Depending on where you live, national averages can seem like a steal. Rene Artale’s four-bedroom house near Newcastle, in Westchester County, N.Y., suffered some damage during a storm last winter. A tree fell in the yard, damaging her fence, arbor and retaining wall. And heavy snow caused her roof to leak. The repair bills just kept piling up. Removing the tree, $3,800. Repairing the wall, $4,000. Fixing the roof, $3,800. Fixing the picket fence, $2,800. “It’s obscene,” Ms. Artale, 47, said.

Of course, some home repairs can wait. If the exterior of your house needs a paint job, you might be able to sand and patch the worst spots while you save up for the larger project. But if a co-op or condo board decides the exterior needs a face-lift and it needs it now, you may just get a letter alerting you to a substantial assessment.

When that happens, all you can do is pay up, as happened with Jason Hark and Kenneth Larivee. In 2016, the couple got hit with a $28,000 assessment to replace the siding on their West Orange, N.J. townhouse in a condo association. This cost came on top of their $610 monthly association fee. After speaking with siding companies, Mr. Hark estimated that if his unit had been a stand-alone property, the project would have cost closer to $7,500. “But we couldn’t fight it,” said Mr. Hark, a director at WCBS-TV.

In theory, the cost of building-wide improvements should be less in a condo because it is spread out across dozens of residents, and the sheer scale of the project could lower the overall price. But it does not always work out that way. Condo board members are volunteers who may not be skilled negotiators or knowledgeable about construction. They may take the first bid they get, or agree to more work than is necessary. If complications increase the cost of the job, residents won’t necessarily know. “This is one of the potential downsides of a condo or co-op scheme,” said Eric D. Sherman, a real estate lawyer and partner in the New York office of Pryor Cashman. “Lots of the time, boards are under the gun. They’re not paid for their efforts, they take the first bid from the first contractor that they see and they say, ‘sign me up.’”

So last year, Mr. Hark and Mr. Larivee, who works in digital marketing, sold their condo and moved to a four-bedroom house, also in West Orange. They now have to mow their lawn and shovel their driveway when it snows, but when they had a drainage problem in the backyard, they got to choose their contractor and schedule the job based on when it made the most financial sense for them.

“It’s not about the monthly costs, it’s really about not having any control,” said Mr. Hark. “Now I can do whatever I want.”

The average single-family homeowner spends around $2,000 a year on maintenance, according to Bankrate.com. That is considerably less than the monthly fees for most condos or co-ops. But even though the monthly outlay for those homeowners might be lower than that of condo or co-op owners, house owners generally are not squirreling away those savings for a rainy day. Nearly half of them have less than $1,000 saved, and a third have nothing saved, according to Liberty Mutual Insurance. So when that sump pump suddenly fails, odds are, we’re scrambling to pay the plumber for a new one.