage has a major operation in the Twin Cities, with about 8,500 employees. Many work in the bank’s south Minneapolis complex where they originate, process and close mortgages, service mortgages, and handle legal, marketing and compliance work. Some also work at the Metropoint complex in St. Louis Park.

Minneapolis-based Ryan Cos. has been exploring options for a mixed-used development on four blocks owned by the Star Tribune near the new Minnesota Vikings site, and there has been speculation that Wells Fargo is interested in space in it.

A Wells Fargo spokeswoman said the bank is always evaluating its space needs, but has no specific information to share about any expansion plans in the Twin Cities.